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CHEMICAL ENGINEERING
health and hygiene hazards. The permissible limits and the precautions to be taken to
ensure the limits are met will be very different for these two classes of toxic materials.
Industrial hygiene is as much a matter of good operating practice and control as of good
design.
The inherent toxicity of a material is measured by tests on animals. It is usually
expressed as the lethal dose at which 50 per cent of the test animals are killed, the
LD 50 (lethal dose fifty) value. The dose is expressed as the quantity in milligrams of the
toxic substance per kilogram of body weight of the test animal.
Some values for tests on rats are given in Table 9.1. Estimates of the LD 50 for man
are based on tests on animals. The LD 50 measures the acute effects; it gives only a crude
indication of the possible chronic effects.

Table 9.1. Some LD 50 values
Compound mg/kg
Potassium cyanide 10
Tetraethyl lead 35
Lead 100
DDT 150
Aspirin 1500
Table salt 3000
Source: Lowrance (1976).

There is no generally accepted definition of what can be considered toxic and non-toxic.
A system of classification is given in the Classification, Packaging and Labelling of
Dangerous Substances, Regulations, 1984 (United Kingdom), which is based on European
Union (EU) guidelines; for example:

LD 50 , absorbed orally in rats, mg/kg
25 very toxic
25 to 200 toxic
200 to 2000 harmful

These definitions apply only to the short-term (acute) effects. In fixing permissible limits
on concentration for the long-term exposure of workers to toxic materials, the exposure
time must be considered together with the inherent toxicity of the material. The “Threshold
Limit Value” (TLV) is a commonly used guide for controlling the long-term exposure of
workers to contaminated air. The TLV is defined as the concentration to which it is
believed the average worker could be exposed to, day by day, for 8 hours a day, 5 days
a week, without suffering harm. It is expressed in ppm for vapours and gases, and in
3
3
mg/m (or grains/ft ) for dusts and liquid mists. A comprehensive source of data on the
toxicity of industrial materials is Sax’s handbook, Lewis (2004); which also gives guidance
on the interpretation and use of the data. Recommended TLV values are published in
bulletins by the United States Occupational Safety and Health Administration. Since
1980 the United Kingdom Health and Safety Executive (HSE) has published values for
the Occupational Exposure Limits (OEL), for both long and short term exposure, in place
of TLV values.