Tyler asbestos workers: mortality experience in a cohort exposed to amosite

An updated evaluation of reported no-observed adverse effect levels for chrysotile, amosite, and crocidolite asbestos for lung cancer and mesothelioma

This analysis updates two previous analyses that evaluated the exposure-response relationships for lung cancer and mesothelioma in chrysotile-exposed cohorts. We reviewed recently published studies, as well as updated information from previous studies. Based on the 16 studies considered for chrysotile (<10% amphibole), we identified the "no-observed adverse effect level" (NOAEL) for lung cancer and/or mesothelioma; it should be noted that smoking or previous or concurrent occupational exposure to amphiboles (if it existed) was not controlled for. NOAEL values ranged from 2.3-<11.5 f/cc-years to 1600-3200 f/cc-years for lung cancer and from 100-<400 f/cc-years to 800-1599 f/cc-years for mesothelioma. The range of best-estimate NOAELs was estimated to be 97-175 f/cc-years for lung cancer and 250-379 f/cc-years for mesothelioma. None of the six cohorts of cement or friction product manufacturing workers exhibited an increased risk at any exposure level, while all but one of the six studies of textile workers reported an increased risk at one or more exposure levels. This is likely because friction and cement workers were exposed to much shorter chrysotile fibers. Only eight cases of peritoneal mesothelioma were reported in all studies on predominantly chrysotile-exposed cohorts combined. This analysis also proposed best-estimate amosite and crocidolite NOAELs for mesothelioma derived by the application of relative potency estimates to the best-estimate chrysotile NOAELs for mesothelioma and validated by epidemiology studies with exposure-response information. The best-estimate amosite and crocidolite NOAELs for mesothelioma were 2-5 f/cc-years and 0.6-1 f/cc-years, respectively. The rate of peritoneal mesothelioma in amosite- and crocidolite-exposed cohorts was between approximately 70- to 100-fold and several-hundred-fold higher than in chrysotile-exposed cohorts, respectively. These findings will help characterize potential worker and consumer health risks associated with historical and current chrysotile, amosite, and crocidolite exposures.

Meta-Analysis of the Association between Asbestos Exposure and Esophageal Cancer

BACKGROUND

We conducted a meta-analysis to quantitatively assess the association between asbestos exposure and esophageal cancer.

METHODS

We systematically collected articles from three electronic databases and calculated the pooled standardized mortality rate (SMR) from the meta-analysis. Subgroup analysis according to the type of asbestos exposure, follow-up years, sample size, industry classification, sex, and high-dose exposure was conducted.

RESULTS

From 242 studies, 34 cohort studies were included in our meta-analysis. Pooled SMR was positively associated with asbestos exposure and esophageal cancer (pooled SMR = 1.28; 95% confidence interval (CI) 1.19-1.38, p < 0.00001). In the subgroup analysis, (1) chrysolite, (2) four groups with follow-up over ten years, (3) the textile industry and shipyard, (4) both male and female, and (5) eight studies on highest asbestos exposure, all the subgroups showed significantly increased pooled SMRs.

CONCLUSION

Asbestos exposure was significantly and positively associated with esophageal cancer, especially chrysolite. Considering the long latency period, we suggest that patients should be followed up for cancer, including esophageal cancer, for over ten years.

Risk of mesothelioma after cessation of asbestos exposure: a systematic review and meta-regression

PURPOSE

A 'risk reversal' has been observed for several human carcinogens following cessation of exposure, but it is unclear whether it also exists for asbestos-related mesothelioma.

METHODS

We conducted a systematic review of the literature and identified nine studies that reported information on risk of mesothelioma after cessation of asbestos exposure, and performed a meta-regression based on random effects models. As comparison we analyzed results on lung cancer risk from four of these studies.

RESULTS

A total of six risk estimates from five studies were included in the meta-analysis. The summary relative risk (RR) of mesothelioma for 10-year interval since cessation of exposure was 1.02 [95% confidence interval (CI) 0.87-1.19; p-heterogeneity 0.01]. The corresponding RR of lung cancer was 0.91 (95% CI 0.84-0.98).

CONCLUSIONS

This analysis provides evidence that the risk of mesothelioma does not decrease after cessation of asbestos exposure, while lung cancer risk does.

Occupational Asbestos Exposure and Incidence of Colon and Rectal Cancers in French Men: The Asbestos-Related Diseases Cohort (ARDCo-Nut)

BACKGROUND

The relationships between asbestos exposure and colorectal cancer remain controversial.

OBJECTIVES

We examined the association between asbestos exposure and colorectal cancer incidence.

METHODS

Volunteer retired workers previously exposed to asbestos were invited to participate in the French ARDCo screening program between 2003 and 2005. Additional data on risk factors for colorectal cancer were collected from the ARDCo-Nut subsample of 3,769 participants in 2011. Cases of colon and rectal cancer were ascertained each year through 2014 based on eligibility for free medical care following a cancer diagnosis. Survival regression based on the Cox model was used to estimate the relative risk of colon and rectal cancer separately, in relation to the time since first exposure (TSFE) and cumulative exposure index (CEI) to asbestos, and with adjustment for smoking in the overall cohort and for smoking, and certain risk factors for these cancers in the ARDCo-Nut subsample.

RESULTS

Mean follow-up was 10.2 years among 14,515 men, including 181 colon cancer and 62 rectal cancer cases (41 and 17, respectively, in the ARDCo-Nut subsample). In the overall cohort, after adjusting for smoking, colon cancer was significantly associated with cumulative exposure (HR = 1.14; 95% CI: 1.04, 1.26 for a 1-unit increase in ln-CEI) and ≥ 20-40 years since first exposure (HR = 4.67; 95% CI: 1.92, 11.46 vs. 0-20 years TSFE), and inversely associated with 60 years TSFE (HR = 0.26; 95% CI: 0.10, 0.70). Although rectal cancer was also associated with TSFE 20-40 years (HR = 4.57; 95% CI: 1.14, 18.27), it was not associated with ln-CEI, but these findings must be interpreted cautiously due to the small number of cases.

