Estimating exposure intensity in an imperfectly mixed room.

The well-mixed room model is traditionally used to predict the concentration of contaminants in indoor environments. To account for imperfect air mixing, the room supply/exhaust air rate Q is frequently multiplied by a mixing factor m, where 0 < m < or = 1, and an effective ventilation rate QE = m . Q is used in place of Q in the well-mixed room equations. However, this procedure is inappropriate because a well-mixed room model, albeit with an adjusted ventilation rate, is still used to describe an imperfectly mixed room. To illustrate the errors that may result, a two-zone model is described in which a room is conceptually divided into an upper zone and lower zone, where the latter is the zone of occupancy. Air is supplied to and exhausted from the upper zone at rate Q, and air exchanges between the two zones at rate beta. The lower zone's true ventilation rate is termed its purging flow rate QL, where QL = [beta/(beta + Q)]Q. Expressions for the steady-state contaminant levels in the two zones and for decay from the steady-state levels are presented. In a two-zone room, if one ignores imperfect air mixing and attempts to estimate QE from a decay curve, QE will usually be greater than QL. Given that contaminant is emitted in the lower zone, subsequent use of QE rather than QL to predict steady-state exposure intensity in the room will cause an underestimation error. For a room with an upper- to lower-zone volume ratio of 2:3, the underestimation error can reach 40%. If a room has a single or dominant point source of contaminant, it is recommended that the purging flow rate near the release point be determined, which permits a more accurate prediction of a worker's exposure intensity near the source. Alternative methods for determining the local purging flow rate are described. It is also shown that age-of-air analysis techniques do not provide information directly relevant to estimating exposure intensity.