Indoor sources Indoor measurements reflect both the fraction of outdoor UFP that infiltrate indoors and indoor-generated UFP - e.g., from cooking, candle and incense burning, and cleaning. These sources result in large but variable magnitude spikes in indoor concentrations and further, the rate of decay of these spikes depends on several factors, such as source strength and duration, room volume and ventilation rate. It is thus challenging to completely separate the contributions from outdoor and indoor UFP sources based on indoor PNC measurements. Nonetheless, to test effects of indoor spikes on associations with biomarkers, the investigators generated a PNC time-series for each home and calculated the six-hour moving median for indoor PNC measurements. The investigators then calculated the standard deviation for the three-week period corresponding to HEPA or sham filtration; indoor measurements that were two standard deviations above this six-hour moving median were classified as spikes and replaced with the last indoor measurement that was not considered to be a spiked value. Even though the investigators' method did not completely remove the contribution from indoor sources, it attenuated the contributions from spikes that skewed the three-week indoor average values used as exposure concentrations.