Filtration in industrial hygiene

Filters used in industrial hygiene are of two basic types, corresponding wi th the two basic airborne hazards: particulate and vapor. They are as diffe rent in their construction as they are in their purpose, and each gives neg ligible protection against the other hazard. By use of the correct type, ad equate filtration efficiency can usually be achieved. Most particulate filt ers are made from fibers, and finer fibers result in higher efficiency. Fil ters can capture particles much smaller than the fiber diameter, as a resul t of diffusional motion of the airborne particles and, in the case of filte rs that hold a permanent electric charge, electrostatic attraction. Most va por filters are made from granules of activated carbon, which have an extre mely large effective surface area, where molecules of contaminant are adsor bed. The performance of all filters tends to alter as the filter material b ecomes loaded. Electrically neutral particulate filters become more efficie nt but at the expense of increased resistance to airflow. Particulate filte rs that act by electric forces may become less efficient, and are often les s inclined to clog. Vapor filters usually have a high initial efficiency, b ut 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 th at the choice of a filter should be made with close reference to the situat ion in which it is to be used, and optimum respiratory protection should be sought, rather than maximum filtration efficiency. Special problems of fil ters are illustrated by some case histories, and finally the use of filters as size selectors for dust samplers is briefly described.