The Danger of the Swimming Pool: Nitrosamine Carcinogens

To prevent epidemics, chemical disinfectants are used in public pools to inhibit bacterial growth. Many current disinfection procedures produce harmful byproducts, however, including N-nitrosodialkylamines (nitrosamines). Professor William Mitch of the Department of Chemical Engineering collected water samples from indoor and outdoor pools, a pool with a retractable dome, and hot and cold tubs in order to analyze the nitrosamine contents of each. The most common nitrosamine detected was N-nitrosodimethylamine (NDMA).

Mitch accurately predicted that higher amine precursors from urine, sweat, feces, and food would contribute to the production of NDMA at greater concentrations in pools and hot tubs than in drinking water. The average level of NDMA in drinking water, 0.7 ng/L, is associated with a one in a million lifetime cancer risk. High levels of NDMA in drinking water, pools, and hot tubs could consequently pose a threat to human health.

Collecting water samples from different pools that received varying amounts of sun exposure, Mitch observed a correlation between NDMA levels and UV exposure. At a temperature of approximately 24oC, the median concentration in indoor pools (32 ng/L) was six times greater than that in outdoor pools (5.3 ng/L). The pool with a retractable roof had a concentration smaller than that of indoor pools and greater than that of outdoor pools. The data indicate that exposure to UV radiation from the sun suppressed the formation of NDMA.

Mitch furthered his studies by comparing water samples from hot and cold tubs to determine whether temperature played a role in the production of NDMA. In the hot tubs (~41 oC), the median NDMA content (313 ng/L) was roughly ten times greater than that in indoor pools. Analysis of a water sample from a “cold” (~23 oC) outdoor hot tub showed NDMA levels comparable to those of the indoor pools. These results imply that higher temperatures may lead to increased production of NDMA.

Using this data, Mitch proposed two pathways for the formation of NDMA from dimethylamine, a compound found in urine and sweat. He noted that dimethylamine is removed from drinking water during treatment, but not regulated in pools, a factor that could contribute to the higher levels of NDMA in pools as compared to drinking water.

It is possible that these processes explain the high concentrations of NDMA in pools and hot tubs. The concentrations of NDMA detected in hot tubs in particular were as much as 500 times greater than those in drinking water. It is, however, important to note that while there are bladder cancer risks associated with drinking NDMA, very little research has been done on the effects of skin absorption of this carcinogen. Theoretical models show that NDMA could be absorbed through the skin on the magnitude of the rate of absorption of hydrocortisone, a common anti-inflammatory drug in ointments and sprays used to treat such conditions as arthritis and irritated skin. Knowing that nitrosamines can cause bladder cancer, the next step of this project would be to conduct epidemiological studies to see if swimming with nitrosamines in high concentrations is positively correlated with increased risk of bladder cancer.

Extra Readings:

Walse, S.S.; Mitch, W.A. Nitrosamine carcinogens also swim in chlorinated pools. Environ. Sci. Technol., 2008, 42 (4), 1032-1037.

Kemper, J.M.; Walse, S.S.; Mitch, W.A. Quaternary amines as nitrosamine precursors: a role for consumer products. Environ. Sci. Technol., 2010, 44 (4), 1224-1231.