Asbestos exposure is usually evaluated by the membrane filter method recommended by the National Institute for Occupational Safety and Health (NIOSH). The method for estimating fiber exposure was originally applied only to environments in which commercially mined asbestos was being used, but it has now been extended to the mining environment and to other environments in which natural materials containing small quantities of asbestos are found. It has been incorporated into state regulations and procedures and adopted by the industrial hygiene community as an appropriate way to evaluate asbestos exposure. Nonetheless, the membrane filter method suffers from the following serious deficiencies: (1) The aspect ratio criterion used to define and count mineral fibers was arbitrarily chosen and has never been shown to have any relevance to the etiology of asbestos-related diseases or to the actual dimensions of airborne asbestos fibers. (2) Common cleavage fragments of many amphiboles are regulated as asbestos. (3) Many fabricated products containing asbestos and many ores that may contain small quantities of asbestos also contain other substances that cannot be distinguished from asbestos by the membrane filter method. (4) Most airborne asbestos fibers are invisible if the membrane filter method is used. (5) There is no predictable relationship between exposure estimates based on the membrane filter method and the total airborne fiber concentration, as established by electron microscopy. The ratio varies between these two estimates of exposure according to (a) the particular asbestos mineral being evaluated, (b) the degree of development of the asbestiform habit, and (c) the degree of processing to which the mineral has been subjected. Therefore, different total exposures are allowed under existing regulations for different minerals, different mineral habits, and different work environments. A revised standard based on electron microscopy and applying appropriate dimensional criteria is recommended.