With the increasing normative trend of clean indoor air laws prohibiting smoking in public places such as worksites and restaurants, the home is becoming the primary source of secondhand smoke (SHS) exposure. However, little empirical data indicate how SHS is distributed throughout homes and whether smoking in segregated areas offers protection. This project studied real-time data on levels of SHS in 9 homes in which smoking was permitted and in 3 smoke-free homes. Active sampling monitors were used to assess levels of PM2.5, a marker for SHS, over a 3-day period. In smoking homes, one monitor was placed in the primary smoking area and another in a distal location, where smoking generally did not occur. Participants logged smoking and other activities that could affect air quality. In smoking homes, without assuming normality, the mean PM2.5 level for the primary smoking areas was statistically significantly higher than that for distal areas (84 and 63 μg/m3, respectively). Both levels far surpassed the U.S. Environmental Protection Agency's annual standard of 15 μg/m3 for outdoor air quality. By contrast, the smoke-free home mean was 9 μg/m3, similar to outdoor air quality. These results suggest that the air in smoking homes was several times more polluted than that in smoke-free homes, regardless of where the measurements were taken, meaning that efforts to confine smoking to only part of the home offer no protection for people anywhere inside the home. Household members can be protected by implementing a smoke-free home policy.