Trihalomethanes (THMs) are chemical compounds found as contaminants in public drinking water supplies. These substances are not intentionally added but are formed as an unintended result of the necessary process of water disinfection. As a common class of disinfection byproducts, THMs are regularly monitored and regulated by public health authorities due to their potential effects on human health.
Defining Trihalomethanes and Their Formation
Trihalomethanes are a family of organic chemical compounds where three of the four hydrogen atoms in a methane (\(\text{CH}_4\)) molecule have been replaced by halogen atoms, such as chlorine or bromine. The four regulated compounds that make up the category of Total Trihalomethanes (TTHMs) are Chloroform, Bromodichloromethane, Dibromochloromethane, and Bromoform. These compounds are highly volatile, meaning they can easily change from a liquid state in water to a gaseous state in the air.
The formation of THMs is a direct consequence of using common disinfectants, typically chlorine, to kill disease-causing pathogens in source water. This public health step prevents outbreaks of waterborne diseases. However, when chlorine reacts with naturally occurring organic matter (NOM)—such as decaying vegetation or algae—it creates Disinfection Byproducts (DBPs), of which THMs are a major component.
The concentration of THMs in a water system is not static; it is influenced by several factors, including the amount of organic matter in the source water, the dosage of disinfectant used, and the temperature and \(\text{pH}\) of the water. For instance, warmer water and higher \(\text{pH}\) levels accelerate the chemical reaction between chlorine and organic matter, leading to increased THM formation. This reaction often continues as the water travels through the distribution system to consumers’ homes.
Common Routes of Human Exposure
Exposure to THMs is not limited solely to drinking contaminated tap water. Because of their volatile nature, THMs readily escape from water and into the air, making inhalation and dermal absorption equally important pathways. This multi-route exposure occurs primarily during common household activities that involve heated water.
Inhalation exposure occurs when THMs volatilize from the warm water used during showering, bathing, or washing dishes. Warm water temperature enhances this volatilization process, creating a concentrated cloud of THM vapor in enclosed spaces like a shower stall. This pathway can be a major contributor to an individual’s total exposure, sometimes accounting for over half of the total daily uptake.
Dermal absorption is the third significant route, involving the absorption of THMs through the skin while bathing, swimming in chlorinated pools, or showering. The skin’s absorption rate for THMs is considerable; dermal uptake during a bath can be equivalent to a large percentage of the daily exposure from ingestion.
Health Risks Associated with THMs
The potential health consequences from THM exposure include acute and chronic effects. Acute exposure to very high concentrations, typically far exceeding regulatory limits, can lead to damage to the liver, kidneys, and central nervous system. Symptoms of high-level exposure can include dizziness, fatigue, headache, and incoordination.
The primary concern for public health revolves around chronic, low-level exposure over many years. Epidemiological studies consistently link long-term exposure to an increased risk of certain cancers, most notably bladder and colorectal cancer. This suggests that risks may exist even at concentrations below the current regulatory standard.
Beyond cancer, chronic THM exposure is also associated with non-cancer risks, particularly concerning reproductive and developmental health. Research has found associations between exposure during pregnancy and adverse outcomes, including miscarriage, small for gestational age, and slight reductions in infant birth weight.
Regulation and Reduction Strategies
Water systems are required to manage THMs under the U.S. Environmental Protection Agency’s (EPA) Disinfectants and Disinfection Byproducts Rule (DBPR). This regulation establishes a maximum contaminant level (MCL) for Total Trihalomethanes (TTHMs) at \(0.080 \text{ milligrams per liter}\) (\(80 \text{ \mu g/L}\)). Compliance is determined by calculating a locational running annual average (LRAA), which requires water systems to monitor THM levels at various points throughout the distribution network over four consecutive quarters.
Consumers can take specific steps to reduce their personal exposure, focusing on both the ingestion and inhalation routes. For drinking water, filtration is the most effective strategy, particularly using systems that incorporate activated carbon filters or reverse osmosis (RO). Activated carbon works by adsorbing the volatile organic compounds, while RO systems use a semipermeable membrane that can remove over \(95\%\) of THMs from the water.
To minimize exposure through inhalation and dermal absorption, consumers can implement strategies that address THM volatilization during hot water use. Because the compounds accumulate in the air of an enclosed space, increasing ventilation is highly effective. Simply running a high-capacity exhaust fan or opening a window while showering can significantly reduce the concentration of THM vapors in the air. Furthermore, shortening the duration of hot showers and baths reduces the overall time available for both inhalation and dermal uptake.