Correlations between short-term indoor and outdoor PM concentrations at residences with evaporative coolers

Airborne particulate matter (PM) was monitored at 10 residences in the El Paso, Texas region in the summer of 2001. Concurrent indoor and outdoor 10-min averaged PM2.5 and PM10 concentrations were recorded for 2 days each to establish the indoor–outdoor PM correlation for typical west Texas residences equipped with evaporative coolers. Indoor PM concentrations stabilize in approximately 10 min in a typical house equipped with evaporative coolers. If the ambient PM concentration remains steady, a 10-min average indoor air sample after the first 10-min period would contain 99% outdoor air and a 1-h average indoor air sample would actually be represented by 95% of the outdoor air. A strong diurnal pattern of PM10 indoor and outdoor was observed in 9 out of the 10 tested houses independent of the possible human activities and other indoor sources at each residence. Consistent with prior regional studies, indoor and outdoor PM10 concentrations at these houses frequently peaked with strong association with each other in the evening hours between 6 and 9 pm. In addition, it is observed that both indoor and outdoor PM10 peaked after the wind speed and wind gust peaked. Indoor PM concentration peaks clearly correlated with documentation of human activities, however, these peaks tended to be of shorter duration due to the high ventilation rates of the evaporative coolers. Evaporative coolers were found to act as PM filters that effectively replace indoor air rapidly creating indoor concentrations approximately 40% of outdoor PM10 and 35% of outdoor PM2.5. Both cooler types, rigid media and aspen pad, appeared to produce similar reduction rates for both PM2.5 and PM10.