CONCLUSIONS

Our findings provide support for an association between occupational exposure to asbestos and colon cancer incidence in men. Citation: Paris C, Thaon I, Hérin F, Clin B, Lacourt A, Luc A, Coureau G, Brochard P, Chamming's S, Gislard A, Galan P, Hercberg S, Wild P, Pairon JC, Andujar P. 2017. Occupational asbestos exposure and incidence of colon and rectal cancers in French men: the Asbestos-Related Diseases Cohort (ARDCo-Nut). Environ Health Perspect 125:409-415; http://dx.doi.org/10.1289/EHP153.

Esophagus cancer and occupational exposure to asbestos: results from a meta-analysis of epidemiology studies

The relationship between occupational asbestos exposure and esophagus cancer (EC) is not fully understood. We performed a meta-analysis to quantitatively assess the association. We systematically searched databases of PubMed, EMBASE, and Web of Science for studies with quantitative estimates of asbestos exposure and EC mortality. Pooled standardized mortality ratios (SMRs) and their corresponding 95% confidence intervals (CIs) were calculated. Twenty cohort studies on EC and asbestos exposure were included in this meta-analysis. Overall, occupational exposure to asbestos was associated with an excess risk of EC (SMR = 1.24, 95% CI: 1.13-1.38, P < 0.001), with little evidence of heterogeneity among studies (I(2) = 0.0%, P = 0.682). Being male, exposure to chrysotile or mixed asbestos, working at textile industry, long study follow-up (≥20 years), Asia, Europe and America cohorts with larger cohort size (>500), and high-exposure group all contribute to significantly higher SMR. Publication bias was not detected (Egger's test P-value = 0.374). This meta-analysis suggested that occupational asbestos exposure might be associated with an increased risk of EC in male. High-exposure level of asbestos could contribute to significantly higher risk of EC mortality.

Risk of Cerebrovascular Events in Pneumoconiosis Patients: A Population-based Study, 1996-2011

Pneumoconiosis is a parenchymal lung disease that develops through the inhalation of inorganic dust at work. Cerebrovascular and cardiovascular events are leading causes of mortality and adult disability worldwide. This retrospective cohort study investigated the association between pneumoconiosis, and cerebrovascular and cardiovascular events by using a nationwide population-based database in Taiwan. The data analyzed in this study was retrieved from the Taiwan National Health Insurance Research Database. We selected 6940 patients with pneumoconiosis from the database as our study cohort. Another 27,760 patients without pneumoconiosis were selected and matched with those with pneumoconiosis according to age and sex as the comparison cohort. We used univariate and multivariate Cox proportional-hazard regression analyses to determine the association between pneumoconiosis and the risk of cerebrovascular and cardiovascular events after adjusting for medical comorbidities. After adjustment for age, sex, and comorbidities, the patients with pneumoconiosis exhibited a significantly higher incidence of ischemic stroke (hazard ratio [HR] 1.14, 95% confidence interval [CI] 1.05-1.24) than did those without pneumoconiosis. The incidence of hemorrhagic stroke was higher, but not significant, in the pneumoconiosis patients (HR 1.20, 95% CI 0.99-1.46). No statistically significant differences were observed between the pneumoconiosis and nonpneumoconiosis groups in acute coronary syndrome (HR 1.10, 95% CI 0.95-1.26). The findings of this study reveal an association between pneumoconiosis and a higher risk of cerebrovascular events after adjustment for comorbidities. Healthcare providers should control the related risk factors for primary prevention of stroke in pneumoconiosis patients.

Tyler asbestos workers: A mortality update in a cohort exposed to amosite

The Tyler asbestos plant produced pipe insulation from 1954 to 1972 and exclusively used amosite asbestos. There were 1130 former workers of this plant during the period of operation. A death certificate mortality analysis was published regarding this plant in 1998 for the period through 1993. This study represents an update of the mortality analysis with additional certificates collected for deaths occurring through 2011.Searches of the National Death Index database were conducted in 2004 and again in 2013. At the time of the latter search, only deaths occurring through 2011 were available. In total, 265 distinct additional death certificates were secured and added to 304 available from the original study. After the new certificates were coded (ICD-9), data were analyzed using the Centers for Disease Control and Prevention Life Table Analysis System (LTAS) and standard mortality ratios (SMR) generated with 95% confidence limits (CL). LTAS constructs cause-specific mortality rates by age, gender, race, and person-time at risk, and compares observed rates with a referent population in order to derive SMR. A significant excess number of deaths due to nonmalignant respiratory disease (asbestosis) and from select malignant neoplasms were identified. There were in total 23 mesothelioma deaths (4% of deaths), with 16 pleural and 7 peritoneal. The SMR for malignant neoplasms of the trachea, bronchus, and lung was 244 (with 95% CL 196, 300), suggesting that exposed workers from this cohort were nearly 2.5-fold (244 %) more likely to die from this cause as the general referent population. The analysis also showed that exposures of short duration (<6 mo) produced significantly elevated SMR for all respiratory cancers, lung cancer, and pleural mesothelioma. There was a significant difference in median duration of exposure for mesothelioma types, confirming association of peritoneal mesothelioma with longer duration of exposure. Deaths due to intestinal cancer (predominantly colon; not including rectum) were also found in excess. The mortality experience of the Tyler cohort continues to be followed with great interest, given the exclusivity of exposure to amosite. Data confirm the inherent pathogenicity of this fiber type for nonmalignant disease as well as select cancers, particularly relevant given the importance of this amphibole's use in the United States.

Occupational exposure to asbestos and cardiovascular related diseases: A meta-analysis

Asbestos has become one of the leading causes of death among occupational workers in the world. The association between asbestos and cardiovascular disease is less reported. We performed a meta-analysis to quantify the association between asbestos exposure and the mortality of cardiovascular related diseases. We performed a systematic review in the PubMed database before December 2014. All cohort studies citing the standardized mortality ratio (SMR) of cardiovascular related diseases in workers exposed to asbestos were collected. We then calculated the pooled standardized mortality ratios of such diseases. Sixteen studies were included. The combined results from all studies indicated the pooled SMR estimate for cardiovascular related diseases was 1.11 (95% CI, 1.01–1.22). This meta-analysis showed that asbestos exposure significantly increased the risk of cardiovascular related diseases in exposed workers.

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Asbestos exposure increased the risk of cardiovascular related diseases.

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Higher risk of cardiovascular related disease mortality was using chrysotile.

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Developing countries obtained a higher SMR compared to those in developed countries.

