Blue-green algae

Blue-green algae occurs naturally in bodies of freshwater, but some kinds can be toxic to humans and lethal to animals.

What does blue-green algae look like?

There’s a wide range of blue-green algae (cyanobacteria). In fresh waters, they’re suspended within the water or attached to rocks and other surfaces. You usually can see them when they’re concentrated into clumps. These clumps can look like green flakes, greenish bundles or brownish dots. They can produce musty, earthy or grassy odours. Blooms can also cause foaming on the shoreline - sometimes confused with sewage pollution. It's impossible to tell if the algae is the dangerous kind just by looking at it, so it's best to not enter the water if you suspect there is algae.

How does blue-green algae affect people and animals?

Bloom and scum forming blue-green algae can produce toxins. Toxin producing blooms are called Harmful Algal Blooms (HABs). These toxins can kill wild animals, farm livestock and domestic pets. Farmers and pet owners should keep their animals away from affected waters.

In humans, they can cause rashes after skin contact and illnesses if swallowed. Illnesses including skin rashes, eye irritation, vomiting, diarrhoea, fever and muscle and joint pain have occurred in people who’ve swallowed or swam through algal scum. These haven’t led to long-term effects or death but, in some cases, the illnesses can be severe.

What should I do to avoid blue-green algae?

Not all blue-green algae blooms and scums are toxic, but you can’t tell just by looking at them, so it’s best to assume they are. Although algal scum isn’t always harmful, avoid contact with it and the water close to it.

To report any sightings of algae to the Environment Agency: telephone 0800 80 70 60.

Don’t enter the water, drink or swallow the water and keep children and pets away from the water.

Read Algal blooms: advice for the public and landowners on GOV.UK.

Last updated: Wednesday, 24 April, 2024.