UDC 614.72:678.675

On the Biological Effects and Hygienic Assessment of Caprolactam as an Atmospheric Air Pollutant

I. M. Krichevskaya

Department of General Hygiene, Kemerovo Medical Institute, and Department of Communal Hygiene, Central Institute for Advanced Medical Training, Moscow

The article presents the results of experimental studies characterizing the degree of harm posed by caprolactam as an atmospheric pollutant and providing a basis for establishing its maximum permissible concentration and a sanitary protection zone.

Caprolactam is widely used as a raw material in the production of the synthetic fiber kapron and polyamide resins. The industrial methods used to synthesize this substance result in the release of caprolactam itself, together with intermediate products, into workplace air and the atmosphere. Caprolactam has been shown to be a convulsant poison that, when inhaled, has relatively high toxicity and a narrow toxic action range (Hohensee; Ceresa and Grazioli; Goldblatt and coauthors; V. A. Savelova; G. V. Lomonova). When inhaled at high concentrations, it has systemic effects, causing degenerative changes in the liver, kidneys, and brain, as well as dermatitis in animals (G. V. Lomonova and A. A. Preobrazhenskaya).

We found no information on the effects of prolonged inhalation of low concentrations of caprolactam on humans and animals, or on its presence as an atmospheric pollutant around the relevant industrial facilities. The aim of our work was to study the biological effects of caprolactam on living organisms and, on that basis, to establish its maximum permissible concentration (MPC) in atmospheric air.

In sanitary practice, caprolactam is determined by a specific colorimetric method, but the method has low sensitivity (0.05 mg in 3.5 ml of solution).

To study low concentrations of the substance in air under experimental conditions, we developed, under the direction of senior research fellow M. D. Manita, a spectrophotometric method based on the absorption of ultraviolet light by an ethanolic solution of caprolactam. Optimal conditions for air sampling were established. Ten milliliters of ethanol are placed in two Gernet absorbers connected in series, and air is drawn through them at a rate of 10 l/min. With this sampling method, only 2 to 10% of the total caprolactam concentration is found in the second absorber.

The sensitivity of the determination is 2.5 µg/ml. Our studies showed that, at the caprolactam concentrations produced, \(77.11 \pm 1.57\%\) is present as a condensation aerosol and \(22.88 \pm 1.82\%\) as vapor.

To establish the maximum one-time permissible concentration of caprolactam in ambient air, we used methods recommended by the Committee for the Sanitary Protection of Ambient Air (V. A. Ryazanov and coauthors). We investigated the thresholds for the olfactory perception of caprolactam and for reflex changes in the light sensitivity of the eyes, as well as the effect of low concentrations of caprolactam on the electrical activity of the brain.

To determine the olfactory perception threshold, we recruited 21 healthy individuals (a total of 529 observations were made). The results are presented in Table 1.

Table 1. Olfactory perception threshold for caprolactam

Table 1 shows that, for the most sensitive individuals, the threshold concentration of caprolactam is 0.30 mg/m³ and the subthreshold concentration is 0.21 mg/m³.

We studied the reflex effect of low concentrations of caprolactam on the light sensitivity of the eyes by the dark adaptation method, using an ADM adaptometer, in three individuals with an individual olfactory perception threshold of 0.30 mg/m³. The minimum concentration of caprolactam that caused a change in the light sensitivity of the eyes was 0.20 mg/m³, and the maximum concentration without an effect was 0.11 mg/m³.

To determine the effect of low concentrations of caprolactam on the electrical activity of the brain, we used electroencephalography in two modifications: the development of an electrocortical conditioned reflex (K. A. Bushtueva and coauthors) and the method of amplifying brain potentials (A. D. Semenenko). Brain biopotentials were recorded with an eight-channel Kaiser encephalograph equipped with an integrator designed by engineer B. N. Balashov.

Studies involving the development of an electrocortical conditioned reflex were conducted in three individuals with a pronounced \(\alpha\)-rhythm. Using this method, the thresholds for the reflex action of caprolactam in these individuals were found to be 0.11 mg/m³; the subthreshold concentration was 0.06 mg/m³.

The method of amplifying brain potentials was also applied in three subjects. Their threshold concentration of caprolactam was 0.18 mg/m³, while a concentration of 0.11 mg/m³ caused no changes. Graphs of changes in the amplitude of the amplified rhythm for subjects G. and D. are presented in Fig. 1.

Thus, the method of establishing an electrocortical conditioned reflex proved more sensitive. The data obtained by this method can serve as the basis for establishing the maximum single-exposure permissible concentration of caprolactam in ambient air.

To establish the permissible 24-hour average concentration of this substance in ambient air, 60 male albino rats weighing 115–150 g were subjected to chronic, continuous inhalation exposure for 82 days. The animals were divided into 4 groups of 15 each. The caprolactam concentrations in the chambers were 6 mg/m³ for group 1, 0.6 mg/m³ for group 2, and 0.06 mg/m³ for group 3 (at the level of the maximum single-exposure permissible concentration); group 4, which received clean air, served as the control. The actual caprolactam concentrations in the experimental groups were, respectively, \(5.96 \pm 0.95\), \(0.66 \pm 0.075\), and \(0.059 \pm 0.024\) mg/m³ \((M \pm \delta)\).

