Current particulate matter (PM) exposure studies are using continuous personal nephelometers (pDR-1000, MIE, Inc.) to measure human exposure to PM. The personal nephelometer is a passive sampler which uses light scattering technology to measure particles ranging in size from 0.1-10 micrometers using a light scattering technique, however, it is more responsive to particles in the fine particle size range (0.3-3 micrometers). While the data from the nephelometer remain semi-quantitative, the instrument is very useful for identifying activities and microenvironments that may significantly enhance human exposure to PM. Based on the use of this instrument in the field, the authors recognize that it is important to identify activities or environments that may have an adverse effect on the instrument's response and subsequent data quality. The authors have tested the nephelometers response to sample vest fabric (cotton/polyester or nylon), sampler location on an individual (shoulder vs. waist), and relative humidity. Repeated scripted activities while wearing a 50-50 cotton/polyester or a nylon vest indicated that significantly more particles (p < 0.01) were introduced by the cotton/polyester vest than the nylon vest. The location of the monitor was weakly significantly different (p < 0.1) for many common activities, and significantly higher particle readings were observed at the waist (p < 0.02) while sweeping.