Odour
Cities measured
87
Detected in
25 / 87
Highest
2 TON — Karachi
Overview
Odour in water originates from biological activity (algae, bacteria), disinfection by-products, dissolved organic matter, and pipe infrastructure degradation. It is assessed by trained sensory panels using the Threshold Odour Number (TON) method.
Health Relevance
Most odorous compounds in treated water are not directly harmful at detected concentrations. Persistent odour reduces consumer trust in tap water — driving bottled water consumption, with significant environmental and financial cost.
Regulatory Limits
Drinking Water Directive 2020/2184
Acceptable to consumers; no abnormal change.
Controversy & Contested Science
Geosmin and 2-methylisoborneol (MIB) — produced by cyanobacteria and soil bacteria — cause the characteristic 'earthy' taste at concentrations as low as 4 ng/L. Humans detect geosmin at sub-nanogram concentrations because it historically signalled moist soil and freshwater — an evolutionary advantage. The human nose outperforms most laboratory instruments at low-range geosmin detection. Cyanobacterial blooms producing geosmin are increasing with lake warming from climate change, creating new treatment challenges for utilities relying on surface water.