Critical issues in benzene toxicity and metabolism: the effect of interactions with other organic chemicals on risk assessment

Shanghai Health Study (2001-2009): What was learned about benzene health effects?

The Shanghai Health Study (SHS) was a large epidemiology study conducted as a joint effort between the University of Colorado and Fudan University in Shanghai, China. The study was funded by members of the American Petroleum Institute between 2001 and 2009 and was designed to evaluate the human health effects associated with benzene exposure. Two arms of the SHS included: an occupational-based molecular epidemiology study and several hospital-based case control studies. Consistent with historical literature, following sufficient exposure to relatively high airborne concentrations and years of exposure, the SHS concluded that exposure to benzene resulted in an increased risk of various blood and bone marrow abnormalities such as benzene poisoning, aplastic anemia (AA), myelodysplastic syndrome (MDS), and acute myeloid leukemia (AML). Non-Hodgkin lymphoma (NHL) was not significantly increased for the exposures examined in this study. Perhaps the most important contribution of the SHS was furthering our understanding of the mechanism of benzene-induced bone marrow toxicity and the importance of identifying the proper subset of MDS relevant to benzene. Investigators found that benzene-exposed workers exhibited bone marrow morphology consistent with an immune-mediated inflammatory response. Contrary to historic reports, no consistent pattern of cytogenetic abnormalities was identified in these workers. Taken together, findings from SHS provided evidence that the mechanism for benzene-induced bone marrow damage was not initiated by chromosome abnormalities. Instead, chronic inflammation, followed by an immune-mediated response, is likely to play a more significant role in benzene-induced disease initiation and progression than previously thought.

Associations between blood BTEXS concentrations and hematologic parameters among adult residents of the U.S. Gulf States

BACKGROUND

Studies of workers exposed to benzene at average air concentrations below one part per million suggest that benzene, a known hematotoxin, causes hematopoietic damage even at low exposure levels. However, evidence of such effects outside of occupational settings and for other volatile organic compounds (VOCs) is limited.

OBJECTIVE

To investigate associations between ambient exposures to five VOCs, including benzene, and hematologic parameters among adult residents of the U.S. Gulf Coast.

MATERIALS AND METHODS

Blood concentrations of selected VOCs were measured in a sample of adult participants in the Gulf Long-term Follow-up Study (GuLF STUDY) during 2012 and 2013. Complete blood counts with differentials were also performed on a subset of participants (n=406). We used these data together with detailed questionnaire data to estimate adjusted associations between blood BTEXS (benzene, toluene, ethylbenzene, o-xylene, m/p-xylene, and styrene) concentrations and hematologic parameters using generalized linear models.

RESULTS

We observed inverse associations between blood benzene concentrations and hemoglobin concentration and mean corpuscular hemoglobin concentration, and a positive association with red cell distribution width among tobacco smoke-unexposed participants (n=146). Among tobacco smoke-exposed participants (n=247), we observed positive associations between blood VOC concentrations and several hematologic parameters, including increased white blood cell and platelet counts, suggestive of hematopoietic stimulation typically associated with tobacco smoke exposure. Most associations were stronger for benzene than for the other VOCs.

CONCLUSIONS

Our results suggest that ambient exposure to BTEXS, particularly benzene, may be associated with hematologic effects, including decreased hemoglobin concentration, mean corpuscular hemoglobin concentration, and increased red cell distribution width.

