Page 280 - Industrial Ventilation Design Guidebook

5.3 TOXICITY AND RISKS INDUCED BY OCCUPATIONAL EXPOSURE TO CHEMICAL COMPOUNDS 24 I

is only suspected of being a carcinogen. When the evidence becomes stronger,
the OELs are usually tightened further. Vinyl chloride provides a good exam-
ple; its OEL was first lowered to 20 ppm from 500 ppm and then further to 3
ppm in Sweden in 1974-5 when its ability to cause a very rare type of cancer,
angiosarcoma of the liver, was detected. The rarity of the disease made it pos-
sible to locate the association; on the other hand, the practical impact of this
carcinogenic potency also remains rather low. It has been estimated that less
than 400 angiosarcoma cases will appear worldwide due to vinyl chloride ex-
posure (in comparison with the number of occupational cancers caused by as-
4 5
bestos which is already about 1000-fold higher). ' Internationally, there is an
ongoing vigorous discussion on whether there are possible thresholds for
genotoxic carcinogens. In many instances these compounds are considered to
have no safe dose. If one assumes that there is some threshold also for geno-
toxic carcinogens, this would have major consequences for the assessment of
6 8
risks of carcinogenic compounds. "
Since the OELs provide the basis for ventilation requirements, an astute
designer tries to find out how secure the OELs of the chemicals which will be
used in the plant he or she is planning. Some of the chemicals used may totally
lack OELs. Therefore, it is advisable to become familiar with the relevant liter-
ature, preferably together with a specialist. It is clear that the ventilation engi-
neer needs to be aware of the possible significance of toxicology for industrial
ventilation construction.
The epidemiological data have the advantage of being based on human ex-
posures. However, the results of epidemiological studies often remain inconclu-
sive because of various confounding factors and poor exposure assessments. In
addition, epidemiological data are available for only a small number of agents.
The target level approach, presented in chapter 6 of this book, uses inherently
large safety margins in relation to OELs. Unfortunately, it is also applicable only
for the most common exposures. Since zero exposure is the best, the ALARA
(As Low As Reasonably Achievable) principle, adopted in radiation protection,
9 10
is, in principle, also a good approach for other exposures. ' However, even
then the question, how low is low enough, may remain unanswered. This chap-
ter has been written with the intention of lowering the threshold for a ventila-
tion engineer to seek a toxicological consultation and to provide the
fundamental background information needed to utilize the available toxicologi-
cal literature. Occupational hygienists may also find the text to be a useful com-
pact overview of the essential concepts of toxicology.

5.3.1.2 Epidemiology
Epidemiological studies usually consist of the knowledge obtained from
human exposures supplementing data derived from experimental studies. Epi-
demiological data often provide the ultimate proof of the deleterious effects of
a chemical compound on humans, and form an important component of the
assessment of the risks of some chemical compounds. In the future, the role of
epidemiological data should be confirmatory rather than decisive in the risk
assessment of existing and, especially, of new chemicals, since toxicology is be-
coming more and more a preventive rather than an observational science in
protecting the health of workers exposed to chemicals and mixtures of chemi-
cals in occupational environments.