The purpose of this study was to assess exposure of four trihalomethanes (THMs-chloroform, dichlorobromomethane, dibromochloromethane, and bromoform) in tap water and indoor air. Measurements of tap water and indoor air during and after showering were taken in residential homes. Houses using private well water (non-chlorinated), public groundwater supply (chlorinated), and public surface water sources (chlorinated) were all evaluated in the study. Water and air samples were collected for each event. THM concentration was taken in the tap water during showering, air samples were collected in the bathroom prior to shower, during showering, and a third air sample was collected in the living room area 30 minutes after showering. Results showed highest THMs in public surface supply, followed by the public groundwater supply. No THMs were detected in the private well water. The public groundwater supply was the only water sampled with all four THMs present. In air samples for the public surface water supply, dibromochloromethane and chloroform were detected. During shower air samples showed an increase in THMs, thus indicating volatilization from the water. In the public groundwater supply, chloroform was the most frequently observed THM. The study reports that while THM concentrations within public groundwater supplies tended be low (5 ppb), THMs were consistently present in the air samples in the living room area. For private wells with no chlorination, THMs were not present in the water, nor were they consistently present in air samples. When there was a positive detection in the air, it was attributed to a running dishwasher in the kitchen that used a detergent containing chlorine bleach. This study shows that showerers are exposed to low levels of THMs in the air during a shower and it may be worthwhile to further quantitate effects of water temperature or bathroom ventilation to be able to recommend ways of minimizing exposure. Technical Report Available.