Aspects of the toxicology of chloroprene: immediate and long-term effects.

The maximum permissible concentration (MPC) of chloroprene was set at 2 mg/m3 in the USSR in the 1940's. The existing MPC is 4 mg/m3. The threshold of systemic effects as a result of chronic chloroprene exposure is l.69 mg/m3. However, the threshold for embryotoxic and mutagenic effects is 0.15 mg/m3. In consideration of this information, setting a new MPC for chloroprene at 0.05 mg/m3 is recommended.

Chloroprene (2-chlorobuta-1,3-diene) is usually obtained by a reaction of vinylacetylene with hydrogen chloride. It is used as a monomer for the production of synthetic rubber and latexes.
Chloroprene may be classified as a polytropic poison since it causes functional and organic disturbances of the nervous system, the cardiovascular system. the liver and the kidneys. In women working with chloroprene (Nairit) latex, pregnancy has been found to take an unfavorable course (1). The maximum permissible concentration (MPC) of chloroprene at 2 mg/m3 was established in the USSR in the 1940's on the basis of calculations and data in the literature. In connection with the appearance of congenital abnormalities in the children of women employed in the polymerization shop in a chloroprene factory (2) women were taken off chloroprene production (3). Clinical and safety investigations in factories and a whole range of experimental studies were subsequently undertaken to establish a corrected MPC.

Clinical and Occupational Safety Investigations
Persons working on chloroprene production, where the concentration of the substance in the * Department of Toxicology, Institute of Industrial Hygiene and Occupational Diseases, Moscow, U.S.S.R. air was several times higher than the former MPC, were given medical examinations (4). Primary changes in chloroprene production workers were found in the nervous system (5)(6)(7). In some workers changes were found in hepatic and renal function (8), (9), the cardiovascular system (6,10), and the morphological composition of the peripheral blood (11).
A special questionnaire was used to study reproductive function in male workers. Altogether, 143 workers from chloroprene shops and 118 controls from factory and office workers in an electrical engineering plant were questioned.
Examinations of chloroprene workers (12) revealed functional disturbances in spermatogenesis after 6-10 yr of work in chloroprene production, and morphological disturbances after 11 yr or more. The questionnaire showed that cases of spontaneous abortion in the wives of chloroprene workers occurred more than three times as frequently as in the control group. In persons employed in chloroprene production (analysis of coded preparation), the frequency of cells with chromosome aberrations in a peripheral blood leukocyte culture (Table 1) considerably exceeded the level of aberrations in the control group and the spontaneous level of this indicator in the blood of healthy persons (4).
In view of the relatively high concentrations of chloroprene in latex and rubber manufac- The results showed that the main hazard to workers was chloroprene vapor which varied in concentration from 1 to 7 mg/m3 in the air of the work areas, which is of the order of the previous MPC. The most frequently encountered other volatile substance in latex was ammonia. However, the ammonia concentration (2.0-4.0 mg/m3) in the air of the work areas was not considered a significant health hazard (MPC is 20 mg/m3) (13).
Work in these occupations is classified as light labor. Most operations have been mechanized. To determine changes in the state of health of the workers, a thorough medical examination was carried out on 12 men and 53 women (14). Two-thirds of the workers in this group had been employed for less than 5 yr in this work.
Examinations of the cardiovascular system showed muiffled heart sounds in 30 persons, reduced arterial presure in 14, and tachycardia in 9. A noteworthy feature was a reduction in the red cell count in the peripheral blood which averaged 3,830,000/mm3. In a considerable number of persons there was a reduction in hemoglobin level below 11.5%o, i.e., below the lower limit of physiological variation.
Worthy of attention was the predominance of a low reticulocyte count, despite anemia. In half the persons examined, the number of reticulocytes was at the lower limit of the physiological range (3-4%o). Together with erythrocytopenia in workers exposed to chloroprene, a tendency towards leukopenia and thrombocytopenia was observed. There was a particu-larly sharp decrease in the thrombocyte count (less than 175,000/mm3) that was quite frequent in women who had been working with chloroprene for 1-5 yr.
Examination of vestibular function, even in persons with less than 5 yr of work with chloroprene, showed moderate, sluggish nystagmus and a relative shortening of postcaloric nystagmus and inhibition of vestibulo-autonomic and vestibulo-sensory reactions. The systems of disturbances in vestibular functions increased with increased length of employment.
Physiological investigations were carried out (13) in a group of women aged 19 to 23 who had worked in this occupation for 2-4 yr and had never worked elsewhere. Women who had had no contact with toxic substances and who were doing light physical labor were used as controls. The results showed that the women wvorkers had an abnormal diurnal variation of arterial presure, with a reduction of its systolic and diastolic components toward the end of the working day. The pulse rate during work (93.1 ± 1.2 beats/min) was considerably higher than in the controls (p <0.01).
