DIRECT-READING INSTRUMENTS FOR AEROSOLS - A REVIEW

Direct-reading instruments for aerosols have not had the popularity wi thin the industrial hygiene community that similar instruments for gas es and vapours have enjoyed. There are several reasons for this: aeros ols have complex properties that are difficult to characterize with a single measurement, commercial instruments often do not provide an acc urate measure of a useful aerosol property and aerosol instruments are relatively expensive for industrial hygiene use. A variety of instrum ents are commercially available and are briefly reviewed. Two general classes of instruments used for industrial hygiene measurements are co vered: field instruments and research instruments. The International S ymposium on Air Sampling Instrument Performance held in Research Trian gle Park, NC, USA, in October, 1991 included a workshop on direct-read ing aerosol instruments that produced several recommendations to advan ce the state of the art. The two primary recommendations approved by t he symposium attendees were to develop voluntary consensus standards f or aerosol mass-measuring instruments and optical particle counters an d to develop an accurate, portable, direct-reading aerosol mass monito r. Some progress is being made on the latter recommendation through a project supported by the US Bureau of Mines. Other instruments have fo und specific application in industrial hygiene measurements. A miniatu rized condensation nucleus counter is being used to estimate fit facto rs for respirators. A fibre monitor is used for monitoring asbestos, e specially in asbestos abatement operations. Optical particle counters are used for low-concentration aerosols, especially in clean rooms. Ae rosol research instruments are being used to evaluate and improve fiel d instrumentation, such as respirable, thoracic and inhalable samplers and cascade impactors. Several such direct-reading instruments are no w commercially available that can rapidly measure aerosol concentratio n and size distribution. These instruments can also be used to make fi eld measurements. Accurate aerosol sampling is often difficult in unco ntrolled atmospheres; many direct-reading instrument manufacturers hav e paid little attention to inlet-characteristics of their instruments. Errors due to sampling-and internal instrument losses an be large.