Fig. 1. Changes in the amplitude of the enhanced rhythm during inhalation of different concentrations of caprolactam (right hemisphere).

Vertical axis: Amplitude of the enhanced rhythm. Horizontal axis: Time (minutes). Gas exposure is marked on each graph.

1—clean air; 2—caprolactam concentration of 0.11 mg/m³; 3—caprolactam concentration of 0.18 mg/m³; а—subject G.; б—subject D.

We monitored the animals’ general condition and weight; the ratio of motor chronaxies of antagonist muscles; whole-blood cholinesterase activity; the sulfhydryl group content of whole-blood proteins; urinary excretion of coproporphyrins and 17-ketosteroids; and ascorbic acid content in the liver, spleen, brain, and adrenal glands at the end of exposure and 14 days after its cessation. In the third month of exposure, a functional challenge was applied: food restriction for 10 days. During this period, the animals received one-third of their daily ration. Drinking water remained available as usual. All tests were performed every 15 days, except chronaximetry, which was repeated every 10 days.

During exposure, the animals were active and gained weight steadily. At the end of the food-restriction period, rats in group 1 showed a statistically significant reduction in weight gain compared with baseline, which was not observed in the other groups. After food restriction ended, animals in all groups gained weight steadily.

In group 1 rats, changes in the normal ratio of extensor to flexor chronaxies were observed as early as the beginning of the sixth week of exposure (0.98). Before exposure, the ratio was 1.17–1.16. This effect became considerably more pronounced after experimental food restriction (0.78). Similar changes were observed in group 2 animals, but they were somewhat less pronounced than in group 1 and appeared at the end of the seventh week of exposure. In group 3 and in group 4 (the control group), the ratio of extensor to flexor chronaxies remained normal throughout exposure. After exposure ended, the changes in groups 1 and 2 returned to normal within 2 weeks.

We studied changes in whole-blood cholinesterase activity in 5 rats from each group using the method of J. Fleisher and E. Pope, modi-

…used in the laboratory of the Department of Community Hygiene at the First Moscow Medical Institute. Caprolactam at a concentration of 6 mg/m³ (group 1) caused a decrease in cholinesterase activity beginning in the fifth week, which persisted throughout the exposure period. In group 2, the decrease in cholinesterase activity, observed from the seventh week of exposure, was less pronounced but persisted until the end of exposure. In group 3, no significant changes in cholinesterase activity were observed compared with the control group. Cholinesterase activity in groups 1 and 2 recovered 11 days after exposure ceased.

We quantified sulfhydryl groups in whole blood spectrophotometrically, using parachloromercuribenzoate and calibration against glutathione (Kh. M. Rubina and L. A. Romanchuk), in the same remaining rats as in the preceding study. The quantity of sulfhydryl groups decreased by an average of 16.6% only in rats exposed to caprolactam at a concentration of 6 mg/m³, at the end of exposure following fasting. No changes were observed in the other experimental animals. Recovery of sulfhydryl groups in animals exposed to caprolactam at the highest concentration was observed on the ninth day after exposure ceased.

Fig. 2. Urinary coproporphyrin content in rats chronically exposed to caprolactam. АБ—exposure period; ВГ—fasting period; К—control group; 1—group 1; 2—group 2; 3—group 3.

Vertical axis: Coproporphyrins (mg per 100 g).
Horizontal axis: Observation dates.

We also quantified urinary coproporphyrin in five rats from each experimental group by a spectrophotometric method (M. I. Gusev and Yu. K. Smirnov). In group 1, urinary coproporphyrin excretion decreased sharply from the fifth week of exposure and remained at that level until the end of exposure (Fig. 2). In group 2, the decrease in coproporphyrin levels from the seventh week of exposure was somewhat less pronounced and persisted until the end of exposure. The greatest decrease in coproporphyrin content in both groups was observed during the final week of exposure, after fasting. In group 3, there were no changes in coproporphyrin excretion. Eleven days after exposure ceased, coproporphyrin levels had returned to normal in group 2 but remained somewhat reduced in group 1.

We determined 17-ketosteroid content in 24-hour urine samples collected from five rats in each group, using O. M. Uvarovskaya’s method followed by colorimetry with a green filter on a ФЭК-М instrument. The results are shown in Fig. 3. In group 1, an increase in urinary 17-ketosteroid content was detected in the fifth week of exposure, but it proved transient: two weeks later, the value had fallen to baseline. It subsequently increased again, and the elevated values persisted until the end of exposure. In group 2, only a tendency toward increased urinary 17-ketosteroid excretion was observed. In group 3, fluctuation…

the level of 17-ketosteroids in the urine did not differ significantly from that in the control group.

We studied ascorbic acid content in the brain, adrenal glands, liver, and spleen of rats at the end of exposure and 14 days after its cessation, using a titrimetric method with 2,6-dichlorophenolindophenol. Three animals from each experimental group were examined (Fig. 4). At the end of exposure, no significant changes in vitamin C content were observed in the brain or spleen of any of the exposed animals compared with the controls, although animals in groups 1 and 2 showed a tendency toward some increase in this vitamin in the spleen. A significant decrease in ascorbic acid content was found in the adrenal glands of animals in groups 1 and 2, more pronounced in the former.