Modulation of microRNA expression by volatile organic compounds in mouse lung

Volatile organic compounds (VOCs) are one of main pollutants indoors. Exposure to VOCs is associated with cancer, asthma disease, and multiple chemical allergies. Despite the adverse health effects of VOCs, the molecular mechanisms underlying VOCs-induced disease remain largely unknown. MicroRNAs (miRNAs), as key post-transcriptional regulators of gene expression, may influence cellular disease state. To investigate whether lung miRNA expression profiles in mice are modified by VOCs mixture exposure, 44 male Kunming mice were exposed in 4 similar static chambers, 0 (control) and 3 different doses of VOCs mixture (groups 1-3). The concentrations of VOCs mixture were as follows: formaldehyde, benzene, toluene, and xylene 3.0 + 3.3 + 6.0 + 6.0 mg/m(3) , 5.0 + 5.5 + 10.0 + 10.0 mg/m(3) , 10.0 + 11.0 + 20.0 + 20.0 mg/m(3) , respectively, which corresponded to 30, 50, and 100 times of indoor air quality standard in China, after exposure to 2 weeks (2 h/day, 5 days/week). Small RNAs in lung and protein isolated from bronchoalveolar lavage fluid (BALF) were collected and analyzed for miRNA expression using microarray analysis and for interleukin-8 (IL-8) protein levels by enzyme-linked immunosorbent assay, respectively. VOCs exposure altered the miRNA expression profiles in lung in mice. Specifically, 69 miRNAs were significantly differentially expressed in VOCs-exposed samples versus controls. Functional annotation analysis of the predicted miRNA transcript targets revealed that VOCs exposure potentially alters signaling pathways associated with cancer, chemokine signaling, Wnt signaling, neuroactive ligand-receptor interaction, and cell adhesion molecules. IL-8 isolated from BALF and nitric oxide synthase of lung increased significantly, whereas GSH of lung decreased significantly in mice exposed to VOCs. These results indicate that inhalation of VOCs alters miRNA patterns that regulate gene expression, potentially leading to the initiation of cancer and inflammatory diseases.

A calibrated human PBPK model for benzene inhalation with urinary bladder and bone marrow compartments

A physiologically-based pharmacokinetic (PBPK) model of benzene inhalation based on a recent mouse model was adapted to include bone marrow (target organ) and urinary bladder compartments. Empirical data on human liver microsomal protein levels and linked CYP2E1 activities were incorporated into the model, and metabolite-specific conversion rate parameters were estimated by fitting to human biomonitoring data and adjusting for background levels of urinary metabolites. Human studies of benzene levels in blood and breath, and phenol levels in urine were used to validate the rate of human conversion of benzene to benzene oxide, and urinary benzene metabolites from Chinese benzene worker populations provided model validation for rates of human conversion of benzene to muconic acid (MA) and phenylmercapturic acid (PMA), phenol (PH), catechol (CA), hydroquinone (HQ), and benzenetriol (BT). The calibrated human model reveals that while liver microsomal protein and CYP2E1 activities are lower on average in humans compared to mice, the mouse also shows far lower rates of benzene conversion to MA and PMA, and far higher conversion of benzene to BO/PH, and of BO/PH to CA, HQ, and BT. The model also differed substantially from existing human PBPK models with respect to several metabolic rate parameters of importance to interpreting benzene metabolism and health risks in human populations associated with bone marrow doses. The model provides a new methodological paradigm focused on integrating linked human liver metabolism data and calibration using biomonitoring data, thus allowing for model uncertainty analysis and more rigorous validation.

Maternal occupational exposure to organic solvents during early pregnancy and risks of neural tube defects and orofacial clefts

OBJECTIVES

Though toxicological experiments demonstrate the teratogenicity of organic solvents in animal models, epidemiologic studies have reported inconsistent results. Using data from the population-based National Birth Defects Prevention Study, the authors examined the relation between maternal occupational exposure to aromatic solvents, chlorinated solvents and Stoddard solvent during early pregnancy and neural tube defects (NTDs) and orofacial clefts (OFCs).

METHODS

Cases of NTDs (anencephaly, spina bifida and encephalocoele) and OFCs (cleft lip ± cleft palate and cleft palate alone) delivered between 1997 and 2002 were identified by birth defect surveillance registries in eight states; non-malformed control infants were selected using birth certificates or hospital records. Maternal solvent exposure was estimated by industrial hygienist review of self-reported occupational histories in combination with a literature-derived exposure database. ORs and 95% CIs for the association between solvent class and each birth defect group and component phenotype were estimated using multivariable logistic regression, adjusting for maternal age, race/ethnicity, education, pre-pregnancy body mass index, folic acid supplement use and smoking.

RESULTS

The prevalence of exposure to any solvent among mothers of NTD cases (n = 511), OFC cases (n = 1163) and controls (n = 2977) was 13.1%, 9.6% and 8.2%, respectively. Exposure to chlorinated solvents was associated with increased odds of NTDs (OR = 1.96, CI 1.34 to 2.87), especially spina bifida (OR = 2.26, CI 1.44 to 3.53). No solvent class was strongly associated with OFCs in these data.

