Contaminant vapors are normally formed by allowing the liquid to evaporate
into the air. A significant source of mercury poisoning is from worker
exposure in laboratories where mercury has been spilled, trapped in cracks, and
then evaporates at room temperature to exceed the TLV of 0.05 mg/m3.
Gases are usually stored or processed in closed systems. Contamination of
air with such gas occurs from fugitive emissions (leaks) or from venting.
Essentially all closed systems leak to some degree. [The Environmental Protection
Agency (EPA) through various studies has determined that emissions from
just the synthetic organic chemical manufacturing industry in the United States
are greater than 80,000 Mg/yr before emission controls are applied.] Obviously,
the tightness of a system is directly related to the engineering and leak
monitoring effort expended. This, in turn, depends on the consequences resulting
from these emissions. High-value and very toxic materials are usually very
tightly controlled. Contaminants that are neither valuable nor toxic but that
create an undesirable atmosphere in neighboring communities are also controlled
to maintain good public relations. Flammable materials likewise are
carefully controlled because a leak may lead to a fire and a possible major loss
in life and facility. Table 1 lists potential sources of air contamination in the
chemical process industry, noting whether these are intermittent or continuous
sources, whether workers are directly involved in the emission operation, the
relative importance of the emission source, and the most probable control of the
emission.