Fifty years following the first U.S. Surgeon General's Report on Smoking there have been significant decreases in national prevalence of smoking and exposure to secondhand smoke. However, tobacco use and exposure remains a leading cause of preventable morbidity and mortality. Community strategies, such as voluntary and legislative smoke-free policies, have been effective tools for protecting the health of people exposed to smoke in public and private spaces. To date these strategies have been less successful at protecting children and others in private homes. Biological evidence suggests that more than fifty percent of children are exposed to toxic and hazardous secondhand smoke, primarily at home, with a disproportionate amount of exposure among children in low-income families. The U.S. Department of Health and Human Services has prioritized reduction of children's exposure to secondhand smoke by identifying it as a leading health indicator and creating a specific Healthy People 2020 objective to address this problem. There is no safe level of exposure to secondhand smoke and home-based interventions to protect children from secondhand smoke are needed. The present n-of-1 clinical trial investigated a real-time intervention to reduce secondhand smoke exposure among young children. A total of 14 families with at least one smoker and one child under the age of five living in the home were recruited. The single case design with repeated measures and group design each provided enough power to detect statistically significant negative differences in cumulative daily geometric mean particle counts and cumulative hours of particle counts above 60µg/m3 in experimental homes using visual analyses, generalized additive models, and multi-levels models with fixed effects. Urine cotinine was positivity and strongly correlated with average log mean particle counts and thus a marker of SHSe. Real-time and delayed real-time feedback resulted in overall and individual home decreases in daily geometric mean fine particle counts thus improving household air quality including probable SHSe reduction.