NIOSH manual of analytical methods, fourth edition - third supplement. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003-154, 2003 Mar; :63-69
This discussion will cover direct-reading aerosol photometers that are self-contained (battery-operated) and portable (can be used while carried by one person). There is a variety of direct-reading aerosol monitors using light-scattering detectors. These instruments generally have advantages in reduced weight, improved ruggedness, and continuous readout when compared with other direct-reading aerosol monitors. These instruments can be used to provide accurate dust concentration measurements as described below, though in most situations they are most useful for approximate or relative concentration measurements. Their principle advantage is that of providing real time information. An aerosol is a group of particles suspended in the air. Aerosols can be introduced into the body primarily through the respiratory system. Total dust measurements indicate concentrations that can enter the nose and mouth of a worker as well as that which can settle on the skin while the respirable fraction of dust is that portion which can reach the lower or gas exchange part of the respiratory system. The respirable fraction of the dust mass has been defined for sampling purposes by the American Conference of Governmental Industrial Hygienists (ACGIH) as that fraction collected by a device with a penetration curve in Figure 1. There are other definitions of respirable dust [2,3,4] as well as empirical data indicating deposition efficiency of dust in the respiratory system. Historically, the most commonly used respirable dust sampling device in the U.S. is the 10-mm nylon cyclone. At a flow rate of 1.7 L/min, the cyclone passes close to 50% of 4-micrometer aerosol particles. However, it has been shown that the 10-mm cyclone has a somewhat sharper cutoff than the ACGIH curve and, with certain size distributions, may introduce a bias with respect to the ACGIH definition . Aerosols are frequently classified according to their physical form and source. Aerosols consisting of solids (e.g., coal, wood, asbestos) are designated dusts. Aerosols consisting of liquid (e.g., oil, water, solvents) droplets are called mists. Submicrometer aerosols that are formed from condensation or combustion processes are generally called fumes or smokes. Some of these aerosols have a significant vapor pressure and will evaporate when aged. The direct-reading photometer may detect these high vapor pressure aerosols while the reference method for respirable dust (Method 0600) will not.