This standard describes the equipment and procedures necessary for sampling and counting fibers, which allow the evaluation of personal exposure and the control of work environments in which the presence of asbestos fibers in the air is assumed. It is also intended that the procedures described in this method can be used in epidemiology. The method is applicable for determining the concentration of asbestos fibers in the air, in a theoretical range of concentrations between 0.02 and 100 fibers/cc. These values are determined by the requirements of sampling and analysis, assuming a maximum sampling time of 8 hours and a minimum acceptable load of 15 fibers/100 fields and a minimum sampling time of 5 minutes and a maximum acceptable load of 10 fibers/field (see section 5.3). Currently, there is insufficient information to determine at what level of concentration the reliability of this method becomes so poor that the results cease to have meaning. This level will not have a single value, but there will be a margin that will depend at least on the absolute and relative concentration of the fibers. There seems to be general agreement among experts in this field that in practice, the lower limit of application is between 0.1 and 0.5 fibers/cc. All particles that match the dimensional conditions for fibers (see section 6.5.1), in the absence of other additional information, must be considered asbestos fibers and counted as such, thus ensuring that the exposure is not underestimated. Therefore other non-asbestiform fibers (e.g. glass fiber, mineral wool) and fibrous and acicular particles (e.g. talc, gypsum) will produce interference and consequently this method should not be used without qualitative knowledge of the sample. Qualitative analysis requires other sampling procedures and different analytical techniques (polarization and dispersion microscopy, electron microscopy, X-ray diffraction, etc.). The main drawback of the method, in addition to the aforementioned interferences, is that it requires a certain skill and training on the part of the analyst. For this reason, attendance at training courses or contact with laboratories experienced in this type of determinations is recommended. Otherwise, errors may be made (due to excess or defect) that may even exceed 100%.