Respiratory effects in toluene diisocyanate manufacture: a multidisciplinary approach

Identifying a reference list of respiratory sensitizers for the evaluation of novel approaches to study respiratory sensitization

The induction of immunological responses that trigger bio-physiological symptoms in the respiratory tract following repeated exposure to a substance, is known as respiratory sensitization. The inducing compound is known as a respiratory sensitizer. While respiratory sensitization by high molecular weight (HMW) materials is recognized and extensively studied, much less information is available regarding low molecular weight (LMW) materials as respiratory sensitizers. Variability of symptoms presented in humans from such exposures, limited availability of (and access to) documented reports, and the absence of standardized and validated test models, hinders the identification of true respiratory sensitizers. This review aims to sort suspected LMW respiratory sensitizers based on available compelling, reasonable, inadequate, or questionable evidence in humans from occupational exposures and use this information to compose a reference list of reported chemical respiratory sensitizers for scientific research purposes. A list of 97 reported respiratory sensitizers was generated from six sources, and 52 LMW organic chemicals were identified, reviewed, and assigned to the four evidence categories. Less than 10 chemicals were confirmed with compelling evidence for induction of respiratory sensitization in humans from occupational exposures. Here, we propose the reference list for developing novel research on respiratory sensitization.

Occupational asthma (OA) with sensitization to diphenylmethane diisocyanate (MDI) presenting at the onset like a reactive airways dysfunction syndrome (RADS)

BACKGROUND

Two types of OA are distinguished: immunological (OA with sensitization) and non-immunological, i.e., irritant induced asthma or reactive airways dysfunction syndrome (RADS).

METHODS

We describe the case of a worker who developed respiratory symptoms after a spill of diphenylmethane diisocyanate (MDI) at the workplace. RADS was initially diagnosed and the worker resumed working. The progressive worsening of symptoms and the appearance of symptoms-work relationship one year later, when concentrations of isocyanates were no longer "irritant," suggested immunological OA.

RESULTS

The diagnosis was confirmed by specific inhalation challenge test, followed by removal from exposure and complete recovery.

CONCLUSIONS

In the case of RADS due to an agent with both irritant and sensitizing properties, history should be repeatedly assessed for a possible symptom-work relationship. If this is found, further investigations should be carried out, including specific inhalation challenges, to confirm the possibility of immunological OA.

Toluene diisocyanate: an assessment of carcinogenic risk following oral and inhalation exposure

Although respiratory sensitization and pulmonary irritation have been the subject of particular studies with toluene diisocyanate (TDI), in recent years the potential carcinogenicity of TDI has been a reason for concern and speculation. This has arisen from the expectation that following exposure to TDI the chemical would hydrolyze at aqueous tissue surfaces to give rise to toluene diamine (TDA), a mutagen and rodent carcinogen. The chemistry of TDI suggests that the reaction with biological NH2 groups such as those on proteins, and polymerization to oligoureas, will compete with the hydrolysis reaction. This has been shown with results of in vitro studies where conjugation to protein occurs without detectable formation of TDA when protein solutions in saline are exposed to TDI vapor. Lower pH levels leading to high protonation of biological NH2 groups facilitate hydrolysis of TDI to TDA and subsequent formation of polyureas. These observations are consistent with comparative toxicokinetic studies in rats, which demonstrate significant levels of TDA following oral dosing with TDI--due to the acidic environment in the stomach--but not after inhalation. These results provide an explanation for the tumors observed in rodents after oral dosing of TDI in corn oil, but not after inhalation. Inhalation is the relevant route of human exposure for TDI and the toxicokinetics of TDI exposure at occupational exposure limits have been studied. These data provide a means by which quantitative estimates of the risk of carcinogenicity possibly resulting from the intermediate formation of TDA during TDI exposure can be obtained. Several calculations have been made, all of which lead to the conclusion that TDI exposure by inhalation at the recommended occupational limits will not give rise to significant carcinogenic risk.

Mortality and cancer morbidity of production workers in the United Kingdom flexible polyurethane foam industry

OBJECTIVE

To describe cause specific mortality and site specific cancer morbidity among workers employed in factories that produce polyurethane foams, and to determine if any part of the experience may be due to occupation, and in particular to exposure to diisocyanates.

DESIGN

Historical prospective cohort study.

SETTING

11 factories in England and Wales.

SUBJECTS

8288 male and female production employees with some employment in the period 1958-79, and with a minimum period of employment of six months.

MAIN OUTCOME MEASURES

Observed and expected numbers of deaths for the period 1958-88, and corresponding figures for cancer registrations for the period 1971-86.

RESULTS

Compared with the general population of England and Wales, standardised mortality ratios (SMRs) for all causes and all neoplasms were 97 (observed deaths (Obs) 816) and 88 (Obs 221) respectively. Statistically significant excesses were found among women for cancer of the pancreas (expected deaths (Exp) 2.2, Obs 6, SMR 271, 95% CI 100-595) and cancer of the lung (Exp 9.1, Obs 16, SMR 176, 95% CI 100-285). Similar excesses were not found among male employees, and the SMRs for cancers of the lung and pancreas among the total study population were 100 (Obs 81) and 136 (Obs 14) respectively. Overall incidence of cancer was also below expectation (SRR 94, Obs 277), although statistically significant excesses among women were found for cancers of the larynx and kidney, based on three and four cases respectively. Incident cancers of the lung and pancreas among women were also in excess, although these findings were not independent of the findings for mortality. Poison regression did not indicate that ever having been employed in jobs attracting either higher or lower exposure to isocyanates was a risk factor for the mentioned cancers. A nested case-control design was used to investigate any associations with nine other occupational exposures. No statistically significant association was found.

