Chlorine doesn't just sanitize — it reacts. When free chlorine encounters organic matter from swimmers (body oils, sweat, urine, skin cells) or natural organic matter from source water, it forms disinfection byproducts (DBPs). The most studied group are trihalomethanes (THMs). Understanding DBPs is increasingly important for pool operators as health research and regulations evolve.
What are THMs?
Trihalomethanes are a class of chemical compounds formed when chlorine reacts with natural organic matter or bather waste. The four THMs of concern in pool and drinking water:
- Chloroform (CHCl₃) — the most common
- Bromodichloromethane (BDCM)
- Dibromochloromethane (DBCM)
- Bromoform (CHBr₃) — more common in brominated systems
THMs are volatile — they off-gas from the water surface. In indoor pools, this concentrates them in the air above the water. Outdoor pools have much lower THM accumulation due to open-air ventilation.
Other significant DBPs
- Haloacetic acids (HAAs) — non-volatile, remain in the water. Formed from similar organic-chlorine reactions as THMs but pose risk through ingestion or skin absorption rather than inhalation.
- Chloramines (trichloramine, NC₃)— while technically a DBP, chloramines are formed specifically from nitrogen-containing compounds (urine, sweat). Trichloramine is the primary cause of respiratory irritation and the “pool smell” in indoor facilities.
- Nitrosamines — formed when chlorine reacts with certain bather-introduced compounds. Extremely low concentrations; research is ongoing.
Health considerations
Long-term, high-exposure risk is primarily a concern for competitive swimmers and indoor pool staff who have substantial daily exposure over years. For recreational swimmers, the health risk from properly treated pool water is very low compared to the risk of waterborne illness in untreated water.
Current EPA limits for THMs in drinking water are 80 ppb (total). Pool water is not regulated at the federal level for THMs; the focus of pool water regulations is on maintaining adequate disinfectant residual to prevent biological illness.
Reducing DBP formation
- Pre-swim showers — removing body oils, sweat, and personal care products before entering reduces organic precursors by 30–70%
- Proper shocking — breakpoint chlorination destroys combined chlorine and organic precursors; UV systems oxidize compounds without adding more chlorine
- Carbon filtration (drinking water application) — activated carbon removes THMs from water but is not practical for large pool volumes
- Secondary disinfection (UV/ozone) — reduces the chlorine demand needed for primary disinfection, lowering overall DBP formation
- Regular water replacement — dilution lowers precursor and byproduct concentration
DBPs are a product of chemistry working as intended — chlorine reacting with contaminants. The solution isn't less chlorine; it's fewer organic precursors. Pre-swim showers and proper shocking are the most effective interventions available at the pool level.
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