Particulate matter (PM) is associated with adverse human respiratory health effects. Although much research has focused on the fine particle component (PM2.5), recent results from the USC Children's Health Study suggest that the coarse fraction may also affect respiratory health in children. Specific components of the coarse fraction responsible for these effects have not been identified, but ambient endotoxin is a strong candidate, based on toxicologic and epidemiologic studies. This study is collecting ambient endotoxin samples in each of the twelve Children's Health Study communities to assess seasonal and inter-community variability. Analysis of the collected samples will be performed at the University of Iowa. The exposure information will then be used to determine its relationship to respiratory effects in children.