If you swim in the sea or in an inland bathing water in the United Kingdom, there is a public data set about the water you are getting into, it is free, and most swimmers have never looked at it. This article is about what it is, what it is not, and where to find it.
What a designated bathing water is
Not every stretch of water is one. A bathing water is a site that has been formally designated, usually on the basis that a large number of people bathe there. Designation brings it into a monitoring regime: samples are taken during a defined bathing season, tested for specified microbiological indicators, and the results are used to classify the site.
Sites that are not designated are not monitored under this regime at all. That is important and frequently missed. A beautiful stretch of river with no designation is not a site that failed a test. It is a site nobody tested.
What the classification actually describes
The classification is derived from sample results across the season and, in the way the regime works, across more than one season. That has two consequences a swimmer should hold in mind:
- It is a summary over time, not a reading for today. A site with a good classification can be poor on a particular day, most commonly after heavy rain.
- It is backward looking. The classification you read describes what has been measured up to now, not what the water is like this morning.
The classification categories used by the regulators run from the best to a category indicating the site did not meet the minimum standard, and where a site is in the lowest category there is normally advice against bathing displayed at the site. We are not going to reproduce the category names and thresholds here, because they are set in regulation and published by the regulators themselves, and the correct source is the regulator rather than a swimming website. Use the Environment Agency bathing water data explorer for England, Scottish Environment Protection Agency, bathing waters for Scotland and Natural Resources Wales for Wales, with policy from Department for Environment, Food and Rural Affairs.
We publish no water quality figures anywhere on this site, including bathing water sample results. The reason is the same as for pool chemistry: a figure without its site, its date, its conditions and its regime is not information, it is decoration. The regulators publish the figures with the context attached. Go there.
Short term pollution and why rain matters
Separately from the annual classification, sites can be subject to short term pollution: a temporary deterioration in water quality with a known cause, typically associated with rainfall. Rain moves material off land and through drainage systems into watercourses and the sea, and at some sites it can also trigger discharges from combined sewer systems.
Where a site is prone to this, the regulator may issue pollution risk forecasts and the site may display advice against bathing on affected days. This is the part of the system that is actually about today, and it is the part worth checking on the morning of a swim rather than the week before.
The practical heuristic that experienced open water swimmers use, and which is consistent with the existence of the forecasting system, is to be cautious about swimming shortly after heavy rain. We are describing why that heuristic exists rather than issuing it as an instruction.
| Annual classification | Short term pollution notice | |
|---|---|---|
| What it describes | Water quality over the season and previous seasons | A specific, current, temporary deterioration |
| Time frame | Backward looking summary | Today, or forecast for today |
| Where it comes from | Sampling programme results | Forecasting, monitoring and incident reporting |
| Where to look | The regulator's bathing water site | The regulator's site and signage at the location |
| What to do with it | Choose where to swim regularly | Choose whether to swim today |
What the regime does not tell you
This matters as much as what it does tell you.
- It says nothing about physical safety. Currents, tides, depth, underwater hazards, boat traffic and cold are not part of it. A site with the best possible classification can be a dangerous place to swim. Physical water safety is the Royal National Lifeboat Institution, water safety's territory.
- It does not cover undesignated water. Most rivers, most lakes and most of the coast are not designated bathing waters and are not in this regime.
- It is not continuous. Sampling happens at intervals during a defined season, not constantly.
- It is not chemistry. The indicators are microbiological. It is not a general test for everything that might be in water.
We are not going to tell you what any symptom means or connect it to any water. If you become unwell after swimming in open water, speak to a pharmacist, a GP or NHS 111, and tell them you have been in open water because it is relevant context for them. Reporting a suspected pollution incident is a separate matter and goes to the environmental regulator.
Is bathing water classification relevant to me?
Not at all. A treated pool is a different regime entirely: continuous treatment, daily testing and records held by the operator. See what gets tested and recorded.
The two systems are not comparable and a pool swimmer should not read across from one to the other.
At an outdoor pool, no, because it is still a treated pool. In open water, yes, and it is the main public information available about the water you are entering.
Check the classification when choosing where to swim regularly, and check for short term pollution on the day.
Whether the water quality at this site is acceptable today
Owns the check. It takes a minute on a phone and it is genuinely the swimmer's job, because nobody else is going to do it for a casual swim. Owns reading the signage at the site, which is where advice against bathing appears.
Owns it for the group and owns it properly. An organised session at a designated site should have a documented water quality check as part of its routine, and at an undesignated site the leader owns the fact that no such data exists. Owns cancelling on the basis of a notice.
Where a site has an operator or a local authority, they own signage, the display of advice against bathing, and site information. Where there is none, nobody does.
Where a duty attachesClassification and monitoring sit with the Environment Agency, Scottish Environment Protection Agency, bathing waters and Natural Resources Wales, under policy from Department for Environment, Food and Rural Affairs. Physical water safety guidance from the Royal National Lifeboat Institution, water safety.
How to check, practically
- Search for the site name on the relevant regulator's bathing water pages. In England that is the bathing water data explorer.
- Look at the current classification and, more usefully, at the history. A site that has moved between categories is telling you something a single year does not.
- Check whether the site is subject to short term pollution and whether a forecast or notice is in place for today.
- Read the signage when you arrive. Advice against bathing is displayed at the site.
- If the site is not designated at all, note that you have no data, and treat that as the fact it is.
None of this replaces the physical safety assessment, which is a separate and more urgent question. Both are worth doing and they are not substitutes for one another.