Handbook of Solvents - George Wypych - ChemTech - Ventech!

20.5 Lymphohematopoietic study of workers exposed to benzene 1367
benzene showed an increased susceptibility to pneumonia and tuberculosis. The experimental
animal data and the epidemiological studies clearly show that 1) benzene is a carcinogen
for the lymphohematopoietic system, 2) benzene has a direct effect on the immune system,
3) benzene has a direct effect on the early development of the blood cells, and 4) benzene is
a pluripotent hematological carcinogen.
20.5.5 RISK ASSESSMENT ESTIMATES
The United States EPA has used several databases in their estimates for benzene exposure
and risk. (Environmental Protection Agency, 5.0 Benzene, 5.1. Chemical and Physical
Properties, EPA, 1988) The data utilized by the EPA to assess the risk included the study by
Rinsky et al. in 1981 57 where the duration of exposure was at least 24 years and exposure
levels are between 10 to 100 ppm (8 hour TWA) with a statistically significant increase incidence
of leukemia. The study of Ott et al. 57 in 1978 showed levels of anywhere from 2 to 25
ppm (8 hour TWA) with increased incidence of leukemia; and Wong et al. 57 1983, where the
exposure was at least 6 months, levels were from 1 ppm to 50 ppm, and there was a statistically
significant increase in the incidence of leukemia, lymphatic and hematopoietic cancers.
The International Agency for Research on Cancer (IARC) has classified benzene as a
Group 1 carcinogen. 58 A Group 1 carcinogen is defined as an agent that is carcinogenic to
humans. This classification is based on sufficient evidence for carcinogenicity in humans.
IARC based this conclusion on the fact that numerous case reports and follow-up studies
have suggested a relationship between exposure to benzene and the occurrence of various
types of leukemia. In addition, IARC considers the evidence for carcinogenicity to animals
to be sufficient. No unit risk was determined by IARC for benzene.
The regulatory agencies in the estimated risk of increased benzene related cancers rely
mainly on the study by Rinsky, 1987, which concluded that the mean annual cumulative exposure
level of less than 1 ppm accumulated over 40 years working life-time would not be
associated with increased death from leukemia. This epidemiological study showed an exponential
decrease in the risk of death from leukemia which could be achieved by lowering
occupational exposure to benzene. According to the model derived in this study, a worker
occupationally exposed to benzene at an average exposure level of 10 ppm for 40 years
would have an increased risk of death form leukemia 154.5. If the average exposure was
lowered to 1 ppm, that excess risk would decrease to 1.7. At 0.1 ppm times 40 years cumu-
lative exposure the risk be virtually equivalent to background risk, 26
Infante et al. 59 have
shown a relative risk of 5.6 with an estimated cohort exposure of 10 to 100 ppm over 8.5
years average, and Vigliani 60 showed a relative risk of 20 estimated cohort exposure to 200
to 500 ppm over 9 years average, and Aksoy showed a relative risk of 25 with an estimated
cohort exposure of 150 to 210 ppm over 8.7 years average. 20,40 These studies clearly show
that the risk of developing lymphohematopoietic cancers is significant, and that benzene is
carcinogenic from an epidemiological point of view at very low levels of 0.1 ppm.
20.5.6 LEVELS OF EXPOSURE
Regulatory levels of exposure to various chemicals, among them solvents and benzene,
have been a subject for constant pressure from industry manufacturers on one hand, regulatory
agencies, health care, and patients on the other hand. The most common question asked
is “Is there a safe level of exposure to benzene?” The answer to that question has been given
by the American Petroleum Institute in their paper on benzene, 1948, 16 and their statement