CONCLUSIONS

The findings suggest that maternal occupational exposure to chlorinated solvents during early pregnancy is positively associated with the prevalence of NTDs in offspring.

Highly efficient and mild electrochemical incineration: mechanism and kinetic process of refractory aromatic hydrocarbon pollutants on superhydrophobic PbO₂ anode

Aqueous aromatic hydrocarbons are chemically stable, high toxic refractory pollutants that can only be oxidized to phenols and quinone on either Pt or traditional PbO(2) electrodes. In this study, a novel method for the electrochemical incineration of benzene homologues on superhydrophobic PbO(2) electrode (hydrophobic-PbO(2)) was proposed under mild conditions. Hydrophobic-PbO(2) can achieve the complete mineralization of aromatic hydrocarbons and exhibit high removal effect, rapid oxidation rate, and low energy consumption. The kinetics of the electrochemical incineration was also investigated, and the results revealed that the cleavage of the benzene ring is a key factor affecting the incineration efficiency. Moreover, on hydrophobic-PbO(2), the decay of intermediates was rapid, and low concentrations of aromatics were accumulated during the reaction. The removal of the initial pollutants and the effects of oxidative cleavage were related to the number of methyl groups on the benzene ring. Specifically, the results of physical experiments and quantum calculations revealed that the charge density of carbon atoms increases with an increase in the number of methyl groups, which promotes the electrophilic attack of ·OH.

Effects of co-exposure of benzene, toluene and xylene to Drosophila melanogaster: alteration in hsp70, hsp60, hsp83, hsp26, ROS generation and oxidative stress markers

Benzene, toluene and xylene are monocyclic aromatic hydrocarbon compounds, used both as individual compound and as mixtures, in industry as well as household. Previous studies involving exposures to these compounds, individually, have shown that benzene was more toxic compared to toluene or xylene. Here, we tested a working hypothesis that toluene and/or xylene in a mixture containing benzene affect benzene induced toxicity in a non-target organism, Drosophila melanogaster. We exposed D. melanogaster larvae transgenic for hsp70, hsp83 or hsp26 and wild type (Oregon R strain) larvae to 25.0-100.0mM benzene, 25.0-100.0mM toluene and 25.0-100mM xylene, individually or in mixtures. Subsequently, we examined the expression of stress genes (encoding heat shock proteins, hsps), generation of reactive oxygen species (ROS), induction of anti-oxidant stress markers and emergence of flies under treatment as well as control conditions. We observed that all these endpoints were significantly altered in all the treatment groups compared to their respective controls. However, the magnitude of toxicity of a benzene-toluene (BT) or benzene-xylene (BX) or benzene-toluene-xylene (BTX) mixture was significantly lower in the organism than that of individual chemical. Our results also show the modulation of toluene toxicity by xylene. Present study suggests antagonistic effect of xylene and toluene on benzene toxicity and additive/synergistic effect of xylene on toluene induced toxicity. Thus, expression of stress genes may be used as an assay for detection of early cellular toxicity. Further, our study supports the use of Drosophila as an alternative animal model for first tier screening of adverse effects of chemical mixtures.

Influence of toluene co-exposure on the metabolism and genotoxicity of benzene in mice using continuous and intermittent exposures

Benzene exposure in occupational settings often occurs with concurrent exposure to toluene, the methyl-substituted derivative of benzene. Toluene is also readily metabolized by CYP450 isozymes although oxidation primarily occurs in the methyl group. While earlier mouse studies addressing co-exposure to benzene and toluene at high concentrations demonstrated a reduction in benzene-induced genotoxicity, we have previously found, using an intermittent exposure regimen with lower concentrations of benzene (50 ppm) and toluene (100 ppm), that toluene enhances benzene-induced clastogenic or aneugenic bone marrow injury in male CD-1 mice with significantly increased CYP2E1, and depleted GSH and GSSG levels. The follow-up study reported here also used the same daily and total co-exposures but over consecutive days and compared the effects of co-exposure on genotoxicity and metabolism in CD-1 mice both with and without buthionine sulfoximine (BSO) treatment to deplete GSH. In this study the toluene co-exposure doubled the genotoxic response (as determined by the erythrocyte micronucleus test) to benzene alone. Further, GSH depletion caused a reduction in this genotoxicity in both benzene exposed and benzene/toluene co-exposed mice. The results are discussed in terms of the analyses of urinary metabolites from this consecutive day study and the intermittent exposure study as well as levels of CYP2E1, epoxide hydrolase, quinone reductase, alcohol dehydrogenase, and aldehyde dehydrogenase activities. The results suggest that the presence of glutathione is necessary for benzene genotoxicity either as a metabolite conjugate or through an indirect mechanism such as TNF-induced apoptosis.

