Water Safety 101 | Risks of Excessive Residual Chlorine in Water

As a common disinfectant component in tap water, swimming pool water and other types of water, residual chlorine can effectively kill pathogens and ensure water safety at a reasonable concentration, serving as an important barrier for maintaining public health. However, excess of anything is harmful. When the residual chlorine concentration in water exceeds the safety limit, its potential health risks will become apparent. Whether it is direct contact during daily bathing or swimming, or long-term drinking or inhalation of related vapors, excessive residual chlorine may cause various impacts on the human body and the environment, with specific manifestations as follows:

1. Direct Effects on the Human Body

   Skin and Mucous Membrane Irritation: Contact with water containing excessive residual chlorine (e.g., >5 mg/L) during bathing or swimming may cause dry skin, itching, eczema, or red, irritated eyes.

   Impact on Respiratory Sensitive Groups: Long-term inhalation of vapor with high concentrations of residual chlorine (e.g., in swimming pool environments) may induce asthma or bronchitis.

   Taste and Digestive System Impact: Excessive residual chlorine in drinking water (e.g., >4 mg/L) will produce a noticeable chlorine odor, impairing the drinking experience. A very small number of sensitive individuals may experience mild gastrointestinal discomfort.

2. Potential Health Risks

   Formation of Disinfection By-Products (DBPs): Residual chlorine reacts with organic substances in water (such as humic acid and fulvic acid) to form carcinogens including trihalomethanes (THMs) and haloacetic acids (HAAs). The International Agency for Research on Cancer (IARC) classifies chloroform (one of the THMs) as a Group 2B carcinogen.

   Risks of Long-Term Exposure: Long-term consumption of water with excessive residual chlorine may increase the risk of bladder cancer and colorectal cancer (with a latency period of several decades and subject to controversy).

3. Harm to the Environment

   Toxicity to Aquatic Organisms: Excessive residual chlorine in industrial wastewater (e.g., >1 mg/L) can damage the gill tissues of fish, leading to suffocation and death. It can also affect microbial communities and impair the self-purification capacity of water bodies.

   Corrosion of Pipes and Equipment: High concentrations of residual chlorine (e.g., >2 mg/L) may accelerate the corrosion of metal pipes, releasing ions such as iron and manganese and causing secondary pollution.

4. Sensory and Practical Impacts

   Odor Issues: When water with excessive residual chlorine is used for making tea or coffee, the chlorine odor can mask the original flavor of the beverages.

   Restrictions on Industrial Water Use: Industries such as food processing and electronic manufacturing have strict limits on residual chlorine (usually requiring<0.1 mg/L), and exceeding the limit may lead to product quality problems.        

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