Toxicological and epidemiological studies on effects of airborne fibers: coherence and public [corrected] health implications

Airborne fibers, when sufficiently biopersistent, can cause chronic pleural diseases, as well as excess pulmonary fibrosis and lung cancers. Mesothelioma and pleural plaques are caused by biopersistent fibers thinner than ∼0.1 μm and longer than ∼5 μm. Excess lung cancer and pulmonary fibrosis are caused by biopersistent fibers that are longer than ∼20 μm. While biopersistence varies with fiber type, all amphibole and erionite fibers are sufficiently biopersistent to cause pathogenic effects, while the greater in vivo solubility of chrysotile fibers makes them somewhat less causal for the lung diseases, and much less causal for the pleural diseases. Most synthetic vitreous fibers are more soluble in vivo than chrysotile, and pose little, if any, health pulmonary or pleural health risk, but some specialty SVFs were sufficiently biopersistent to cause pathogenic effects in animal studies. My conclusions are based on the following: 1) epidemiologic studies that specified the origin of the fibers by type, and especially those that identified their fiber length and diameter distributions; 2) laboratory-based toxicologic studies involving fiber size characterization and/or dissolution rates and long-term observation of biological responses; and 3) the largely coherent findings of the epidemiology and the toxicology. The strong dependence of effects on fiber diameter, length, and biopersistence makes reliable routine quantitative exposure and risk assessment impractical in some cases, since it would require transmission electronic microscopic examination, of representative membrane filter samples, for determining statistically sufficient numbers of fibers longer than 5 and 20 μm, and those thinner than 0.1 μm, based on the fiber types.

Stomach cancer and occupational exposure to asbestos: a meta-analysis of occupational cohort studies

Background:

A recent Monographs Working Group of the International Agency for Research on Cancer concluded that there is limited evidence for a causal association between exposure to asbestos and stomach cancer.

Methods:

We performed a meta-analysis to quantitatively evaluate this association. Random effects models were used to summarise the relative risks across studies. Sources of heterogeneity were explored through subgroup analyses and meta-regression.

Results:

We identified 40 mortality cohort studies from 37 separate papers, and cancer incidence data were extracted for 15 separate cohorts from 14 papers. The overall meta-SMR for stomach cancer for total cohort was 1.15 (95% confidence interval 1.03–1.27), with heterogeneous results across studies. Statistically significant excesses were observed in North America and Australia but not in Europe, and for generic asbestos workers and insulators. Meta-SMRs were larger for cohorts reporting a SMR for lung cancer above 2 and cohort sizes below 1000.

Conclusions:

Our results support the conclusion by IARC that exposure to asbestos is associated with a moderate increased risk of stomach cancer.

Occupational asbestos exposure and lung cancer--a systematic review of the literature

The objective of this study was to evaluate the scientific literature concerning asbestos and lung cancer, emphasizing low-level exposure. A literature search in PubMed and Embase resulted in 5,864 citations. Information from included studies was extracted using SIGN. Twenty-one statements were evidence graded. The results show that histology and location are not helpful in differentiating asbestos-related lung cancer. Pleural plaques, asbestos bodies, or asbestos fibers are useful as markers of asbestos exposure. The interaction between asbestos and smoking regarding lung cancer risk is between additive and multiplicative. The findings indicate that the association between asbestos exposure and lung cancer risk is basically linear, but may level off at very high exposures. The relative risk for lung cancer increases between 1% and 4% per fiber-year (f-y)/mL, corresponding to a doubling of risk at 25-100 f-y/mL. However, one high-quality case-control study showed a doubling at 4 f-y/mL.

Lung cancer risk at low cumulative asbestos exposure: meta-regression of the exposure-response relationship

PURPOSE

Existing estimated lung cancer risks per unit of asbestos exposure are mainly based on, and applicable to, high exposure levels. To assess the risk at low cumulative asbestos exposure, we provide new evidence by fitting flexible meta-regression models, a notably new and more robust method.

METHODS

Studies were selected if lung cancer risk per cumulative asbestos exposure in at least two exposure categories was reported. From these studies (n = 19), we extracted 104 risk estimates over a cumulative exposure range of 0.11-4,710 f-y/ml. We fitted linear and natural spline meta-regression models to these risk estimates. A natural spline allows risks to vary nonlinearly with exposure, such that estimates at low exposure are less affected by estimates in the upper exposure categories. Associated relative risks (RRs) were calculated for several low cumulative asbestos exposures.

RESULTS

A natural spline model fitted our data best. With this model, the relative lung cancer risk for cumulative exposure levels of 4 and 40 f-y/ml was estimated between 1.013 and 1.027, and 1.13 and 1.30, respectively. After stratification by fiber type, a non-significant three- to fourfold difference in RRs between chrysotile and amphibole fibers was found for exposures below 40 f-y/ml. Fiber-type-specific risk estimates were strongly influenced by a few studies.

CONCLUSIONS

The natural spline regression model indicates that at lower asbestos exposure levels, the increase in RR of lung cancer due to asbestos exposure may be larger than expected from previous meta-analyses. Observed potency differences between different fiber types are lower than the generally held consensus. Low-exposed industrial or population-based cohorts with quantitative estimates of asbestos exposure a required to substantiate the risk estimates at low exposure levels from our new, flexible meta-regression.

Overreliance on a single study: there is no real evidence that applying quality criteria to exposure in asbestos epidemiology affects the estimated risk

A critical need exists for reliable risk management policies and practices that can effectively mitigate asbestos-related health threats, and such policies and practices need to be based on sound science that adequately distinguishes hazardous situations from those that are not. Toward that end, the disparate means by which study quality has been addressed in recent meta-analyses used to establish potency factors (K ( L ) and K ( M ) values) for asbestos cancer risks were compared by conducting additional sensitivity analyses. Results suggest that, other than placing undue emphasis on the influence of the K ( L ) and K ( M ) values reported from a single study, there appears to be little to no evidence of a systematic effect of study quality on K ( L ) or K ( M ) values; none of the findings warrant excluding studies from current or future meta-analyses. Thus, we argue that it is better to include as much of the available data as possible in these analyses while formally addressing uncertainty as part of the analysis itself, rather than sequentially excluding studies based on one type of limitation or another. Throwing out data without clearly proving some type of bias is never a good idea because it will limit both the power to test various hypotheses and the confidence that can be placed in any findings that are derived from the resulting, truncated data set. We also believe that it is better to identify the factors that contribute to variation between studies included in a meta-analysis and, by adjusting for such factors as part of a model, showing that the disparate values from individual studies can be reconciled. If such factors are biologically reasonable (based on other evidence) and, if such a model can be shown to fit the data from all studies in the meta-analysis, the model is likely to be predictive of the parameters being evaluated and can then be applied to new (unstudied) environments.