Urinary biomarkers and low-level environmental benzene concentration: assessing occupational and general exposure

The categories of urban workers undergoing prolonged exposure to gasoline due to vehicle emissions, evaporation and traffic fumes are considered occupationally exposed to benzene, even if at low concentrations. The aim of this study was to evaluate the specificity of unmetabolized benzene excreted in urine (UBz) and S-phenylmercapturic acid (SPMA) as biomarkers of exposure to very low levels of benzene, and to study the impact of putative individual confounders like smoking and alcohol habits, co-exposure to other toxicants and body mass index on the exposure assessment. Environmental and biological monitoring of exposure to benzene were performed in 114 Urban Policemen. The mean value of UBz in non-smokers was significantly lower than in either groups of light to moderate smokers and heavy smokers (0.24, 1.82 and 2.82 microg L(-1), respectively). On the contrary, SPMA values did not discriminate exposure resulting from smoking habits. Moreover, the concentration of UBz in non-smokers appears to be correlated with environmental benzene concentration (BenzA) (R(2)=0.13, beta=0.37, p=0.002). On the other hand, no significant correlation was found between SPMA concentration (corrected for creatinine excretion and log transformed, LogSPMA) and LogBenzA (R(2)=0.003, beta=0.05, p=0.6). Our findings reinforce previous research on the use of unmetabolized urinary benzene as a specific and sensitive biomarker of low-level exposure to benzene and confirm that smoking habits strongly influence the excretion of UBz.

Genotoxicity of intermittent co-exposure to benzene and toluene in male CD-1 mice

Benzene is an important industrial chemical. At certain levels, benzene has been found to produce aplastic anemia, pancytopenia, myeloblastic anemia and genotoxic effects in humans. Metabolism by cytochrome P450 monooxygenases and myeloperoxidase to hydroquinone, phenol, and other metabolites contributes to benzene toxicity. Other xenobiotic substrates for cytochrome P450 can alter benzene metabolism. At high concentrations, toluene has been shown to inhibit benzene metabolism and benzene-induced toxicities. The present study investigated the genotoxicity of exposure to benzene and toluene at lower and intermittent co-exposures. Mice were exposed via whole-body inhalation for 6h/day for 8 days (over a 15-day time period) to air, 50 ppm benzene, 100 ppm toluene, 50 ppm benzene and 50 ppm toluene, or 50 ppm benzene and 100 ppm toluene. Mice exposed to 50 ppm benzene exhibited an increased frequency (2.4-fold) of micronucleated polychromatic erythrocytes (PCE) and increased levels of urinary metabolites (t,t-muconic acid, hydroquinone, and s-phenylmercapturic acid) vs. air-exposed controls. Benzene co-exposure with 100 ppm toluene resulted in similar urinary metabolite levels but a 3.7-fold increase in frequency of micronucleated PCE. Benzene co-exposure with 50 ppm toluene resulted in a similar elevation of micronuclei frequency as with 100 ppm toluene which did not differ significantly from 50 ppm benzene exposure alone. Both co-exposures - 50 ppm benzene with 50 or 100 ppm toluene - resulted in significantly elevated CYP2E1 activities that did not occur following benzene or toluene exposure alone. Whole blood glutathione (GSH) levels were similarly decreased following exposure to 50 ppm benzene and/or 100 ppm toluene, while co-exposure to 50 ppm benzene and 100 ppm toluene significantly decreased GSSG levels and increased the GSH/GSSG ratio. The higher frequency of micronucleated PCE following benzene and toluene co-exposure when compared with mice exposed to benzene or toluene alone suggests that, at the doses used in this study, toluene can enhance benzene-induced clastogenic or aneugenic bone marrow injury. These findings exemplify the importance of studying the effects of binary chemical interactions in animals exposed to lower exposure concentrations of benzene and toluene on benzene metabolism and clastogenicity. The relevance of these data on interactions for humans exposed at low benzene concentrations can be best assessed only when the mechanism of interaction is understood at a quantitative level and incorporated within a biologically based modeling framework.

