What does a harmful algal bloom look like?

Look for water that resembles pea soup, spilled green or blue-green paint, or has a surface scum, mats, or foam. Blooms can also appear brown, red, or even pinkish. Discolored water, paint-like streaks, dead fish, or a musty/foul smell are all reasons to keep your family out.

What happens if my child swims in blue-green algae?

Symptoms depend on contact type: skin rash and eye irritation from touching, vomiting, diarrhea, headache, fever, or muscle weakness from swallowing, and coughing or wheezing from inhaling spray. Rinse off immediately with clean water and call your pediatrician or Poison Control (1-800-222-1222) if symptoms appear.

Why are children more at risk from algal blooms?

Children swallow more water relative to body weight, play in shallow areas where scum concentrates, stay in longer, and put wet hands and toys in their mouths. The CDC notes children and dogs are the most frequent victims of bloom-related illness.

Can a lake with algae be safe in some spots?

Blooms move with wind and currents, so a clear-looking beach on a lake with an active bloom advisory is not reliably safe — toxins can persist in clear water even after visible scum disperses. Follow posted advisories for the whole water body, not just what you can see.

How do I check if my lake has a bloom advisory?

Check your state health or environmental department's beach and HAB advisory page before lake days — most states maintain interactive maps in summer. At the beach, look for posted warning signs, and when in doubt, choose a different swim spot.

Harmful Algal Blooms: When Lake Water Isn't Safe to Swim

What is a harmful algal bloom?

A harmful algal bloom — HAB for short — happens when algae or cyanobacteria (often called "blue-green algae," though they're actually bacteria) multiply explosively in warm, nutrient-rich water. Most algae are harmless, but certain species produce potent toxins: microcystins that damage the liver, anatoxins that attack the nervous system, and cylindrospermopsin that affects multiple organs.

Blooms thrive on the exact conditions of an American summer: hot weather, strong sun, still water, and nutrient runoff from lawns and farms. That's why advisories spike from June through September — peak family swimming season — and why freshwater lakes and ponds, the slow-moving and warm ones especially, are the most common sites. Marine blooms (like "red tide" on Gulf and coastal beaches) follow the same logic with different organisms.

Climate trends are making blooms more frequent, longer-lasting, and more widespread — lakes that never had advisories a decade ago post them now. The EPA and CDC both track this growth, which is why bloom-awareness now belongs in every family's lake and ocean safety toolkit.

How do you recognize a bloom?

You don't need a test kit — you need to know what "wrong" looks like. Keep your family out of the water if you see:

Pea-soup green water — the classic cyanobacteria look, sometimes blue-green, sometimes brownish or reddish.
Paint-like streaks or swirls on the surface, as if someone spilled green or turquoise paint along the shoreline.
Scum, foam, or floating mats — especially collected in coves, boat ramps, and the downwind shore where kids wade.
A musty, earthy, or foul smell coming off the water.
Dead fish or sick wildlife along the shore — treat this as a hard stop.
Posted advisory signs — they're sometimes small and faded; look for them at boat launches and beach entrances.

Two traps to avoid. First, blooms move: wind pushes surface scum around the lake, so the beach that looked clean at 10 a.m. can be scummy by 2 p.m. — and a clean-looking spot on a lake under advisory isn't reliably safe, because toxins can linger in clear water after the visible bloom breaks up. Second, not all blooms float: some toxin-producing mats grow on the bottom in shallow water, exactly where toddlers play.

Key statistic: In CDC summaries of harmful algal bloom–associated illness reports, children account for a disproportionate share of human cases — and dogs are the most commonly reported animal victims, often fatally. Kids swallow more water, play where scum concentrates, and stay in longer; dogs drink the water and lick contaminated fur.

What does exposure do to a child?

Symptoms depend on how the toxins got in:

Skin contact: rash, hives, itching, or blistering — sometimes within hours, especially under swimsuits where wet fabric holds water against skin.
Swallowing: stomach pain, vomiting, diarrhea, headache, fever; in larger exposures, liver injury (microcystins) or neurological signs like dizziness, tingling, and muscle weakness (anatoxins).
Inhaling spray or mist — from tubing, skiing, or splashing: coughing, wheezing, sore throat, irritated eyes.

If your child swam in suspect water: get them out, rinse thoroughly with clean water (including under the suit), wash the suit, and watch for symptoms over the next 24–48 hours. If symptoms appear, call your pediatrician or Poison Control at 1-800-222-1222 and mention possible algal exposure — it's not on most doctors' radar unless you say it. For pets, rinse immediately and call a vet urgently if you see drooling, weakness, or stumbling; cyanotoxin poisoning in dogs progresses fast.

As always, this is general safety information, not medical advice — when in doubt about any symptom, consult your pediatrician.

How to check before you load the car

Most states publish HAB advisory maps through their health or environmental departments, updated weekly or better in summer. Search "[your state] harmful algal bloom advisory" — many states offer interactive maps. The EPA's CyAN project even tracks large blooms by satellite. Make the check part of your lake-day routine, alongside weather and the packing list from our open water safety checklist.

At the water, do your own 30-second scan even if no advisory is posted — monitoring lags reality, and small ponds often aren't monitored at all. Color, scum, smell, dead fish, signs. When the water fails the eye test, swim somewhere else; "the kids were so excited" is how most bad exposures happen.

After the bloom: when is it safe again?

Toxins can outlast the visible bloom by days to weeks, which is why advisories often stay posted after the water clears. Respect the posted status rather than the appearance. Once an advisory lifts, normal swimming is fine — blooms don't permanently contaminate a lake.

And remember the boring basics still apply on every lake day: life jackets for weak swimmers, designated watchers, and awareness of the open-water hazards we cover in natural swimming hole safety — cold water, drop-offs, and limited visibility don't take summers off either.

Five habits for bloom-season lake families

If lakes are part of your summer, fold these into the routine and bloom risk drops to nearly nothing:

Check the map the night before. Pull up your state's HAB advisory page while you pack the cooler. Thirty seconds, done.
Do the shoreline scan on arrival. Color, scum, smell, dead fish, posted signs — teach older kids to run the scan themselves; they love being the family water inspector.
Keep mouths closed and toys tracked. Coach younger kids not to swallow lake water in general (it carries more than algae), and rinse water toys before they go back in the car — toxins and germs ride home on wet plastic.
Rinse everyone — including the dog — after every natural-water swim. A jug of clean water in the trunk handles beaches without showers. For dogs, rinse before they groom themselves; licking contaminated fur is how most canine poisonings happen.
Report what you see. If you spot a suspicious bloom at an unposted beach, call the lake manager or your county health department. Your report may be the reason another family's toddler doesn't wade into it.

These habits stack neatly on top of the supervision and life-jacket rules that govern every lake day — the bloom check is simply the first item on the same mental list.

The bottom line

Harmful algal blooms are the rare water hazard you can usually see coming — green, scummy, smelly water announcing itself. Teach your kids the pea-soup test, check the advisory map before lake days, rinse everyone after natural-water swims, and apply the CDC's five words without negotiation: when in doubt, stay out. There's always another swim spot; there's no good version of a sick kid or a lost dog.

Harmful Algal Blooms: When Lake Water Isn't Safe for Your Family to Swim