Estimating the asbestos-related lung cancer burden from mesothelioma mortality

BACKGROUND

Quantifying the asbestos-related lung cancer burden is difficult in the presence of this disease's multiple causes. We explore two methods to estimate this burden using mesothelioma deaths as a proxy for asbestos exposure.

METHODS

From the follow-up of 55 asbestos cohorts, we estimated ratios of (i) absolute number of asbestos-related lung cancers to mesothelioma deaths; (ii) excess lung cancer relative risk (%) to mesothelioma mortality per 1000 non-asbestos-related deaths.

RESULTS

Ratios varied by asbestos type; there were a mean 0.7 (95% confidence interval 0.5, 1.0) asbestos-related lung cancers per mesothelioma death in crocidolite cohorts (n=6 estimates), 6.1 (3.6, 10.5) in chrysotile (n=16), 4.0 (2.8, 5.9) in amosite (n=4) and 1.9 (1.4, 2.6) in mixed asbestos fibre cohorts (n=31). In a population with 2 mesothelioma deaths per 1000 deaths at ages 40-84 years (e.g., US men), the estimated lung cancer population attributable fraction due to mixed asbestos was estimated to be 4.0%.

CONCLUSION

All types of asbestos fibres kill at least twice as many people through lung cancer than through mesothelioma, except for crocidolite. For chrysotile, widely consumed today, asbestos-related lung cancers cannot be robustly estimated from few mesothelioma deaths and the latter cannot be used to infer no excess risk of lung or other cancers.

A meta-analysis of asbestos and lung cancer: is better quality exposure assessment associated with steeper slopes of the exposure-response relationships?

BACKGROUND

Asbestos is a well-recognized cause of lung cancer, but there is considerable between-study heterogeneity in the slope of the exposure-response relationship.

OBJECTIVE

We considered the role of quality of the exposure assessment to potentially explain heterogeneity in exposure-response slope estimates.

DATA SOURCES

We searched PubMed MEDLINE (1950-2009) for studies with quantitative estimates of cumulative asbestos exposure and lung cancer mortality and identified 19 original epidemiological studies. One was a population-based case-control study, and the others were industry-based cohort studies.

DATA EXTRACTION

Cumulative exposure categories and corresponding risks were abstracted. Exposure-response slopes [KL (lung cancer potency factor of asbestos)] were calculated using linear relative risk regression models.

DATA SYNTHESIS

We assessed the quality of five exposure assessment aspects of each study and conducted random effects univariate and multivariate meta-regressions. Heterogeneity in exposure-response relationships was greater than expected by chance (I2 = 64%). Stratification by exposure assessment characteristics revealed that studies with well-documented exposure assessment, larger contrast in exposure, greater coverage of the exposure history by exposure measurement data, and more complete job histories had higher meta-KL values than did studies without these characteristics. The latter two covariates were most strongly associated with the KL value. Meta-KL values increased when we incrementally restricted analyses to higher-quality studies.

CONCLUSIONS

This meta-analysis indicates that studies with higher-quality asbestos exposure assessment yield higher meta-estimates of the lung cancer risk per unit of exposure. Potency differences for predominantly chrysotile versus amphibole asbestos-exposed cohorts become difficult to ascertain when meta-analyses are restricted to studies with fewer exposure assessment limitations.

Occupational exposures to asbestos, polycyclic aromatic hydrocarbons and solvents, and cancers of the oral cavity and pharynx: a quantitative literature review

PURPOSE

The role of occupational risk factors in oral and pharyngeal cancer is not well known and is possibly underestimated. This quantitative review summarizes epidemiological findings on exposure to asbestos, polycyclic aromatic hydrocarbons and solvents, and cancers of the oral cavity and pharynx.

METHODS

A systematic literature search was performed. We analyzed 63 publications: 8 from case-control studies and 55 from cohort studies. For agents with at least five available studies with homogenous exposure, a series of meta-analyses was conducted to provide quantitative pooled estimates of risks, using random effect models.

RESULTS

Exposure to asbestos (meta-RR 1.25; 95% CI 1.10-1.42) and to polycyclic aromatic hydrocarbons (meta-RR 1.14; 95% CI 1.02-1.28) was found to be associated with oral and pharyngeal cancer risk. On the other hand, no association was found with exposure to solvents in general (meta-RR 0.98; 95% CI 0.77-1.23) but the strong heterogeneity between studies suggested differences in exposures. The small number of studies with homogeneous exposure did not allow meta-analyses for specific solvents.

CONCLUSIONS

Future investigations should overcome common weaknesses of past studies, in terms of sample size, characterization of exposure, and classification of cancer sites.

Comparing milled fiber, Quebec ore, and textile factory dust: has another piece of the asbestos puzzle fallen into place?

