The use of chlorine in water sanitation represents one of the most significant public health achievements of the past two centuries. The widespread adoption of chlorination in municipal water supplies dramatically reduced the incidence of deadly waterborne diseases. Chlorine remains the most common disinfectant globally due to its effectiveness against pathogens, low cost, and ability to maintain a disinfectant residual throughout the water distribution system. Since the 1970s, scientific attention has focused on an unintended consequence of this disinfection process.
The core question is whether the chemical used for disinfection, or the compounds it creates, poses a long-term risk to human health, specifically an increased risk of cancer. Research shows the primary concern is not the elemental chlorine itself, but a family of chemical compounds that form as byproducts of the disinfection process. This article explores the current scientific understanding of the link between chlorine-based water disinfection and cancer risk.
The Formation of Disinfection Byproducts
The health concern stems from a chemical reaction that occurs when chlorine is introduced into water containing organic matter. Naturally occurring organic materials, such as decaying vegetation and humic substances found in source waters, act as precursors. When the chlorine disinfectant reacts with these organic compounds, it creates hundreds of different chemical compounds collectively known as Disinfection Byproducts (DBPs).
The two most frequently studied and regulated groups of DBPs are Trihalomethanes (THMs) and Haloacetic Acids (HAAs). THMs include chemicals like chloroform, bromoform, bromodichloromethane, and chlorodibromomethane. HAAs comprise compounds such as monochloroacetic acid, dichloroacetic acid, and trichloroacetic acid.
The concentration of these byproducts is influenced by several factors in the water treatment process. Water sources with high organic content, typically surface water, tend to form more DBPs than groundwater sources. Higher chlorine dosage, longer contact time, and warmer water temperatures generally lead to greater DBP formation. Water treatment plants often employ methods like enhanced coagulation to remove the organic precursors before chlorination to minimize the DBP yield.
Exposure Pathways and Associated Health Data
Exposure to DBPs occurs through multiple routes during public water usage. The most direct route is the ingestion of tap water. However, significant exposure also occurs through non-ingestion pathways, including dermal absorption through the skin and inhalation of volatile compounds.
Drinking Water Consumption
Epidemiological studies focused on long-term exposure to chlorinated drinking water suggest an association with an increased risk for specific cancers. The most consistent findings link DBP exposure, primarily measured by THM concentrations, to an elevated risk of bladder cancer. Studies also indicate an association between DBP exposure and colorectal cancer.
These risks are considered dose-dependent, meaning individuals with higher lifetime exposure to DBP concentrations over many years have the greatest potential for concern. Exposure is amplified in enclosed spaces like showers and baths, where volatile THMs like chloroform can evaporate from the warm water and be inhaled or absorbed through the skin.
Swimming Pools and Recreational Water
Recreational water environments, especially indoor swimming pools, represent another distinct pathway of DBP exposure. In pools, chlorine reacts with organic substances introduced by swimmers, such as sweat and skin cells. This reaction generates a complex mixture of DBPs, including volatile compounds like trichloramine and various trihalomethanes.
Exposure in indoor pools is dominated by the inhalation of these volatile DBPs that concentrate in the air above the water surface. Research shows that swimming in chlorinated indoor pools can lead to short-term increases in biomarkers associated with potential DNA damage. The findings highlight the need for improved pool water quality and ventilation to mitigate inhalation risk.
Official Classifications of Carcinogenic Risk
Major health organizations have formally reviewed the scientific evidence to classify the carcinogenic potential of chlorine and its byproducts. The International Agency for Research on Cancer (IARC) has not classified chlorine itself regarding human carcinogenicity. Instead, the risk assessment focuses on the individual Disinfection Byproducts.
The IARC has classified some common DBPs based on evidence from animal and human studies. Chloroform, the most prevalent trihalomethane, is classified as Group 2B, meaning it is “possibly carcinogenic to humans.” Dichloroacetic acid, a common HAA, is also classified as Group 2B, based primarily on sufficient evidence of carcinogenicity in experimental animals.
In the United States, the Environmental Protection Agency (EPA) addresses potential health risks through the Disinfectants and Disinfection Byproducts Rules (DBPRs). These regulations establish enforceable limits, called Maximum Contaminant Levels (MCLs), for the total concentration of common THMs and HAAs. These limits are intended to protect against long-term health effects, including cancer.
Practical Steps to Minimize Exposure
For the individual consumer, several practical steps can be taken to reduce personal exposure to DBPs in the home environment. The most effective method for treating drinking water is installing a filtration system certified to remove DBPs. Filters containing Granular Activated Carbon (GAC) are recognized for their ability to adsorb and remove both THMs and HAAs from tap water. When taking a shower or bath, improving ventilation is an effective mitigation strategy against inhalation and dermal exposure. Taking shorter showers also limits the time spent exposed to the steam and its concentrated byproducts.
In the context of recreational swimming, showering thoroughly before entering the water significantly reduces the amount of organic matter that reacts with the chlorine. This simple action helps to reduce the formation of new DBPs in the pool water. Choosing indoor pools with high-quality, effective ventilation systems can further reduce the inhalation of volatile DBPs that hover above the pool surface.