The safety of hydroquinone

Hydroquinone is one of the most effective molecules for the treatment of hyperpigmentary disorders, with over 40 years of efficacy and safety data. Concerns over its safety have been raised because of the fact that it is a derivative of benzene and because of the long-term side-effects observed with cosmetic products containing high concentrations of hydroquinone. However, despite 40-50 years use of hydroquinone for medical conditions, there has not been a single documented case of either a cutaneous or internal malignancy associated with this drug. This article reviews the evidence for the safety of hydroquinone in the treatment of hyperpigmentation conditions.

A proposed methodology for setting occupational exposure limits for hydrocarbon solvents

Occupational exposure limit (OEL) development for hydrocarbon solvents is complicated because most of these solvents have complex compositions and only a few representative constituents have been studied in detail. A proposed solution to this problem is to group constituents with similar physical, chemical, and toxicological properties and to assign "guidance values" to each group. A unique OEL can then be calculated for each solvent, using a reciprocal calculation procedure (RCP) based on the liquid composition. This procedure follows the American Conference of Governmental Industrial Hygienists' (ACGIH) generic advice for complex mixtures and is recommended by the U.K. Health and Safety Executive for OEL calculations by hydrocarbon solvent manufacturers. The RCP is justified, as the toxicological properties of the constituents are additive and the differences between the vapor and liquid compositions do not substantially affect the calculated exposure limits. The guidance values are based principally on acute central nervous system depression and eye and respiratory tract irritation, effects that are the most sensitive indicators of hydrocarbon solvent exposure. One benefit of this procedure is that it is a relatively simple but practical procedure that requires limited compositional information. Further, it provides OEL recommendations that are consistent with occupational experience and current regulatory advice. Groupings and guidance values are proposed, and sample calculations are provided.

The interaction effects of binary mixtures of benzene and toluene on the developing heart of medaka (Oryzias latipes)

The United States Environmental Protection Agency (USEPA) has pursued the estimation of risk of adverse health effects from exposure to chemical mixtures since the early 1980s. Methods used to calculate risk estimates of mixtures were often based on single chemical information that required assumptions of dose-addition or response-addition and did not consider possible changes in response due to interaction effects among chemicals. Full factorial designs for laboratory studies can produce interactions information, but these are expensive to perform and may not provide the information needed to evaluate specific environmentally relevant mixtures. In this research, groups of Japanese medaka (Oryzias latipes) embryos were exposed to binary mixtures of benzene and toluene as well as to each of these chemicals alone. Endpoint specific dose-response models were built for the hydrocarbon mixture under an assumption of dose-additivity, using the single chemical dose-response information on benzene and toluene. The endpoints included heart rate, heart rate progression, and lethality. Results included a synergistic response for heart rate at 72 h of development, and either additivity or antagonism for all other endpoints at 96 h of development. This work uses an established statistical method to evaluate the toxicity of an environmentally relevant mixture to ascertain whether interaction effects are occurring, thus providing additional information on toxicity.

The toxicology of hydroquinone--relevance to occupational and environmental exposure