CONCLUSIONS

In general, cancer rates in this population were lower than those for the general population. All increased cancer rates among women occurred at sites of cancer known to be related to cigarette smoking, and these excesses are probably due to a combination of cigarette smoking, chance, and factors unrelated to the industry under study.

Volume adjustment of maximal midexpiratory flow. Importance of changes in total lung capacity

In 21 of 100 consecutive subjects demonstrating greater than or equal to 10 percent increase in one second forced expired volume following 200 micrograms inhaled salbutamol, the forced expired flow over the middle half of the vital capacity (FEF25-75% was unchanged (n = 16) or fell greater than or equal to 10 percent (n = 5). Volume adjustment of FEF25-75%, to the same volume below total lung capacity (TLC) before and after administration of the bronchodilator resulted in significant increases in 18 of these 21 subjects. The volume-adjusted FEF25-75% increased more (98 percent) following 1500 micrograms inhaled metaproterenol in ten asthmatic subjects, and decreased more (53 percent) following inhalation of histamine in eight subjects, than did the standard FEF25-75% (44 percent increase and 34 percent decrease respectively). When the small changes in TLC which were seen following metaproterenol (4.4 +/- 4.1 percent reduction, p < 0.01) and histamine (11.0 +/- 7.2 percent increase, p < 0.001) were considered, and FEF25-75% volume adjusted so it was measured at the same absolute lung volume, greater changes were seen (198 percent increase after use of metaproterenol, and 75 percent decrease after use of histamine). When FEF25-75% is being examined following administration of a bronchodilator or a bronchoprovocation challenge, it should be volume-adjusted to absolute lung volume. When TLC cannot easily be determined repeatedly (eg routine pulmonary function studies), volume adjustment to the same volume below TLC is advised.

Radioallergosorbent testing of toluene diisocyanate-reactive individuals using p-tolyl isocyanate antigen

Serum from 26 persons shown to react to provocative inhalation challenge with low concentration of toluene diisocyanate (TDI) was tested by radioallergosorbent test (RAST) using a p-tolyl isocyanate-human serum albumin (TMI-HSA) conjugate. Results were evaluated by the RAST ratio and by the mean +3 standard deviation methods. Positive results were obtained in 15% to 19% of TDI reactors, depending on the method of evaluation. These findings suggest that TDI asthma may be a complex syndrome with several types of reaction possible, including an IgE mechanism in some individuals, but they do not support the theory that TDI asthma is based solely on an immune mechanism. However, they do not disagree with the hypothesis that TDI reactivity may be mediated by a pharmacologic mechanism.

Toluene diisocyanate pulmonary disease: immunopharmacologic and mecholyl challenge studies

Selected workers exhibiting clinical "sensitivity" to toluene diiosocyanate (TDI) (wheezing, cough, and dyspnea upon entering a TDI-containing area) were studied for : (1) in vitro TDI-induced leukocyte histamine release; (2) determination of cyclic 3',5' adenosine monophosphate (cAMP) levels of lymphocytes exposed to TDI; (3) effect of TDI on the isoproterenol-induced increase of lymphocyte cAMP levels: and (4) acetyl-beta-methylcholine (mecholyl) inhalation challenge. TDI did not induce histamine release from leukocytes of "sensitive" or "nonsensitive" individuals, nor were lymphocyte cAMP levels affected by in vitro TDI exposure, TDI did, however, diminish in vitro stimulation of cAMP by isoproterenol. This effect, seen with cells of "sensitive" and "nonsensitive" individuals, appeared to be dose-dependent; there were no significant differences between the two groups. When challenged with mecholyl, 7 of 10 "sensitive" but only 1 of 10 "nonsensitive" individuals showed a greater than 20% decrease in FEV1. These results suggest that TDI-induced obstructive airways disorders may be associated with altered beta-adrenergic function.

Toluene diisocyanate (TDI) pulmonary disease: immunologic and inhalation challenge studies

Clinical and serologic effects of TDI exposure were studied in 112 occupationally exposed plant workers. Sera were obtained before and after commencement of TDI production. All subjects were skin-tested with common inhalant allergens and a TDI-HSA conjugate. Total eosinophil counts, immunoglobulin quantitations, and specific antibody assays by PCA, P-K, and radioimmunoassay were performed. Clinically "sensitive" individuals were tested by provocative inhalation challenge with from 0.005 ppm to the threshold limit value of 0.02 ppm TDI. No TDI-induced immunologic changes were noted with the exception of 3 individuals who demonstrated small positive wheal-and-erythema reactions to TDI-HSA but not to HSA alone. Inhalation challenge with TDI vapor produced airways obstruction, as measured by FEF (25-75). These responses were of the immediate, delayed, and dual type, and were provoked in some cases with levels as low as 0.005 ppm TDI.