The effect of room size and general ventilation on the relationship between near and far-field concentrations

Measurements of personal exposure level are generally greater than or equal to concentrations found at some fixed location in the body of the workroom. This article presents a theoretical analysis of such differences using a simple model of exposure, implemented as a two-compartment mass balance model with a constant emission rate. Simulated exposure levels (termed the near-field) and the concentration at a fixed location (the far-field) were obtained for five room sizes (30 to 3000 m3), each with five general ventilation rates (0.3 to 30 air-changes per hour) and three levels of air movement in the vicinity of the worker (generally 3 to 30 m3/min). The ratio of near- to far-field concentrations from the simulations ranged from unity in small poorly ventilated rooms to 24 in large well ventilated areas, which compares favorably with actual measurement data. The predicted concentrations obtained in small rooms (< 100 m3) with less than one air-change per hour were almost 40 times higher than in larger rooms. The results from these simulations have been used to modify the original exposure model to better reflect the likely effect of general ventilation on workers' exposure.