975173 - Well-Mixed Room Model Applicability for Assessing Inhalation Exposure

Revised ABSTRACT:
The well-mixed room (WMR) model is a valuable tool for assessing inhalation exposure after the release of a volatile product or pollutant. The model assumes that vapor is instantaneously mixed into the room air to a uniform concentration. Although, a release may not be instantaneously mixed, it may be become well-mixed after sufficient time. The time required for vapor to become well-mixed in a mechanically ventilated room was estimated using the Foat equation. There are other factors that help mix the room air which are not be accounted for in the Foat equation. Factors that may increase the AER for a room are: natural ventilation through open windows/doors, infiltration, etc. Factors that increase the air mixing in a room without increasing the AER are: the presence of sunlight, use of fans, turbulent air flow created by active people, if the source location is moving, electric heaters, etc. There may also be air cleaners or sinks removing vapor pollutants.
A simple sensitivity analysis predicted the time for room air to become well-mixed: increasing the air exchange rate (AER) or room volume decreased the mixing time, but increasing the supply air vent area increased the mixing time. Use of the WMR model and the complete mixing assumption are reasonable for an exposure time significantly greater than the time for the room to become well-mixed. Other caveats for the use of the WMR model are discussed. This paper is directed to risk assessors who want a practical, scientific analysis but do not want to become an expert in exposure modeling.