Fig. 3. 17-Ketosteroid content in the urine of white rats during chronic exposure to caprolactam.

Vertical axis: 17-ketosteroids (mg per 100 g). Horizontal axis: observation dates.

The symbols are the same as in Fig. 2.

Fig. 4. Ascorbic acid content in the organs of white rats during chronic exposure to caprolactam.

Vertical axis: vitamin C (mg %).

a—brain; b—liver; c—spleen; d—adrenal glands; 1—control group; 2—group 1; 3—group 2; 4—group 3.

In the liver, its content increased in animals in groups 1 and 2 compared with the controls (the changes were statistically significant). In the adrenal glands and liver of animals in group 3, it was the same as in the controls.

Fourteen days after exposure ceased, the ascorbic acid content in the adrenal glands, liver, brain, and spleen of animals in groups 1 and 2 did not differ from the control level.

To characterize atmospheric pollution by caprolactam, we surveyed one of the plants where it is produced. The sources of atmospheric pollution there are industrial emissions released at heights of 22 and 12 m above ground level. Only one of these emissions is treated.

During the field study, a colorimetric method was used to determine caprolactam, because other impurities in the air around the production facility interfered with the spectrophotometric method. Samples around the facility were collected in February 1967 on the downwind side at temperatures ranging from 0 to 26°. The studies showed (Table 2) that single-sample caprolactam concentrations exceeded the maximum single-exposure permissible concentration proposed by us (0.06 mg/m³) at a distance of 300 m from the emission source. At 500 m, these concentrations did not exceed that level. The proportions of caprolactam aerosol and vapor under field conditions were close to those in the experimental mixtures (aerosol \(66.84 \pm 0.44\%\), vapor \(33.15 \pm 0.6\%\); \(M \pm m\)).

Table 2

Atmospheric air pollution by caprolactam around the plant

Conclusions

1. Experimental studies have established that inhaled caprolactam has a pronounced biological effect on humans and animals at concentrations of 0.11 mg/m³ and above.

2. A maximum single-exposure permissible concentration of 0.06 mg/m³ is recommended for caprolactam in atmospheric air.

3. The same value is recommended as the permissible average daily concentration of caprolactam.

4. Atmospheric air pollution by caprolactam was found around one of the enterprises producing it. In establishing the sanitary protection zone for this enterprise, it should be taken into account that the zone must extend at least 500 m on account of caprolactam.

References

Bushtueva K. A., Polezhaev E. F., Semenenko A. D. Gig. i san., 1960, no. 1, p. 57. — Gusev M. I., Smirnov Yu. K. In: Maximum Permissible Concentrations of Atmospheric Pollutants. Moscow, 1960, issue 4, p. 139. — Lomonova G. V. Materials on the Toxicology of Certain Products of Caprolactam Production. Candidate’s dissertation. Gorky, 1964. — Lomonova G. V., Preobrazhenskaya A. A. Proceedings of the Gorky Research Institute of Occupational Hygiene and Occupational Diseases, 1962, collection 9, p. 34. — Ryazanov V. A., Bushtueva K. A., Novikov Yu. V. In: Maximum Permissible Concentrations of Atmospheric Pollutants. Moscow, 1957, issue 3, p. 117. — Rubina Kh. M., Romanchuk L. A. Vopr. med. khimii, 1961, vol. 6, p. 652. — Savelova V. A. In: Sanitary Protection of Water Bodies from Pollution by Industrial Wastewater. Moscow, 1960, issue 4, p. 156. — Semenenko A. D. Gig. i san., 1963, no. 7, p. 49. — Ceresa C., Grazioli C., Med. d. Lavoro, 1952, v. 43, p. 124. — Goldblatt M. W., Farquharson M. E., Bennet G. et al., Brit. J. industr. Med., 1954, v. 11, p. 1. — Hohensee F., Faserforschung u. Textiltechnik, 1951, Bd 2, S. 299.

Received June 5, 1967.

The Biological Effect of Kaprolaktam and Its Sanitary Hygienic Assessment as Pollutant of Atmospheric Air

I. M. Krichevskaya

A pronounced biological effect was produced on man and animals on inhalation of kaprolaktam at concentrations of 0.11 mg/m³ and higher. The olfactory threshold value of kaprolaktam was at a level of 0.30 mg/m³ and the subthreshold value amounted to 0.21 mg/m³. Its threshold and subthreshold concentrations affecting the light sensitivity of eyes equaled 0.20 and 0.11 mg/m³ accordingly. The threshold value of the reflex action, as determined by the method of intensification of the cerebral biocurrents potentials, amounted to 0.18 mg/m³; the subthreshold concentration — 0.11 mg/m³. This

method proved to be more sensitive than the method of obtaining the electrocortical conditioned reflexes.

The maximum one-time and the daily average maximum permissible concentration of Kaprolaktam are recommended to be set at a level of 0.06 mg/m³.