The Middlesex-London Health Unit monitors beaches weekly from June to September. The health unit works closely with owners and operators of local beaches. The Middlesex-London Health Unit monitors the water for bacteria levels, as well as physical or chemical dangers. Some beaches are permanently posted wit advisories alerting people that the beach is not monitored.

Lake Ontario Waterkeeper updates Swim Guide daily with Middlesex-London Health Unit’s test results.

Five water samples are taken from each monitored beach and brought to the Public Health Lab for testing.

As of 2018, Ontario beaches are following the Operational Approaches for Recreational Water Guideline, 2018, and the Recreational Water Protocol, 2018, for geometric mean and single-sample maximum parameters. These are now equivalent to those set out in the Guidelines for Canadian Recreational Water Quality – Third Edition, 2012:

Geometric mean concentration (minimum of five samples): ? 200 E. coli/100 mL.
Single-sample maximum concentration: ? 400 E. coli/100 mL.

(These values replace the previous, more stringent Ontario standard: a geometric mean of ? 100 E. coli/100 mL.)

A beach is marked Green when when the geometric mean of 5 samples collected within a 30-day period is below 200 E. coli / 100 ml of water and the single sample value is below 400 E.coli/100mL. A beach is marked Red when the geometric mean of 5 samples collected within a 30-day period exceeds 200 E. coli / 100 ml of water and/or exceeds the single sample value of 400 E.coli/100mL. A beach is marked Grey when reliable or up-to-date information is not available.


Blue Green Algae
Middlesex-London Health Unit also post advisories and closure if toxic blue-green algae is detected at a beach.

Other causes for closures or advisories:
Chemical, oil, manure or sewage spills
Waste water treatment plant bypasses
Blue-green algae blooms
Heavy algae growth
Fish or other wildlife die-off at the beach
Visible debris, metal or sharp objects found in the water
High levels of bacteria