Page 279 - Industrial Ventilation Design Guidebook

240 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGiCAL CONSIDERATIONS

the number of individuals suffering occupational diseases has declined rather
slowly. In addition, the number of new cases of registered occupational health
diseases depends on employment circumstances; there is a natural decline dur-
ing times of economic recession. This gradual change is due to several factors:
the diagnostic criteria have become less stringent, physicians have learned to
recognize occupational diseases better, and many occupational diseases de-
velop slowly and thus the present situation reflects, at least to some extent,
past exposures. Furthermore, the significance of occupational allergies has in-
creased and allergic reactions can be caused even by low exposure levels,
Toxic chemicals, such as benzene and carbon disulfide, which in the past
were common causes of occupational diseases, are nowadays generally well
controlled. Most of the current occupational diseases are caused by exposures
which are not particularly acutely toxic, but cause allergies. Typical exposures
causing allergies are animal and flour dusts. If one considers actual chemicals,
then isocyanates have become a major problem, principally via their ability to
cause sensitization. Historical exposure to asbestos and other mining dusts
still leads to numerous new diseases, many of which are very serious, even fa-
tal. Solvents and pesticides are the groups of chemicals probably causing the
largest amount of acute poisoning-type occupational diseases. ^
Traditionally, the greatest risk due to chemicals has been considered to occur
via inhalation. Chemicals may also penetrate through the skin. Water-based
products are increasingly replacing solvent-based products in many applications,
such as painting, printing, and gluing. The water-based products may, however,
contain glycol derivatives which penetrate through the skin with ease. Many
chemicals also irritate or sensitize the skin. Chromium, nickel, and epoxy resins
are examples of common occupational skin allergens.
Ventilation engineers and occupational hygienists must be aware of the
risks of chemicals with a high acute toxicity. Chemicals which are odorless (e.g.,
carbon monoxide), paralyze the sense of smell (e.g., hydrogen sulfide), or cause
pulmonary edema as a delayed effect (e.g., nitrogen dioxide and ozone) are es-
pecially insidious. Often these gases are produced as unwanted by-products. For
example, nitrogen dioxide and ozone may be formed due to oxidation of air
during welding. Welding near sources of chlorinated solvents, such as perchlo-
roethylene, may cause pyrolysis and the formation of phosgene.
Occupational exposure limits (OELs) have been set in most industrial coun-
tries to prevent excessive exposures. The limits for the most common exposures
are based on experimental animal and epidemiological studies. Most novel
agents have now generally gone through extensive toxicological testing. For the
older chemicals, usually a plethora of epidemiological data is available.
When the incidence of occupational diseases was compared with the fre-
quency of OEL violations in Finland, a rather good correlation was observed.
This indicates that these OELs are reasonably well defined. This is also natural
because they are based on long-term exposure history of a large number of
people. However, the OELs for many chemicals are still only educated guesses,
and numerous and often large changes have been made when the OEL lists
have been revised. In addition, most chemicals still have no OEL. Only about
2000 chemicals have an OEL in some country. 3
A particularly strict exposure-control policy is applied for carcinogenic
chemicals. The OELs are usually lowered considerably even when a chemical