An indication of the effect of chloroprene on the central nervous system function was a lengthening of the latent period in the sensorimotor response to a visual cue, as compared with the control group. There was also a reduction in olfactory sensibility, the olfactory threshold being considerably higher among workers with a long employment record than among those who had been in contact with chloroprene for a shorter time.
A cytogenetic examination of the peripheral blood lymphocytes was carried out on 20 female workers (aged 19-23, length of employment 1-4 yrs, and aged 19-50, length of employment 1-30 yr) (15). Results are summarized in Table 2. In botlh groups there was Environmental Health Perspectives an increased number of cells with aberrations, regard to the concentrations typifying the upamounting to 3.49 ± 0.51% and 2.5 ± 0.49%, per levels of chloroprene toxicity (Table 3).
respectively, as against the spontaneous level In experiments on two species of laboratory of 1.19 ± 0.06% (16).
animals (white rats and C57BL/6 mice) a To determine the effect of working condi-study was made of the systemic, gonadotropic, tions on specific functions in female workers, and mutagenic effects of chloroprene following a special investigation was carried out on 147 chronic exposure (4.5 months for rats, 2 women workers in a chloroprene shop and 100 months for mice) to low concentrations, i.e., women in a control group (17). The break-the level of the previous MPC (2 mg/m3) and down of noninflammatory morbidity was as concentrations 10 to 40 times lower than that. follows: the prevalence of tumors among The embryotoxic effect of chloroprene was women chloroprene workers was 8.2% and studied in white rats following exposure duramong the controls 1% (p <0.05); primary ing the whole period of pregnancy in concenand secondary sterility in the chloroprene trations about equal to the previous MPC and workers was 6.1 %o and in the controls was 3.0, 10, and 40 times lower. More than 800 2 % (17). rats and 600 mice were used, about 3000 fetuses were analyzed, and 700 offspring were examined. The studies of the embryotoxic, mutagenic, and gonadotropic effects of chloroprene Experimental Studies were reproducible.
It was established that following 4.5 months The parameters of acute toxicity have been exposure to chloroprene at a concentration established by a number of investigators, and close to the former MPC (1.69 ± 0.087 mg/ the results are generally in good agreement in m3) signs of a systemic effect were observed October 1976 in male rats. There was an increase in the "summation threshold index" after 2.5 months (Table 4), and after 4.5 months a decrease in the synthesis of hippuric acid from sodium benzoate (Quick's test) and an inhibition of gas exchange. Chloroprene had no effect on the selected indicators at concentrations of 0.15 -0.0059 mg/m3 and 0.051 + 0.006 mg/m3. With the indicators used in the tests, exposure to chloroprene produced no systemic effect in mice at any of the following concentrations: 35 ± 0.7 mg/i3; 1.85 + 0.18 mg/i3; 0.32 + 0.06 mg/m3; 0.13 ± 0.01 mg/m3; 0.064 -0.01 mg/m3; and 0.054 ± 0.02 mg/m3. Thus, the threshold for the chronic systemic effect of chloroprene in rats, as established by the indicators used in this study, appears to be 1.69 ± 0.087 mg/m3, which is close to the previous MPC (18).
Investigation of the gonadotropic effect of chloroprene showed that a concentration close to the former MPC (1.69 ± 0.087 mg/m3), after 4.5 months exposure in male rats, caused a reduction in the number of normal sperma-togonia, an increase in the number of dead spermatozoa, an increased susceptibility of spermatoza to inactivation in an acid medium, and a decrease in their period of motility. Some of the indicators showed values outside the normal physiological range (Tables 5 and 6). A less marked but similar effect was observed after exposure to a chloroprene concentration (0.15 +-0.0059 mg/m3) one order of magnitude lower than the MPC. A concentration of 0.051 -+-0.006 mg/m3 caused no functional or morphological changes in spermatogenesis. Consequently, a chloroprene concentration of 0.15 + 0.0059 mg/m3, i.e., one order of magnitude lower than the existing MPC (based on indicators of systemic effect), can be considered to be close to the threshold value for the effect on rat gonads. Study of the gonadotropic effect of chloroprene in C57BL/6 mice has shown that concentrations equal to and one order of magnitude below the MPC caused adverse changes in spermatogenesis (increase in the number of tubules with desquamating germinal epithelium) ( Table 1). A concentration of   (19).
The mutagenicity of chloroprene was studied in germ cells (by counting dominant lethal mutations) and in somatic tissue in the bone marrow (metaphase method). It was estab-lished that chloroprene, at concentrations close to and one order of magnitude below the previous MPC, increased the frequency of dominant lethal mutations in the germ cells of male rats after 2.5 months of exposure. Chloroprene did not induce such changes in rats in concentrations of 0.057 ± 0.003 mg/M3 (Table 8).