Results of a meta-analysis indicate that the variation in potency factors observed across published epidemiology studies can be substantially reconciled (especially for mesothelioma) by considering the effects of fiber size and mineral type, but that better characterization of historical exposures is needed before improved exposure metrics potentially capable of fully reconciling the disparate potency factors can be evaluated. Therefore, an approach for better characterizing historical exposures, the Modified Elutriator Method (MEM), was evaluated to determine the degree that dusts elutriated using this method adequately mimic dusts generated by processing in a factory. To evaluate this approach, elutriated dusts from Grade 3 milled fiber (the predominant feedstock used at a South Carolina [SC] textile factory) were compared to factory dust collected at the same facility. Elutriated dusts from chrysotile ore were also compared to dusts collected in Quebec mines and mills. Results indicate that despite the substantial variation within each sample set, elutriated dusts from Grade 3 fiber compare favorably to textile dusts and elutriated ore dusts compare to dusts from mines and mills. Given this performance, the MEM was also applied to address the disparity in lung cancer mortality per unit of exposure observed, respectively, among chrysotile miners/millers in Quebec and SC textile workers. Thus, dusts generated by elutriation of stockpiled chrysotile ore (representing mine exposures) and Grade 3 milled fiber (representing textile exposures) were compared. Results indicate that dusts from each sample differ from one another. Despite such variation, however, the dusts are distinct and fibers in Grade 3 dusts are significantly longer than fibers in ore dusts. Moreover, phase-contrast microscopy (PCM) structures in Grade 3 dusts are 100% asbestos and counts of PCM-sized structures are identical, whether viewed by PCM or transmission electron microscope (TEM). In contrast, a third of PCM structures in ore dusts are not asbestos and only a third that are counted by PCM are also counted by TEM. These distinctions also mirror the characteristics of the bulk materials themselves. Perhaps most important, when the differences in size distributions and PCM/TEM distinctions in these dusts are combined, the combined difference is sufficient to completely explain the difference in exposure/response observed between the textile worker and miner/miller cohorts. Importantly, however, evidence that such an explanation is valid can only be derived from a meta-analysis (risk assessment) covering a diverse range of epidemiology study environments, which is beyond the scope of the current study. The above findings suggest that elutriator-generated dusts mimic factory dusts with sufficient reliability to support comparisons between historical exposures experienced by the various cohorts studied by epidemiologists. A simulation was also conducted to evaluate the relative degree that the characteristics of dust are driven by the properties of the bulk material processed versus the nature of the mechanical forces applied. That results indicate it is the properties of bulk materials reinforces the theoretical basis justifying use of the elutriator to reconstruct historical exposures. Thus, the elutriator may be a valuable tool for reconstructing historical exposures suitable for supporting continued refinements of the risk models being developed to predict asbestos-related cancer risk.

Developments in asbestos cancer risk assessment

BACKGROUND

Efforts have been made for 25 years to develop asbestos risk assessments that provide valid information about workplace and community cancer risks. Mathematical models have been applied to a group of workplace epidemiology studies to describe the relationships between exposure and risk. EPA's most recent proposed method was presented at a public meeting in July 2008.

METHODS

Risk assessments prepared by USEPA, OSHA, and NIOSH since 1972 were reviewed, along with related literature.

RESULTS AND CONCLUSIONS

None of the efforts to use statistical models to characterize relative cancer potencies for asbestos fiber types and sizes have been able to overcome limitations of the exposure data. Resulting uncertainties have been so great that these estimates should not be used to drive occupational and environmental health policy. The EPA has now rejected and discontinued work on its proposed methods for estimating potency factors. Future efforts will require new methods and more precise and reliable exposure assessments. However, while there may be genuine need for such work, a more pressing priority with regard to the six regulated forms of asbestos and other asbestiform fibers is to ban their production and use.

Update of potency factors for asbestos-related lung cancer and mesothelioma

The most recent update of the U.S. Environmental Protection Agency (EPA) health assessment document for asbestos (Nicholson, 1986, referred to as "the EPA 1986 update") is now 20 years old. That document contains estimates of "potency factors" for asbestos in causing lung cancer (K(L)'s) and mesothelioma (K(M)'s) derived by fitting mathematical models to data from studies of occupational cohorts. The present paper provides a parallel analysis that incorporates data from studies published since the EPA 1986 update. The EPA lung cancer model assumes that the relative risk varies linearly with cumulative exposure lagged 10 years. This implies that the relative risk remains constant after 10 years from last exposure. The EPA mesothelioma model predicts that the mortality rate from mesothelioma increases linearly with the intensity of exposure and, for a given intensity, increases indefinitely after exposure ceases, approximately as the square of time since first exposure lagged 10 years. These assumptions were evaluated using raw data from cohorts where exposures were principally to chrysotile (South Carolina textile workers, Hein et al., 2007; mesothelioma only data from Quebec miners and millers, Liddell et al., 1997) and crocidolite (Wittenoom Gorge, Australia miners and millers, Berry et al., 2004) and using published data from a cohort exposed to amosite (Paterson, NJ, insulation manufacturers, Seidman et al., 1986). Although the linear EPA model generally provided a good description of exposure response for lung cancer, in some cases it did so only by estimating a large background risk relative to the comparison population. Some of these relative risks seem too large to be due to differences in smoking rates and are probably due at least in part to errors in exposure estimates. There was some equivocal evidence that the relative risk decreased with increasing time since last exposure in the Wittenoom cohort, but none either in the South Carolina cohort up to 50 years from last exposure or in the New Jersey cohort up to 35 years from last exposure. The mesothelioma model provided good descriptions of the observed patterns of mortality after exposure ends, with no evidence that risk increases with long times since last exposure at rates that vary from that predicted by the model (i.e., with the square of time). In particular, the model adequately described the mortality rate in Quebec chrysotile miners and millers up through >50 years from last exposure. There was statistically significant evidence in both the Wittenoom and Quebec cohorts that the exposure intensity-response is supralinear(1) rather than linear. The best-fitting models predicted that the mortality rate varies as [intensity](0.47) for Wittenoom and as [intensity](0.19) for Quebec and, in both cases, the exponent was significantly less than 1 (p< .0001). Using the EPA models, K(L)'s and K(M)'s were estimated from the three sets of raw data and also from published data covering a broader range of environments than those originally addressed in the EPA 1986 update. Uncertainty in these estimates was quantified using "uncertainty bounds" that reflect both statistical and nonstatistical uncertainties. Lung cancer potency factors (K(L)'s) were developed from 20 studies from 18 locations, compared to 13 locations covered in the EPA 1986 update. Mesothelioma potency factors (K(M)'s) were developed for 12 locations compared to four locations in the EPA 1986 update. Although the 4 locations used to calculate K(M) in the EPA 1986 update include one location with exposures to amosite and three with exposures to mixed fiber types, the 14 K(M)'s derived in the present analysis also include 6 locations in which exposures were predominantly to chrysotile and 1 where exposures were only to crocidolite. The K(M)'s showed evidence of a trend, with lowest K(M)'s obtained from cohorts exposed predominantly to chrysotile and highest K(M)'s from cohorts exposed only to amphibole asbestos, with K(M)'s from cohorts exposed to mixed fiber types being intermediate between the K(M)'s obtained from chrysotile and amphibole environments. Despite the considerable uncertainty in the K(M) estimates, the K(M) from the Quebec mines and mills was clearly smaller than those from several cohorts exposed to amphibole asbestos or a mixture of amphibole asbestos and chrysotile. For lung cancer, although there is some evidence of larger K(L)'s from amphibole asbestos exposure, there is a good deal of dispersion in the data, and one of the largest K(L)'s is from the South Carolina textile mill where exposures were almost exclusively to chrysotile. This K(L) is clearly inconsistent with the K(L) obtained from the cohort of Quebec chrysotile miners and millers. The K(L)'s and K(M)'s derived herein are defined in terms of concentrations of airborne fibers measured by phase-contrast microscopy (PCM), which only counts all structures longer than 5 microm, thicker than about 0.25 microm, and with an aspect ratio > or =3:1. Moreover, PCM does not distinguish between asbestos and nonasbestos particles. One possible reason for the discrepancies between the K(L)'s and K(M)'s from different studies is that the category of structures included in PCM counts does not correspond closely to biological activity. In the accompanying article (Berman and Crump, 2008) the K(L)'s and K(M)'s and related uncertainty bounds obtained in this article are paired with fiber size distributions from the literature obtained using transmission electron microscopy (TEM). The resulting database is used to define K(L)'s and K(M)'s that depend on both the size (e.g., length and width) and mineralogical type (e.g., chrysotile or crocidolite) of an asbestos structure. An analysis is conducted to determine how well different K(L) and K(M) definitions are able to reconcile the discrepancies observed herein among values obtained from different environments.

