While municipal water systems deliver water treated to be safe for drinking, the act of taking a hot shower introduces unique methods of exposure that change how we assess water quality. The combination of heat, aerosolization, and home plumbing means the risks associated with showering water differ significantly from those of drinking water. Because showering involves both skin contact and the inhalation of steam, contaminants that pose little threat when ingested can become a greater concern when breathed in.
Defining “Clean” Water Standards
Public water systems are held to legally enforceable standards designed to protect public health from contaminants. The Environmental Protection Agency (EPA) sets National Primary Drinking Water Regulations, which limit the levels of various contaminants, including microorganisms, disinfectants, and inorganic chemicals. Disinfection is a primary method used to ensure safety, typically involving chlorine or chloramines. The EPA also enforces rules specifically targeting disinfection byproducts (DBPs), such as trihalomethanes (THMs), which form when disinfectants react with organic matter. These regulations establish a baseline quality for the water leaving the treatment plant, but this quality can change once the water is heated and aerosolized in a shower.
How Heating Water Changes the Risk Profile
Showering dramatically alters the chemical environment of the water due to the effect of heat on volatile chemicals. When chlorinated water is heated, the temperature increase causes volatile organic compounds (VOCs) and disinfection byproducts, particularly THMs, to rapidly off-gas into the surrounding air. This process, known as volatilization, turns liquid contaminants into airborne gases and fine aerosols that are easily inhaled. For these compounds, the inhalation exposure during a shower can be greater than the exposure from drinking the same water. The small water droplets generated by the showerhead are easily inhaled deep into the respiratory tract, making aerosolized exposure the main reason showering contributes significantly to the overall exposure risk for volatile DBPs.
Contamination from Household Plumbing and Fixtures
Water meeting all public standards can become contaminated after it enters the home, a phenomenon often called the “last mile” problem. Internal plumbing, including pipes, water heaters, and shower fixtures, can degrade water quality through both biological growth and physical leaching. Biological contaminants, such as the bacterium Legionella pneumophila, thrive in the warm, stagnant conditions often found in hot water tanks and shower infrastructure. When a shower is run, these bacteria can be released into the air within aerosolized water droplets, which, if inhaled, can cause Legionnaires’ disease. To minimize this risk, hot water systems are required to store water at temperatures high enough to kill the bacteria, typically at or above 140°F (60°C).
Furthermore, older household plumbing may introduce heavy metals into the water supply. Pipes installed before the mid-1980s, or even newer plumbing with brass fixtures, can leach metals like lead and copper into the water, particularly when water sits unused for several hours.
Improving Water Quality Through Filtration and Maintenance
Consumers can take practical steps to mitigate the risks associated with shower water quality using point-of-use filtration systems. These filters attach directly to the shower arm and primarily address chemical issues caused by municipal disinfection. While carbon filtration media reduces chlorine and volatile organic compounds, its efficiency decreases significantly in high-temperature water. Kinetic Degradation Fluxion (KDF) media is effective in hot water because it uses an oxidation-reduction process to neutralize chlorine. Combining carbon and KDF media offers a balanced approach, targeting both chlorine and other chemical contaminants.
Beyond filtration, regular maintenance is necessary to prevent biological contamination within fixtures. Showerheads should be cleaned periodically to remove limescale and biofilm, which can harbor bacteria like Legionella. Additionally, any shower or faucet that is rarely used should be flushed with hot water for at least 15 seconds weekly to prevent water stagnation. Replacing the filter cartridge regularly is also essential.