We investigated how UV treatment of indoor pool water affects the concentration of trihalomethanes (THMs) and chloramines (NCl3) in the air above one therapy pool. One-hundred and sixty-two samples of tTHMs (sum of the four most common THMs in swimming facilities) and thirty-six samples of NCl3 were collected simultaneously at poolside and in the extract channel in a room with one therapy pool for 2 days a week over a period of 5 weeks. When the UV lamp was on, the concentration of combined chlorine in the water decreased 58%, the concentration of tTHMs in the air increased 37%, and the concentration of NCl3 in the air decreased 15%. Between 42% and 56% of the gases in the air are recirculated back into the poolroom along with the recycled air. The correlation between NCl3 and THMs in the air was stronger when the UV treatment was on (r2 = 0.963) compared to when the UV treatment was off (r2 = 0.472). Using a linear mixed model, 30% of the variability in THMs was attributed to UV treatment. For NCl3, the number of bathers was the most important predictor variable. UV treatment has a limited effect on airborne NCl3 but increases the air concentration of tTHMs.