A meta-analysis of asbestos-related cancer risk that addresses fiber size and mineral type

Quantitative estimates of the risk of lung cancer or mesothelioma in humans from asbestos exposure made by the U.S. Environmental Protection Agency (EPA) make use of estimates of potency factors based on phase-contrast microscopy (PCM) and obtained from cohorts exposed to asbestos in different occupational environments. These potency factors exhibit substantial variability. The most likely reasons for this variability appear to be differences among environments in fiber size and mineralogy not accounted for by PCM. In this article, the U.S. Environmental Protection Agency (EPA) models for asbestos-related lung cancer and mesothelioma are expanded to allow the potency of fibers to depend upon their mineralogical types and sizes. This is accomplished by positing exposure metrics composed of nonoverlapping fiber categories and assigning each category its own unique potency. These category-specific potencies are estimated in a meta-analysis that fits the expanded models to potencies for lung cancer (KL's) or mesothelioma (KM's) based on PCM that were calculated for multiple epidemiological studies in our previous paper (Berman and Crump, 2008). Epidemiological study-specific estimates of exposures to fibers in the different fiber size categories of an exposure metric are estimated using distributions for fiber size based on transmission electron microscopy (TEM) obtained from the literature and matched to the individual epidemiological studies. The fraction of total asbestos exposure in a given environment respectively represented by chrysotile and amphibole asbestos is also estimated from information in the literature for that environment. Adequate information was found to allow KL's from 15 epidemiological studies and KM's from 11 studies to be included in the meta-analysis. Since the range of exposure metrics that could be considered was severely restricted by limitations in the published TEM fiber size distributions, it was decided to focus attention on four exposure metrics distinguished by fiber width: "all widths," widths > 0.2 micro m, widths < 0.4 microm, and widths < 0.2 microm, each of which has historical relevance. Each such metric defined by width was composed of four categories of fibers: chrysotile or amphibole asbestos with lengths between 5 microm and 10 microm or longer than 10 microm. Using these metrics three parameters were estimated for lung cancer and, separately, for mesothelioma: KLA, the potency of longer (length > 10 microm) amphibole fibers; rpc, the potency of pure chrysotile (uncontaminated by amphibole) relative to amphibole asbestos; and rps, the potency of shorter fibers (5 microm < length < 10 microm) relative to longer fibers. For mesothelioma, the hypothesis that chrysotile and amphibole asbestos are equally potent (rpc = 1) was strongly rejected by every metric and the hypothesis that (pure) chrysotile is nonpotent for mesothelioma was not rejected by any metric. Best estimates for the relative potency of chrysotile ranged from zero to about 1/200th that of amphibole asbestos (depending on metric). For lung cancer, the hypothesis that chrysotile and amphibole asbestos are equally potent (rpc = 1) was rejected (p < or = .05) by the two metrics based on thin fibers (length < 0.4 microm and < 0.2 microm) but not by the metrics based on thicker fibers. The "all widths" and widths < 0.4 microm metrics provide the best fits to both the lung cancer and mesothelioma data over the other metrics evaluated, although the improvements are only marginal for lung cancer. That these two metrics provide equivalent (for mesothelioma) and nearly equivalent (for lung cancer) fits to the data suggests that the available data sets may not be sufficiently rich (in variation of exposure characteristics) to fully evaluate the effects of fiber width on potency. Compared to the metric with widths > 0.2 microm with both rps and rpc fixed at 1 (which is nominally equivalent to the traditional PCM metric), the "all widths" and widths < 0.4 microm metrics provide substantially better fits for both lung cancer and, especially, mesothelioma. Although the best estimates of the potency of shorter fibers (5 < length < 10 microm) is zero for the "all widths" and widths < 0.4 microm metrics (or a small fraction of that of longer fibers for the widths > 0.2 microm metric for mesothelioma), the hypothesis that these shorter fibers were nonpotent could not be rejected for any of these metrics. Expansion of these metrics to include a category for fibers with lengths < 5 microm did not find any consistent evidence for any potency of these shortest fibers for either lung cancer or mesothelioma. Despite the substantial improvements in fit over that provided by the traditional use of PCM, neither the "all widths" nor the widths < 0.4 microm metrics (or any of the other metrics evaluated) completely resolve the differences in potency factors estimated in different occupational studies. Unresolved in particular is the discrepancy in potency factors for lung cancer from Quebec chrysotile miners and workers at the Charleston, SC, textile mill, which mainly processed chrysotile from Quebec. A leading hypothesis for this discrepancy is limitations in the fiber size distributions available for this analysis. Dement et al. (2007) recently analyzed by TEM archived air samples from the South Carolina plant to determine a detailed distribution of fiber lengths up to lengths of 40 microm and greater. If similar data become available for Quebec, perhaps these two size distributions can be used to eliminate the discrepancy between these two studies.

