True Colour
Cities measured
87
Detected in
86 / 87
EU limit
20 mg/L Pt
Highest
12 mg/L Pt — Lagos
Overview
True colour measures absorption by dissolved organic matter — humic and fulvic acids from decaying vegetation, dissolved iron, and manganese. Stockholm's source water (Lake Mälaren) contains moderate natural organic matter, which treatment must reduce.
Health Relevance
High natural organic matter (NOM), when chlorinated, forms disinfection by-products (DBPs) — trihalomethanes (THMs) and haloacetic acids (HAAs). These are regulated carcinogens. The EU limit for total THMs is 100 μg/L.
Regulatory Limits
Drinking Water Directive 2020/2184
Acceptable to consumers; no abnormal change. No numerical colour limit.
Controversy & Contested Science
The tension between disinfection and DBP formation is drinking water's central dilemma. Chlorinating NOM-rich water reliably kills pathogens but creates THMs. Epidemiological studies persistently link chlorinated water to a small increase in bladder cancer risk — approximately 1–2 extra cases per 100,000 per year. Regulators accept this as a reasonable trade-off for microbial safety. Critics argue the limit should be lowered; utilities argue that the alternative — using less chlorine — creates greater pathogen risk.