Chlorine is the most widely used disinfectant in public water systems, a practice that has protected global health for over a century. This chemical is added to treated water to destroy harmful pathogens like bacteria and viruses before they reach the consumer’s tap. While chlorination is highly effective at preventing waterborne disease, the residual chemical presence and the byproducts it creates have led to public concerns regarding its long-term safety.
Why Chlorine is Necessary for Safe Drinking Water
The widespread use of chlorine began in the early 1900s as a direct response to devastating outbreaks of waterborne illnesses such as cholera and typhoid fever. Before chlorination, these diseases were common in urban areas, and its introduction dramatically reduced mortality rates from contaminated water supplies. Chlorine’s primary function is to serve as a powerful oxidizing agent that inactivates or kills nearly all harmful microorganisms present in the source water.
The disinfection process includes maintaining a residual level of chlorine as the water travels through the distribution system. This persistent disinfectant, known as free chlorine, prevents the regrowth of pathogens and the formation of bacterial biofilms on pipe walls. Some utilities use chloramine, a compound of chlorine and ammonia, because it is more stable and provides a longer-lasting residual effect, particularly in large water systems. While chloramine is a weaker disinfectant than free chlorine, its stability helps maintain water safety over greater distances.
Health Concerns from Chlorinated Water and Byproducts
Immediate concerns related to residual chlorine are generally aesthetic, causing the familiar chemical taste and odor in tap water. Direct exposure to higher-than-normal levels can also cause mild, temporary irritation to the eyes and skin, which is more noticeable when bathing or swimming. The more significant public health discussion centers on the chemical compounds that form when chlorine reacts with natural organic matter in the water source, such as decaying vegetation. These substances are called Disinfection Byproducts (DBPs).
The two most regulated groups of DBPs are Trihalomethanes (THMs) and haloacetic acids (HAAs), which form when chlorine reacts with organic carbon molecules. Long-term exposure to elevated concentrations of these byproducts has been linked to potential health risks, including an increased incidence of bladder cancer and possible adverse reproductive outcomes. Exposure occurs not only through drinking but also through inhalation of steam and skin absorption while showering or bathing, as THMs are volatile and can escape into the air. Regulatory bodies emphasize that the risk of disease from consuming undisinfected water is significantly greater than the potential long-term risks posed by DBPs at regulated levels.
Regulatory Guidelines for Chlorine Levels
Governmental agencies establish strict regulations to manage the balance between effective disinfection and the formation of potentially harmful byproducts. These standards define the acceptable concentrations of both the disinfectant and the resulting DBPs in public drinking water. The Maximum Residual Disinfectant Level (MRDL) for free chlorine and chloramine is set at 4.0 milligrams per liter (mg/L), which is the highest level considered safe to prevent adverse health effects.
For the byproducts, enforceable standards called Maximum Contaminant Levels (MCLs) are set to limit long-term exposure. The MCL for Total Trihalomethanes (TTHMs) is 0.080 mg/L, and the MCL for the five regulated haloacetic acids (HAA5) is 0.060 mg/L. Water systems must routinely monitor these levels, typically calculated as a running annual average, to ensure compliance with public health standards.
Methods for Reducing Chlorine in Drinking Water
For individuals concerned about the taste, odor, or residual chemical content of their tap water, several methods can effectively reduce chlorine and its byproducts at home. One of the simplest approaches to address free chlorine is to let an open container of water sit for a few hours, allowing the volatile chlorine gas to dissipate through a process called off-gassing. Briefly boiling the water and then allowing it to cool will also accelerate this process, though this is less effective for chloramine.
The most comprehensive and effective method for removing both disinfectants and DBPs is filtration using activated carbon. These filters, whether in pitcher form, faucet-mounted units, or whole-house systems, work by adsorbing the chemical compounds onto the porous surface of the carbon material. Activated carbon is particularly effective at removing free chlorine, significantly improving taste and odor, and also reduces the concentration of Trihalomethanes and haloacetic acids.