Chrysotile as a Cause of Mesothelioma: An Assessment Based on Epidemiology

There has been a longstanding debate about the potential contribution of chrysotile asbestos fibers to mesothelioma risk. The failure to resolve this debate has hampered decisive risk communication in the aftermath of the collapse of the World Trade Center towers and has influenced judgments about bans on asbestos use. A firm understanding of any health risks associated with natural chrysotile fibers is crucial for regulatory policy and future risk assessments of synthesized nanomaterials. Although epidemiological studies have confirmed amphibole asbestos fibers as a cause of mesothelioma, the link with chrysotile remains unsettled. An extensive review of the epidemiological cohort studies was undertaken to evaluate the extent of the evidence related to free chrysotile fibers, with particular attention to confounding by other fiber types, job exposure concentrations, and consistency of findings. The review of 71 asbestos cohorts exposed to free asbestos fibers does not support the hypothesis that chrysotile, uncontaminated by amphibolic substances, causes mesothelioma. Today, decisions about risk of chrysotile for mesothelioma in most regulatory contexts reflect public policies, not the application of the scientific method as applied to epidemiological cohort studies.

Occupational exposure to asbestos and mortality among asbestos removal workers: a Poisson regression analysis

The asbestos industry has shifted from manufacture to stripping/removal work. The aim of this study was to investigate early indications of mortality among removal workers. The study population consisted of 31 302 stripping/removal workers in the Great Britain Asbestos Survey, followed up to December 2005. Relative risks (RR) for causes of death with elevated standardised mortality ratios (SMR) and sufficient deaths were obtained from Poisson regression. Risk factors considered included dust suppression technique, type of respirator used, hours spent stripping, smoking status and exposure length. Deaths were elevated for all causes (SMR 123, 95% CI 119–127, n=985), all cancers including lung cancer, mesothelioma, and circulatory disease. There were no significant differences between suppression techniques and respirator types. Spending more than 40 h per week stripping rather than less than 10, increased mortality risk from all causes (RR 1.4, 95% CI 1.2–1.7), circulatory disease and ischaemic heart disease. Elevated mesothelioma risks were observed for those first exposed at young ages or exposed for more than 30 years. This study is a first step in assessing long-term mortality of asbestos removal workers in relation to working practices and asbestos exposure. Further follow-up will allow the impact of recent regulations to be assessed.

Risk of gastrointestinal cancers from inhalation and ingestion of asbestos

This paper summarizes the weight of epidemiological evidence to evaluate the hypothesis that asbestos exposure is causally associated with increased risk of gastrointestinal (GI) cancers as suggested by Selikoff in an early study of insulation workers. This review looks at populations that develop GI cancers, namely stomach, colorectal, colon and rectal. Guidelines for assessing causality are strength of association, biological gradient and consistency of the associations. Exposure-response (E-R) was evaluated using three methods to estimate exposure. Rate Ratios (RRs) for lung cancer and percent of mesothelioma are used as surrogate measures of asbestos exposure for all the cohorts of exposed workers. Quantitative or semi-quantitative estimates of cumulative exposure to asbestos were also used to assess E-R trends and were compared to E-R trends for lung cancer and mesothelioma in individual studies. Surrogate measures are important since there are few individual studies that have assessed E-R. None of the various methods to estimate asbestos exposure yielded consistent E-R trends and the strength of the associations were consistently weak or non-existent for the four types of GI cancers. The epidemiological evidence detracts from the hypothesis that occupational asbestos exposure increases the risk of stomach, colorectal, colon, and rectal cancer. Findings are briefly summarized below.

Mesothelioma and asbestos

The current state of knowledge concerning mesothelioma risk estimates is reviewed. Estimates of the risk of mesothelioma exist for the commercial asbestos fiber types chrysotile, amosite and crocidolite. Data also exist on which to assess risks for winchite (sodic tremolite) and anthophyllite asbestos. Uncertainty in estimates is primarily related to limitations in measurements of exposure. Differences in the dimensions of the various fiber types and of the same fiber types at different stages of processing add a further complication. Never-the-less, in practical terms, crocidolite presents the highest asbestos related mesothelioma risk. The risk associated with sodic tremolite (winchite) appears to be similar. In chrysotile miners and millers, the mesothelioma risk has been linked with exposure to asbestiform tremolite. Exposure to chrysotile in a pure form seems likely to present a very low if any risk of mesothelioma. While the majority of mesothelial tumors result from exposure to the asbestos minerals, there are other well established and suspected etiological agents. While a practical threshold seems to exist for exposure to chrysotile, it is unlikely to exist for the amphibole asbestos minerals, especially for crocidolite. To date there is no indication of an increased risk of mesothelioma resulting from non-commercial fiber exposure in the taconite industry.

Mortality and cancer incidence among tire manufacturing workers hired in or after 1962

OBJECTIVE

This study evaluated mortality during 1962 through 2003 and cancer incidence during 1995 through 2003 at a tire manufacturing plant.

METHODS

The mortality study included 3425 men and women, employed for at least one year. Of these, 3069 were eligible for the cancer incidence study.

RESULTS

Employees experienced 390 deaths compared with 608 expected (standardized mortality ratio (SMR)=64; 95% confidence interval (CI)=58-71). Total cancer mortality (123 observed, SMR=75, CI=62-89) and lung cancer mortality (47 observed, SMR=72, CI=53-96) were lower than expected. Hourly white men had small increases in stomach cancer, bladder cancer, and leukemia deaths. During 1995 through 2003, 169 incident cancers were observed compared with 197 expected (SIR=86, 95% CI=74-100). Three mesothelioma cases occurred among hourly white men (SIR=653, CI=135-1907); all were exposed potentially to asbestos before starting at the rubber plant.

CONCLUSIONS

Small numbers and limited information on jobs, occupational agents, and lifestyle preclude attribution of observed increases to workplace exposures.

Molecular biology of malignant mesothelioma: a review

Malignant mesothelioma (MM) is an uncommon tumor with high mortality and morbidity rates. It arises from mesothelial cells that line the pleural, pericardial, peritoneal, and testicular cavities. This is a disease with an indolent course because tumors arise 20 to 40 years after exposure to an inciting agent. Extensive research has shown that mesothelial cells are transformed into MM cells through various chromosomal and cellular pathway defects. These changes alter the normal cells' ability to survive, proliferate, and metastasize. This article discusses the alterations that occur in transforming normal mesothelial cells into MM. It also details some of the signal transduction pathways that seem to be important in MM with the potential for novel targeted therapeutics.

