Filtration in industrial hygiene.

Filters used in industrial hygiene are of two basic types, corresponding with the two basic airborne hazards: particulate and vapor. They are as different in their construction as they are in their purpose, and each gives negligible protection against the other hazard. By use of the correct type, adequate filtration efficiency can usually be achieved. Most particulate filters are made from fibers, and finer fibers result in higher efficiency. Filters can capture particles much smaller than the fiber diameter, as a result of diffusional motion of the airborne particles and, in the case of filters that hold a permanent electric charge, electrostatic attraction. Most vapor filters are made from granules of activated carbon, which have an extremely large effective surface area, where molecules of contaminant are adsorbed. The performance of all filters tends to alter as the filter material becomes loaded. Electrically neutral particulate filters become more efficient but at the expense of increased resistance to airflow. Particulate filters that act by electric forces may become less efficient, and are often less inclined to clog. Vapor filters usually have a high initial efficiency, but the penetration of vapor increases as the filters become saturated with adsorbed vapor, and the performance of these filters is normally expressed in terms of their lifetime rather than their efficiency. It is important that the choice of a filter should be made with close reference to the situation in which it is to be used, and optimum respiratory protection should be sought, rather than maximum filtration efficiency. Special problems of filters are illustrated by some case histories, and finally the use of filters as size selectors for dust samplers is briefly described.