How do we know if a beach or river is clean enough to swim in?

We know a beach or river is clean enough to swim in by counting a bacterium that lives in the guts of warm-blooded animals: Escherichia coli, usually called E. coli. Officials do not search the water for sewage itself, which would be hopeless. Instead they look for signs of faecal pollution, and E. coli is a good one because it is expelled into the environment in faecal matter. In England, the Environment Agency tests bathing waters for E. coli and intestinal enterococci, sources of pollution known to be a risk to bathers' health.

Most E. coli strains are harmless, part of the normal gut microbiota of warm-blooded animals. The point is not that the bacterium itself is the danger. Some types are pathogenic and cause serious food poisoning, so a high count is treated as a warning sign.

Each bathing water gets up to 20 samples during a season that runs from May to September, and its classification reflects the latest four years of data. Sites are labelled Excellent, Good, Sufficient or Poor. In the 2025 classifications, published in November 2025, 92.9% of England's bathing waters met at least the Sufficient standard, up from 91.8% in 2024, and 32 were classed as Poor.

The system is a long-term report card, not a live alarm: the Environment Agency says the sampling results are not designed to give instant information about whether it is safe to swim on a given day.