Hydroquinone (HQ) is a high-volume commodity chemical used as a reducing agent, antioxidant, polymerization inhibitor, and chemical intermediate. It is also used in over-the-counter (OTC) drugs as an ingredient in skin lighteners and is a natural ingredient in many plant-derived products, including vegetables, fruits, grains, coffee, tea, beer, and wine. While there are few reports of adverse health effects associated with the production and use of HQ, a great deal of research has been conducted with HQ because it is a metabolite of benzene. Physicochemical differences between HQ and benzene play a significant role in altering the pharmacokinetics of directly administered when compared with benzene-derived HQ. HQ is only weakly positive in in vivo chromosomal assays when expected human exposure routes are used. Chromosomal effects are increased significantly when parenteral or in vitro assays are used. In cancer bioassays, HQ has reproducibly produced renal adenomas in male F344 rats. The mechanism of tumorigenesis is unclear but probably involves a species-, strain-, and sex-specific interaction between renal tubule toxicity and an interaction with the chronic progressive nephropathy that is characteristic of aged male rats. Mouse liver tumors (adenomas) and mononuclear cell leukemia (female F344 rat) have also been reported following HQ exposure, but their significance is uncertain. Various tumor initiation/promotion assays with HQ have shown generally negative results. Epidemiological studies with HQ have demonstrated lower death rates and reduced cancer rates in production workers when compared with both general and employed referent populations. Parenteral administration of HQ is associated with changes in several hematopoietic and immunologic endpoints. This toxicity is more severe when combined with parenteral administration of phenol. It is likely that oxidation of HQ within the bone marrow compartment to the semiquinone or p-benzoquinone (BQ), followed by covalent macromolecular binding, is critical to these effects. Bone marrow and hematologic effects are generally not characteristic of HQ exposures in animal studies employing routes of exposure other than parenteral. Myelotoxicity is also not associated with human exposure to HQ. These differences are likely due to significant route-dependent toxicokinetic factors. Fetotoxicity (growth retardation) accompanies repeated administration of HQ at maternally toxic dose levels in animal studies. HQ exposure has not been associated with other reproductive and developmental effects using current USEPA test guidelines. The skin pigment lightening properties of HQ appear to be due to inhibition of melanocyte tyrosinase. Adverse effects associated with OTC use of HQ in FDA-regulated products have been limited to a small number of cases of exogenous ochronosis, although higher incidences of this syndrome have been reported with inappropriate use of unregulated OTC products containing higher HQ concentrations. The most serious human health effect related to HQ is pigmentation of the eye and, in a small number of cases, permanent corneal damage. This effect has been observed in HQ production workers, but the relative contributions of HQ and BQ to this process have not been delineated. Corneal pigmentation and damage has not been reported at current exposure levels of <2 mg/m3. Current work with HQ is being focused on tissue-specific HQ-glutathione metabolites. These metabolites appear to play a critical role in the renal effects observed in F344 rats following HQ exposure and may also be responsible for bone marrow toxicity seen after parenteral exposure to HQ or benzene-derived HQ.

Antigenotoxic effects of cimetidine against benzene induced micronuclei in mouse bone marrow erythrocytes

An in vivo micronucleus assay using Balb/C male mice was used to examine antigenotoxic effects of cimetidine (CM) on benzene (BZN) induced genotoxic effects. CM not only has therapeutic and immunomudolatory role, but it has also been shown to protect bone marrow stem cells from radiation induced clastogenic effects. Therefore, in the present study we attempt to investigate the protective effects and possible mechanisms involved in the effects of CM. An 8-week-old male Balb/C mice (22+/-4 g weight) were treated with different doses of BZN (400, 600 and 800 mg/kg body weight), i.p. and sampled at 24, 48 and 72 h after treatment by cervical dislocation. Various doses of CM (10, 15, 30 mg/kg) were used in association with BZN and 1-2 h prior to BZN treatment. Results show that BZN effectively induced micronuclei in polychromatic erythrocytes (PCEs). Application of CM led to a significant reduction of micronuclei in PCEs, i.e. 2-fold after 10 mg/kg and 3-fold after 30 mg/kg CM treatment. Results also indicate CM was more effective when used in combination with BZN. Therefore, results indicate that CM could reduce clastogenic effects of BZN. Although further investigations are needed to reveal the mechanistical background behind the effect, the most probable mechanism involved might be free radical scavenging. This mechanism might be associated with amplification of glutathione system and cytochrome P-450 inhibition.

The National Exposure Registry: analyses of health outcomes from the benzene subregistry

