Page 66 - Plant design and economics for chemical engineers

48 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

effect is only apparent after long exposure at low concentrations is considered
as an industrial health and hygiene hazard. The permissible limits and the
precautions to be taken to ensure that such limits will not be exceeded are quite
different for these two classes of toxic materials. Information on the effects of
many chemicals and physical agents is accessible through computer databases
such as MEDLAR and TOXLINE. A number of health effects noted in these
sources besides that of cancer are dermatitis, neuropathy, irritation, reproduc-
tive damage, and acute poisoning.
The inherent toxicity of a material is measured by tests on animals. The
short-term effect is expressed as LDsO, the lethal dose at which 50 percent of
the test animals do not survive. Estimates of the LD,, value for humans are
extrapolated from the animal tests. On the other hand, the permissible limits of
concentration for the long-term exposure of humans to toxic materials is set by
the threshold limit value (TLV). The latter is defined as the upper permissible
concentration limit of the material believed to be safe for humans even with an
exposure of 8 hr per day, 5 days per week over a period of many years. The
handbook prepared by Sax? provides a comprehensive source of data as well as
guidance on the interpretation and use of the data. Recommended TLV values
are published in bulletins by the Occupational Safety and Health Agency
(OSHA), the American Conference of Governmental Industrial Hygienists
(ACGIH), the American Industrial Hygiene Association (ARIA), the National
Institute for Occupational Safety and Health (NIOSH), and the United Ring-
dom Health and Safety Executive (HSE).
With the uncertainties involved in the designation of occupational expo-
sure standards and the variability of the occupational environment, it would be
unreasonable to interpret occupational limits as rigidly as one might interpret
an engineering standard or specification. Fortunately, there has been a recent
effort to make these rather subjective judgements more scientific and uniform
by the application of statistics. The latter makes it possible to develop decision-
making strategies that can prescribe how many samples to take, where and
when to take them in the workplace, and how to interpret the results.

Sources of Exposure

The main objective of health-hazard control is to limit the chemical dosage of a
chemical by minimizing or preventing exposure. It is not practical to measure or
control the chemical dosage directly; rather, exposure is measured and limits
are set for the control of such exposure.
The most common and most significant source of workplace exposure to
chemicals and also the most difficult to control is inhalation. Workers become
exposed when the contaminant is picked up by the air they breathe. Thus, an

TN. T. Sax, “Dangerous Properties of Industrial Materials,;’ 6th ed., Van Nostrand Reinhold,
New York, 1984.