Estimating exposure intensity in an imperfectly mixed room

Area of Science:

• Indoor air quality
• Environmental engineering
• Occupational hygiene

Background:

• The well-mixed room model is standard for predicting indoor contaminant concentrations.
• This model often uses an adjusted ventilation rate (QE) to account for imperfect air mixing.
• However, applying a well-mixed model to imperfectly mixed rooms is scientifically inappropriate.

Purpose of the Study:

• To evaluate the accuracy of the well-mixed room model in predicting indoor contaminant levels under imperfect mixing conditions.
• To introduce and analyze a two-zone model for a more realistic representation of indoor environments.
• To quantify the potential underestimation errors in worker exposure assessments.

Main Methods:

• Development of a two-zone model dividing a room into upper and lower (occupancy) zones with air exchange (beta) and supply/exhaust rates (Q).
• Derivation of the true ventilation rate for the lower zone (purging flow rate, QL).
• Comparison of contaminant levels predicted by the two-zone model versus the adjusted well-mixed model (QE).

Main Results:

• The effective ventilation rate (QE) derived from decay curves often exceeds the true purging flow rate (QL).
• Using QE instead of QL for contaminants emitted in the lower zone leads to underestimation of steady-state exposure.
• For a 2:3 upper- to lower-zone volume ratio, underestimation errors can reach 40%.

Conclusions:

• The well-mixed room model, even with an adjusted ventilation rate, is inadequate for accurately assessing indoor contaminant exposure in imperfectly mixed spaces.
• Determining the local purging flow rate (QL) near contaminant sources is crucial for accurate worker exposure prediction.
• Age-of-air analysis is not directly relevant for estimating exposure intensity.