Chlorine is utilized in public water systems as a powerful disinfectant, a practice that has significantly reduced waterborne diseases and protected public health for over a century. This chemical addition is designed to kill harmful bacteria and viruses, making the water safe for daily consumption. Despite its success, a common public concern remains whether this routinely treated water can cause digestive upset, such as diarrhea. The answer involves separating the effects of acute, high-concentration exposure from the theoretical effects of chronic, low-level exposure.
Direct Effects of Chlorine Ingestion
The immediate cause of diarrhea from chlorine exposure is nearly always connected to ingesting the chemical at a concentration far exceeding the regulated levels found in drinking water. Chlorine is a strong oxidizing agent. When consumed in high amounts, this agent acts as a direct irritant to the mucous membranes lining the gastrointestinal tract.
Regulated municipal drinking water contains a residual chlorine level, typically kept at or below 4 milligrams per liter (mg/L), which is deemed safe for lifelong consumption. However, accidental ingestion of water with a much higher concentration—such as from improperly treated swimming pools or household cleaning products—can trigger a corrosive response. This acute irritation can lead to immediate symptoms like nausea, vomiting, abdominal pain, and diarrhea as the body attempts to flush out the irritant. These symptoms are a sign of chemical injury, not a reaction to the safe amounts of disinfectant in tap water.
The Role of Gut Microbiota Disruption
The concern regarding everyday chlorinated drinking water focuses on a potential long-term, low-level biological effect rather than acute chemical irritation. Since chlorine is antimicrobial, the question arises whether it also kills the beneficial bacteria residing in the human gut microbiome. The theory suggests that chronic exposure to low doses of chlorine could alter the delicate balance of intestinal flora, a condition known as dysbiosis, which may result in chronic digestive issues.
Scientific studies investigating this link often focus on disinfection byproducts (DBPs) that form when chlorine reacts with organic matter in the water. Some research, primarily in animal models, suggests that long-term DBP exposure may cause subtle shifts in bacterial groups and lead to oxidative stress in the gut. However, a large-scale human study involving children who received chlorinated water showed no substantial negative impact on the overall diversity of their gut microbiome. The immense public health benefit of chlorination—a significant reduction in diarrheal disease cases—outweighed the theoretical risk of microbial disruption from the low residual levels.
Distinguishing Recreational Water Illnesses
Diarrhea is often incorrectly attributed to chlorine when it is actually caused by pathogens in recreational water, known as Recreational Water Illnesses (RWIs). When individuals swallow water from a swimming pool, hot tub, or water park, the resulting diarrhea is most frequently a sign of infection, not chemical exposure. This occurs because certain germs are remarkably resistant to the chlorine levels typically maintained in pools.
Parasites such as Cryptosporidium (Crypto) and Giardia are protected by an outer shell that allows them to survive for days in properly chlorinated water. If an infected person contaminates the water, these pathogens can infect others who swallow it. Cryptosporidium is considered the leading cause of diarrhea outbreaks linked to swimming, as it can survive for over seven days even in water meeting standard chlorine requirements. To prevent these infections, public health experts advise against swimming when experiencing diarrhea and recommend avoiding swallowing the water.