Evaluation of a steady-state method to estimate indoor PM2.5 concentration of outdoor origin

Exposure to PM2.5 of outdoor origin, which leads to millions of global premature deaths annually, occurs mainly indoors. A steady-state method has been used to estimate averaged indoor PM2.5 concentration of outdoor origin, but its relative error has not been clarified. This study addresses that problem by comparing the results of a steady-state method to those of a dynamic method. For the cases evaluated, the steady-state method can model indoor annual-, seasonal-, monthly-, and weekly-average PM2.5 concentration of outdoor origin with relative error less than 30%. The relative error of 95% of the daily-average concentrations by the steady-state method is less than 31%, while the maximum is 57%. Sensitivity analysis indicates that both pattern (determined by window opening/closing behavior) and strength of air exchange rate significantly impact the error. Therefore, the averaged concentration of a period longer than one week using the steady-state method can be used to estimate the corresponding health risk. The averaged concentration of a short time period (daily or hourly) has a large relative error and should not be used for estimating the corresponding heath risk or controlling the operation of the air cleaning systems.