In C57BL/6 mice the frequency of dominant lethal mutations increased in the germ cells after exposure to chloroprene at a concentration close to the MPC (Table 9). An increase in chromosome aberrations in mouse bone marrow cells occurred after exposure to chloroprene for 2 months at concentrations near to (3.5 ± 0.7 and 1.85 ± 0.18 mg/m3) and one order of magnitude below (0.32 + 0.06 and 0.13 + 0.01 mg/m3) the previous MPC. A chloroprene concentration 40 times lower than the former MPC (0.054 ± 0.024 and 0.065 + 0.01 mg/m3) proved inactive in relation to the selected indicators (Table 10).
Thus, the threshold concentration of chloroprene for mutagenicity, as established in chronic experiments on rats and mice, can be considered to be one order of magnitude below the former MPC.
The results of studies of the embryotropic effect showed that a chloroprene concentration of 4 ± 0.7 mg/m3 (the level of the existing MPC) had a systemic effect on pregnant rats (inhibition of spontaneous motor activity, an increase in hippuric acid level in the urine after administration of sodium benzoate, hypoproteinemia, an increase in oxygen consumption, a reduced weight gain, and an increase in the weight coefficients of the brain, lungs, liver, and kidneys). In nonpregnant experimental animals only a reduction in spontaneous motor activity was observed (Table 11). Exposure  to a concentration (0.6 + 0.08 mg/m3) one third the previous MPC produced hypoproteinemia in pregnant animals (6.2 ± 0.02 as against 7.3 ± 0.01 mg% in the controls, p <0.01), but no abnormalities were found in nonpregnant animals. Chloroprene in concentrations one order of magnitude lower (0.13 ± 0.009 mg/rm3) and 40 times lower than the previous MPC (0.056 + 0.006 mg/m3) caused no abnormalities in pregnant animals so far as it could be judged by the selected tests (20,21). Thus, the threshold concentration of chloroprene based on the indicators of systemic effects in nonpregnant rats is about 4 + 0.7 mg/m3, i.e., close to the MPC, which coincides with the threshold of chronic effects found in males. The limCh for pregnant animals was three times lower than the former MPC (0.6 ± 0.08 mg/m3). The results obtained indicate that pregnant rats are highly sensitive to chloroprene.
An analysis of embryonic material obtained from animals exposed to chloroprene during the entire pregnancy showed that in a concen-tration of 4 ± 0.7 mg/m3 the substance caused an increase in overall embryonic mortality, a decrease in fetal weight (Table 12), and a disturbance in vascular permeability as shown in histological sections. On exposure to the substance in concentration 3, 10, and 40 times lower than the former MPC, no changes of this kind were found, as compared with the controls (22).
In offspring whose mothers had been exposed to 4 ± 0.7 mg/m3 of chloroprene during the entire pregnancy, a decrease in the rate of weight gain and in the urinary content of hippuric acid (Quick's test) were found. At concentrations three times lower than the former MPC (0.6 + 0.98 mg/m3), chloroprene caused increased numbers of deaths in the first 3 weeks after birth, a reduction in the rate of weight gain, and a disturbance in liver function following administration of a hepatotoxin. After exposure to a concentration of 0.13 ± 0.009 mg/M3, an increase was also noted in postnatal deaths of offspring, together with a decrease in the urinary protein level. Chloroprene failed to produce any abnormalities in the offspring at a concentration of 0.056 + 0.006 mg/m3, irrespective of the indicators used. Consequently, a chloroprene concentration of 0.13 ± 0.009 mg/m3 is close, under our experimental conditions, to the threshold value for embryotropic effect (22).

Conclusion
The changes found in worIkers occurred following exposure to chloroprene in the air in concentrations slightly above the MPC established earlier (2 mg/3).
The threshold for chronic effects of chloroprene on animals (the tests were reproduced many times on males and females) based upon the indicators of general systemic effect, is 1.69 ± 0.087 mg/m3, i.e. about the same as the MPC formerly adopted. The threshold concentration based on specific indicators (embryotropic, gonadotropic, and mutagenic effects) was 0.15 ± 0.0059 mg/m3, i.e., one order of magnitude below the former MPC. The corre-lation of specific effects was shown previously by Sanotskii et al. (19) The general systemic effect of chloroprene observed at a concentration close to the former MPC, which was based on clinical and epidemiological studies, and its marked specific effect (Z,P -12) on the reproductive function and the development of offspring (in man and animal experiments), provide substantiation for a new MPC for chloroprene of 0.05 mg/mi3.