260525.fig.004
Figure 4: Average concentrations of winter (2005, 2006, and 2007) indoor PM2.5 over 24 hours at 20 homes. Home numbers 1–19 were burning coal during sampling period; home number 20 had an alternate heating source. Red dotted line indicates the 24-hour ambient US Environmental Protection Agency standard of 35  𝜇 g/m3 for comparison.