Is the Water Safe to Swim In? Algae Blooms and Water Quality

Before you swim in a lake, river or ocean beach, check two things: is it safe from drowning hazards, and is the water itself clean enough? Stay out of water that looks discolored or scummy or smells bad, check official advisories, and skip swimming for at least a day after heavy rain.

This page is about the second question. It explains what harmful algal blooms are, where to find advisories, and what to do if someone gets sick.

All 50 states

Drowning risk and water quality are different questions

A lifeguard, a calm surface and a designated swimming area make drowning less likely. None of them tell you whether the water will make you sick. A beach can be fully staffed and still be closed for bacteria.

In New York City, the Health Department gives every public beach one of three classifications based on water quality: open, advisory (warning) or closed (opens in a new tab). Under an advisory, which uses a yellow sign, swimming and wading are not recommended (opens in a new tab). When a beach is closed, with a red sign, swimming and wading are not allowed (opens in a new tab).

For the drowning side of the question, see choosing a safe place to swim outdoors and lake swimming safety.

What harmful algal blooms look like

CDC defines a harmful algal bloom as the rapid growth of algae or cyanobacteria in water (opens in a new tab) that can harm people, animals or the environment. Blooms happen in fresh water, salt water and brackish water (opens in a new tab), and they have been reported in all 50 states (opens in a new tab).

What to look for:

• Foam, scum, mats or paint on the surface of the water (opens in a new tab) (CDC).
• A change in the water's color to green, blue, brown or red (opens in a new tab) (CDC).
• Parallel streaks, usually green, on the surface (opens in a new tab), blue, green or white spilled paint (opens in a new tab), or water that looks bright green or like pea soup (opens in a new tab) (New York State DEC).
• Water that smells bad.

You can't tell by looking whether a bloom is toxic. New York's DEC notes that harmless green algae can look similar, and its advice is short: "Know it, Avoid it, Report it!" (opens in a new tab) CDC's rule is just as simple: stay out of discolored, scummy or smelly water and keep pets away (opens in a new tab).

Beach and lake advisories

CDC advises you to check for swimming advisories before visiting lakes, oceans or other bodies of water (opens in a new tab), and to avoid fishing, swimming, boating or playing water sports in water with signs of a harmful algal bloom (opens in a new tab).

Where to look:

• New York City beaches. The NYC Health Department suggests you sign up for "Beach Notifications" on Notify NYC or visit the NYC Beach Water Quality Map (opens in a new tab). NYC Parks also points beachgoers to the Health Department for water quality information before you head to the beach (opens in a new tab).
• New York State lakes and ponds. DEC's NYHABS system lets you view the map of current HAB locations (opens in a new tab). A water body that isn't listed may still have a bloom nobody has reported.
• Anywhere else. Check your state or county health department or environmental agency, and read the signs at the beach.

If you spot a possible bloom in New York, you can report it to DEC with a Suspicious Algal Bloom Report Form, ideally with photos (opens in a new tab).

Kids, pets and algae

Children love to play at the water's edge, and dogs drink lake water and lick their fur. Both need extra care.

• Keep pets out of bloom water. CDC warns that animals can get very sick or even die within hours of coming into contact with harmful algae or cyanobacteria (opens in a new tab).
• Rinse right away. CDC says if you or your animal go in water that may have a bloom, rinse off with tap water right after (opens in a new tab). For pets, do not let them lick their fur before you rinse them (opens in a new tab).
• Check fish advisories. CDC says to follow local advisories before eating fish or shellfish you catch yourself (opens in a new tab). It also notes that cooking or preserving food does not remove toxins (opens in a new tab).
• Don't fill pools from lakes. CDC advises: do not fill pools with water directly from lakes, rivers or ponds (opens in a new tab).

Stormwater ponds near homes are a special case. Lake County, Indiana's Health Department warns that nutrient-rich runoff can cause cyanobacteria blooms that often look like green paint or scum (opens in a new tab) in these ponds. Keep children and pets away from them.

After heavy rain

Rain changes water quality fast. Washington State's Department of Ecology explains that water from heavy rainfall runs off the land into nearby lakes, rivers and saltwater beaches, and waste from humans, pets and wildlife can easily be washed downstream (opens in a new tab). Heavy rain can also cause sewage systems to overflow (opens in a new tab).

Its advice:

• Avoid water recreation for at least 24 hours after heavy rainfall. (opens in a new tab)
• Avoid areas where you see pipes or streams that drain directly to the beach. (opens in a new tab)

In NYC, the Health Department places a beach under advisory when rainfall exceeds the rainfall thresholds (opens in a new tab) set for that beach. Heavy rain also raises drowning risk in rivers and creeks, which can rise quickly: see river safety.

If someone gets sick

We can't diagnose illness from a web page, and neither should anyone else. If you or your child feel unwell after swimming:

• People. CDC says to contact a healthcare provider or the Poison Control hotline at 1-800-222-1222 (opens in a new tab).
• Pets and livestock. CDC says to contact a veterinarian right away (opens in a new tab).
• In NYC. The Health Department asks that if you suspect you got sick after swimming at a beach, you seek medical attention promptly and report your illness by calling 311 (opens in a new tab).

What you can do today

• Find the official water-quality page for the beaches or lakes you visit most, and bookmark it.
• If you live in NYC, sign up for Beach Notifications on Notify NYC.
• Teach your family the rule: if the water is discolored, scummy or smelly, we stay out.
• Pack a jug of tap water in the car for a quick rinse after lake swims, for people and dogs.
• Save the Poison Control number in your phone: 1-800-222-1222.

For NYC beaches, see NYC beach safety.