The purpose of the National Exposure Registry is to assess the long-term health consequences to a general population from long-term, low-level exposures to specific substances in the environment. This study investigates the health outcomes of 1,143 persons (1,127 living, 16 deceased) living in south central Texas who had documented environmental exposure to benzene (up to 66ppb) in tap water. As with all subregistries, face-to-face interviews were used to collect self-reported information for 25 general health status questions. Using computer-assisted telephone interviewing, the same health questions were asked 1 year (Followup 1, F1) and 2 years later (Followup 2, F2). The health outcome rates for Baseline and Followup 1 and 2 data collections for the Benzene Subregistry were compared with national norms, that is, the National Health Interview Survey (NHIS) rates. For at least one of the three reporting periods, specific age and sex groups of the Benzene Subregistry population reported more adverse health outcomes when compared with the NHIS population, including anemia and other blood disorders, ulcers, gall bladder trouble, and stomach or intestinal problems, stroke, urinary tract disorders, skin rashes, diabetes, kidney disease, and respiratory allergies. Statistically significant deficits for the Benzene Subregistry population overall were found for asthma, emphysema, or chronic bronchitis; arthritis, rheumatism, or other joint disorders; hearing impairment; and speech impairment. No statistically significant differences between the two populations were seen for the outcomes hypertension; liver disease; mental retardation; or cancer. These results do not identify a causal relationship between benzene exposure and adverse health effects; however, they do reinforce the need for continued followup of registrants.

Potential health effects of gasoline and its constituents: A review of current literature (1990-1997) on toxicological data

We reviewed toxicological studies, both experimental and epidemiological, that appeared in international literature in the period 1990-1997 and included both leaded and unleaded gasolines as well as their components and additives. The aim of this overview was to select, arrange, and present references of scientific papers published during the period under consideration and to summarize the data in order to give a comprehensive picture of the results of toxicological studies performed in laboratory animals (including carcinogenic, teratogenic, or embryotoxic activity), mutagenicity and genotoxic aspects in mammalian and bacterial systems, and epidemiological results obtained in humans in relation to gasoline exposure. This paper draws attention to the inherent difficulties in assessing with precision any potential adverse effects on health, that is, the risk of possible damage to man and his environment from gasoline. The difficulty of risk assessment still exists despite the fact that the studies examined are definitely more technically valid than those of earlier years. The uncertainty in overall risk determination from gasoline exposure also derives from the conflicting results of different studies, from the lack of a correct scientific approach in some studies, from the variable characteristics of the different gasoline mixtures, and from the difficulties of correctly handling potentially confounding variables related to lifestyle (e.g., cigarette smoking, drug use) or to preexisting pathological conditions. In this respect, this paper highlights the need for accurately assessing the conclusive explanations reported in scientific papers so as to avoid the spread of inaccurate or misleading information on gasoline toxicity in nonscientific papers and in mass-media messages.

Benzene--a review of the literature from a health effects perspective

A literature review of the impact on human health of exposure to benzene was conducted. Special emphasis in this report is given to the health effects reported in excess of national norms by participants in the Benzene Subregistry of the National Exposure Registry--people having documented exposure to benzene through the use of benzene-contaminated water for domestic purposes. The health effects reported in excess (p < or = .01) by some or all of the sex and age groups studied were diabetes, kidney disease, respiratory allergies, skin rashes, and urinary tract disorders; anemia was also increased for females, but not significantly so.

Quantifying health effects from the combined action of low-level radiation and other environmental agents: can new approaches solve the enigma?

Efforts to assess the quantify deleterious effects from toxicants are directed mainly towards single agents, whereas real world environmental and occupational exposures to natural and anthropogenic agents quite often entail the concomitant presence of several toxicants. These combined exposures may lead to health risks that differ from those expected from simple addition of the individual risks. For example, combined exposures to physical and chemical agents such as radon and smoking or asbestos and smoking produce over-additive effects at exposure levels typical for earlier workplaces. In tumour therapy, the modulation of radiation effects by cytotoxic drugs is widely used to enhance the therapeutic gain. Whether interactions occurring at high exposure levels are important at the low exposure levels set for the public and for modern workplaces is difficult to answer. A scientifically sound extrapolation from these high to low-dose levels should be based on dose-effect relationships of the relevant agents alone and in combination. In general this information is not available. The existing data base on combined effects is rudimentary, mainly descriptive and rarely covers exposure ranges large enough to make direct inferences to present day low-dose exposure situations. In view of the multitude of possible interactions between the large number of potentially harmful agents in the human environment, descriptive approaches will have to be supplemented by the use of mechanistic models for critical health endpoints such as cancer. To generalise and predict the outcome of combined exposures, agents will have to be grouped depending on their physical or chemical mode of action on the molecular and cellular level. Such a grouping must be guided by specific mechanistic studies designed to examine the underlying hypothesis regarding how various classes of agents interact.