Can You Swim in a Lake With Algae? When It's Safe to Go In

What algae advisory levels mean

Each state uses its own names, but they work the same way: the higher the level, the less contact with the water is advised. In 2019 the US Environmental Protection Agency recommended swimming advisories when microcystins reach 8 µg/L or cylindrospermopsin reaches 15 µg/L, and many states base their levels on similar values.

State	Lower level	Higher levels
California	Caution: stay away from scum	Warning: no swimming. Danger: stay out of the water and off the shore
Kansas	Watch: avoid swimming near blooms	Warning: avoid all water contact. Hazard: closure recommended
Oregon	Precaution: keep pets away from blooms	Health advisory: avoid swimming and water activities
New York	Suspicious bloom	Confirmed bloom: avoid contact where the bloom is
Florida	Bloom seen	Toxins found, judged against the EPA level of 8 µg/L

No advisory doesn't mean a lake has been tested. Most lakes are checked only when someone reports a problem, so use your eyes as well as the official list.

Before you get in: a quick checklist

Check the latest report. Search for your lake on this site, then check the state's live map linked from its page.
Read the signs at the shore. They are updated by lake managers and can be newer than any website.
Look at the water. Scum, foam, mats, paint-like streaks or water that's green, blue-green, brown or red are all reasons to stay out.
Check the wind. Blooms pile up along the shore the wind blows towards. A beach on the other side of the lake may be clear.
Don't swallow lake water, and rinse off with clean water when you get out.

Blooms are most common in warm, calm, sunny weather. Across the lakes on this site, most reports fall between May and September, but blooms have been recorded in every month.

Children and pets need extra care

Young children are at higher risk because they are small, play at the water's edge where scum collects and are more likely to swallow water. Dogs are at the greatest risk of all: they drink the water, eat algae on the shore and lick it off their fur. A small amount can kill a dog within hours. Read our guide to toxic algae and dogs before you take your dog to a lake.

Lakes with an advisory or recent report now

These lakes have a current official advisory or a bloom report in the last 30 days, from the states that publish live data (California and Kansas). For everything else, see the US toxic algae map.

Questions

Is it safe to swim if the algae is only in one part of the lake?

Sometimes, but be careful. Blooms drift with the wind and collect along downwind shores and coves, so conditions can change within hours. Stay well away from any visible scum, and follow the advisory signs for the area you are in.

Can you swim in a lake after an algae bloom has gone?

Toxins can stay in the water for days or weeks after a bloom looks like it has cleared. States usually lift an advisory only after tests show toxin levels have dropped, so wait until the official advisory is lifted.

Is green water always dangerous?

No. Many lakes are naturally green or brown, and plenty of algae is harmless. But you can't tell toxic blue-green algae apart by looking, so treat scummy, paint-like or unusually discoloured water as unsafe.

What should I do after swimming in a lake with algae?

Rinse off with clean tap water as soon as you can, and rinse any dog that went in before it licks its coat. If you feel unwell, contact a doctor or Poison Control on 1-800-222-1222.

Can you boat or fish on a lake with an algae advisory?

It depends on the level. Under lower advisories, such as a Kansas Watch or a California Caution, boating and fishing are usually fine if you avoid touching the water. Under higher advisories, such as a Kansas Warning or a California Danger, states advise avoiding all contact with the water.

Sources

Health information on this page is based on: the US Centers for Disease Control and Prevention, the US EPA recreational criteria for cyanotoxins and the Kansas Department of Health and Environment. Lake data comes from official state records, last checked on 24 October 2026.

This guide is for information only and isn't medical advice. If you feel unwell after contact with lake water, contact a doctor or Poison Control on 1-800-222-1222.

More guides

What does toxic algae look like?The colours, scums and mats that signal a harmful bloom.
Blue-green algae symptoms in peopleSigns of exposure, who is most at risk and when to get help.
Blue-green algae vs green algaeHow to tell harmless algae from a possible toxic bloom.
Toxic algae and dogsSymptoms, what to do in the first minutes and how to keep your dog safe.

Can you swim in a lake with algae?

Is it safe to swim in a lake with algae?