Relations between indoor and outdoor PM2.5 and constituent concentrations

Outdoor PM2.5 influences both the concentration and composition of indoor PM2.5. People spend over 80% of their time indoors. Therefore, to assess possible health effects of PM2.5 it is important to accurately characterize indoor PM2.5 concentrations and composition. Controlling indoor PM2.5 concentration is presently more feasible and economic than decreasing outdoor PM2.5 concentration. This study reviews modeling and measurements that address relationships between indoor and outdoor PM2.5 and the corresponding constituent concentrations. The key factors in the models are indooroutdoor air exchange rate, particle penetration, and deposition. We compiled studies that report I/O ratios of PM2.5 and typical constituents (sulfate (SO42–), nitrate (NO3–), ammonium (NH4+), elemental carbon (EC), and organic carbon (OC), iron (Fe), copper (Cu), and manganese (Mn)). From these studies we conclude that: 1) sulfate might be a reasonable tracer of non-volatile species (EC, Fe, Cu, and Mn) and PM2.5 itself; 2) particulate nitrate and ammonium generally desorb to gaseous HNO3 and NH3 when they enter indoors, unless, as seldom happens, they have strong indoor sources; 3) indoor-originating semi-volatile organic compounds sorb on indoor PM2.5, thereby increasing the PM2.5 OC load. We suggest further studies on indoor-outdoor relationships of PM2.5 and constituents so as to help develop standards for healthy buildings.