Evidence for excess colorectal cancer incidence among asbestos-exposed men in the Beta-Carotene and Retinol Efficacy Trial

The relation between asbestos exposure and colorectal cancer remains controversial. The authors of this 1984-2004 US study examined the association among 3,897 occupationally exposed participants in the Beta-Carotene and Retinol Efficacy Trial (CARET) for chemoprevention of lung cancer, followed prospectively for 10-18 years. When a Cox stratified proportional hazards model was used, risks of colorectal cancer were elevated among male heavy smokers exposed to asbestos. Their relative risk was 1.36 (95% confidence interval: 0.96, 1.93) when compared with that for CARET heavy smokers not exposed to asbestos, after adjusting for age, smoking history, and intervention arm. The presence of asbestos-induced pleural plaques at baseline was associated with a relative risk of 1.54 (95% confidence interval: 0.99, 2.40); colorectal cancer risk also increased with worsening pulmonary asbestosis (p = 0.03 for trend). A dose-response trend based on years of asbestos exposure was less evident. Nonetheless, these data suggest that colorectal cancer risk is elevated among men occupationally exposed to asbestos, especially those with evidence of nonmalignant asbestos-associated radiographic changes.

Asbestos fiber length as related to potential pathogenicity: a critical review

BACKGROUND

Asbestos inhalation is recognized as an exposure that increases the risk for the development of lung disease. It is unique among dusts in that it is both a carcinogen and capable of inducing extrapulmonary responses such as pleural thickening and fibrosis as well as malignancy. One feature of asbestos suggested as crucial in its pathological activity is its fibrous morphology. Long fibers that have been inhaled are cleared less readily and are thus more persistent in the body. Furthermore certain experimental models link fiber length to levels of risks for development of certain diseases. The present review will survey the data on this subject.

METHODS

The review considers experimental models that have been used to assess the response to various lengths of fibers in animal models in addition to data obtained from studies of human materials. The review also emphasizes the importance in defining the method by which a sample is categorized.

RESULTS

Data are offered which support the potential for longer fibers as well as shorter fibers to contribute to pathological responses.

CONCLUSIONS

The data presented argue that asbestos fibers of all lengths induce pathological responses and that caution should be exerted when attempting to exclude any population of inhaled fibers, based on their length, from being contributors to the potential for development of asbestos-related diseases.

The epidemiology of mesothelioma

It has been more than 40 years since occupational crocidolite exposure in South African miners was found to be associated with development of malignant mesothelial tumors 30 to 40 years later. Similar cases were not seen in the amosite and chrysotile miners. Since then, epidemiological and toxicological knowledge have increased enormously, but mortality continues to rise steeply (5% to 10% per year) in most industrialized countries. Even with widespread asbestos abatement efforts, this increase is likely to continue in Western Europe and the United State well into the next century, at least until 2020. Unregulated use of asbestos in less industrialized countries may cause the epidemic to continue throughout the next century in those regions. Asbestos abatement seems to be successful as evidenced by a decline in the proportion of patients with peritoneal tumors, which are the most common malignancies in heavily exposed individuals. Whereas in the 1960s peritoneal tumors comprised up to 30% of the total, in recent years the proportion has fallen to about 10%, This changing ratio could also be due to the steady increase in pleural tumors. The difficulty in formulating the connection as to the etiology of mesothelioma resulted from an unforeseeable difference in the carcinogenicity of various asbestos and mineral fiber types and was compounded by the very long latency of the disease. Unfortunately, the use of a single term, "asbestos," to describe at least five fibrous silicate minerals, each with unique physical, chemical, and biological properties and not infrequently and naturally admixed, severely hampered scientific investigation into the occupational health risks. The field became confused and filled with debate. At the heart of the fiber type controversy lies a fundamentally differing view of the importance of biopersistence of various asbestos fibers in carcinogenesis. This review will deal with the epidemiology of mesothelioma with particular attention to the studies that elucidate the impact of various asbestos fiber types on the etiology of the disease.

A case-control study of the relationship between the risk of colon cancer in men and exposures to occupational agents

BACKGROUND

We conducted a population-based case-control study in Montreal, Canada, to explore associations between hundreds of occupational circumstances and several cancer sites, including colon.

METHODS

We interviewed 497 male patients with a pathologically confirmed diagnosis of colon cancer, 1514 controls with cancers at other sites, and 533 population-based controls. Detailed job histories and relevant potential confounding variables were obtained, and the job histories were translated by a team of chemists and industrial hygienists into a history of occupational exposures.

RESULTS

We found that there was reasonable evidence of associations for men employed in nine industry groups (adjusted odds ranging from 1.1 to 1.6 per a 10-year increase in duration of employment), and in 12 job groups (OR varying from 1.1 to 1.7). In addition, we found evidence of increased risks by increasing level of exposures to 21 occupational agents, including polystyrene (OR for "substantial" exposure (OR(subst)) = 10.7), polyurethanes (OR(subst) = 8.4), coke dust (OR(subst) = 5.6), mineral oils (OR(subst) = 3.3), polyacrylates (OR(subst) = 2.8), cellulose nitrate (OR(subst) = 2.6), alkyds (OR(subst) = 2.5), inorganic insulation dust (OR(subst) = 2.3), plastic dusts (OR(subst) = 2.3), asbestos (OR(subst) = 2.1), mineral wool fibers (OR(subst) = 2.1), glass fibers (OR(subst) = 2.0), iron oxides (OR(subst) = 1.9), aliphatic ketones (OR(subst) = 1.9), benzene (OR(subst) = 1.9), xylene (OR(subst) = 1.9), inorganic acid solutions (OR(subst) = 1.8), waxes, polishes (OR(subst) = 1.8), mononuclear aromatic hydrocarbons (OR(subst) = 1.6), toluene (OR(subst) = 1.6), and diesel engine emissions (OR(subst) = 1.5). Not all of these effects are independent because some exposures occurred contemporaneously with others or because they referred to a group of substances.

CONCLUSIONS

We have uncovered a number of occupational associations with colon cancer. For most of these agents, there are no published data to support or refute our observations. As there are few accepted risk factors for colon cancer, we suggest that new occupational and toxicologic studies be undertaken focusing on the more prevalent substances reported herein.