A diffusion stage for the omnicount to measure personal ultrafine particle number concentrations

Exposure to smoke among wildland firefighters has been associated with adverse health effects, including lung cancer and heart disease. Wildland firefighters are especially at risk with long shifts, little respiratory protection, and tasks that move them to different areas of a fire. There is a need for a wearable device to assess personal exposure to ultrafine and fine particles independently with a high time resolution (< 5 min). The objective of this work was to develop, through theory and experimentation, a diffusion stage for an OmniCount™ prototype (OmniCount™ Portable Water-based Condensation Particle Counter (PWCPC) Model 3002, TSI inc., Shoreview, MN) that may be used to assess personal exposure to ultrafine and fine particle number concentrations. A diffusion stage was first developed for one channel of the OmniCount™ based on theory. A stack of three 400-square-mesh, stainless-steel screens in a 37-mm polypropylene cassette was selected as having the best traits of those tested for the OmniCount™ diffusion stage, including a cutoff diameter of 98 nm, a pressure drop of 0.18 Pa, and a maximum penetration of 89% at 1100 nm. The performance of the diffusion stage was evaluated experimentally and penetration measured experimentally agreed with theory. The bias, or the relative discrepancy between the measured number concentration and the true number concentration, in the assessment of ultrafine particle number concentration using the diffusion stage was calculated. Bias was estimated from theory as -0.2 to -0.03 for hot burns and -0.2 to 0.3 for smolders. In bench top tests, the bias for the OmniCount™ with the diffusion stage was dependent on particle concentration: 0.3 to 0.7 for less than 50,000 cm-3; and 0 to -0.4 for concentrations greater than the manufacturers recommended operating range of the OmniCount™ (>100,000 cm-3). The source of the overestimate for low concentrations is unclear, although the underestimate at high particle number concentration is likely due to particle coincidence in the sensing zone of the OmniCount™. The coefficient of variation increased as the particle number concentration increased. The coefficient of variation ranged from 2% to 13% in the operating range of the OmniCount™.