Is Green Algae Harmful to Humans

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Written By Mark Washburn

Mark has 20 years of experience as a professional pond management consultant.

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Algae are common aquatic plants that float on bodies of water. Many are classified as green algae, or algae that have enough chlorophyll to turn the plant green.

While most algae are harmless, there are certain types of toxic algae that can be dangerous to animals and humans.

One of the most common is blue-green algae or cyanobacteria. This organism can form on fresh bodies of water and harm organisms that it touches, including families.

Knowing the difference between toxic and harmless types of algae can help save your life. Here is everything that you need to know about dangerous algae.

What Are Green Algae?

Green algae are an informal grouping within the algae family that encompasses many different species of algae plants. They are united in their color, which is by definition green.

All of these algae are marine plants with large amounts of chlorophyll in their cellular structures, which gives them that distinctive green color. Common species include filamentous algae and seaweed.

Green algae are not inherently harmful to humans or animals. Although they can damage marine ecosystems if there are too many of them, these plants are usually not dangerous.

However, you should proceed with caution if you see a body of water covered in algae as it is easy to confuse the harmless green type for the more dangerous blue-green blooms.

What Is Blue-Green Algae?

Despite its colloquial name, blue-green algae are not an algae at all. Instead, it is a cyanobacteria, a type of bacteria that lives in water.

These single-cell organisms clump together to mimic the appearance of algae and behave in the same way as algae do, photosynthesizing sunlight to produce nutrients.

You can find this organism in freshwater and saltwater.

Cyanobacteria is not typically harmful. However, in large quantities, or higher densities, this organism can be toxic to marine ecosystems, animals, and even humans.

How Can You Tell What Is a Harmful Algae?

It can be difficult to tell the difference between green and blue-green types of algae because they form similar films on the surface of the water.

Blue-green algae has a bluer color and will discolor the water around it, sometimes turning it blue, green, or purple.

Algae blooms of both kinds tend to form mats or carpets on the surface of the water, although the blue-green kind often look more like paint spills than solid colors.

Smell is another way to distinguish between these types of algae. The blue-green variety usually smells worse, like a swamp, especially when the cells start to decompose.

There are a few tests that can help you tell the difference between blue-green and green types of algae.

One is the stick test. While wearing gloves, insert a stick into the water where there is a surface bloom. If there are strands of plants gripping to the stick, then you are dealing with green algae. If the residue looks more like paint, it is the blue-green variety.

However, there are no at-home tests that can tell you whether you are dealing with toxic algae or a harmless variety.

Certain species of dangerous algae look just like harmless algae, while others are only dangerous in large quantities, and others are completely undetectable in the water because they float below the surface.

When in doubt, avoid the water or take precautions.

What Is an Algae Bloom?

An algae bloom occurs when algae rapidly grow, overtaking the surface of the water. Both green and blue-green varieties of algae follow this behavior.

Algae blooms occur when the water is warm, so usually in the summer and early fall. They are most common when the water is rich in nutrients such as nitrogen and phosphorus, which is why algae blooms increase when there is runoff or pollution in the water.

Although green algae are normally harmless, large blooms are dangerous to animals because they block sunlight from entering the water and deplete oxygen levels, often killing off all the fish in a lake.

Large blooms can harm humans by entering the drinking water, although they usually just add a foul taste or odor that needs to be treated instead of any serious health risks.

Blue-green blooms often release cyanotoxins. This is the compound that can make humans or animals sick if ingested.

How Can Algae Make You Sick?

If you come in contact with toxic algae, it can cause several health problems.

Humans who come in contact with cyanotoxins can experience digestive problems such as vomiting or diarrhea. It can also cause respiratory issues such as coughs and sore throats.

Other symptoms of exposure include rashes, headaches, and irritated eyes.

Most healthy adults will become mildly to moderately ill after exposure to dangerous algae. In very rare cases, algae can cause permanent liver and brain damage.

However, exposure to algae can be dangerous and even fatal for children and pets. Their organs are smaller and less able to withstand the impact of toxins.

Be vigilant about letting your children and animals near the water and monitor them carefully—sometimes symptoms don’t appear until two days after exposure.

If you are concerned about exposure, call a veterinarian or doctor right away.

What to Do If You Encounter Toxic Algae

The most dangerous thing to do with algae-infested waters is to ingest them.

If you are visiting a body of water with an algae bloom, don’t drink the water and avoid swimming as water could accidentally get into your mouth (animals and children should especially avoid swimming as they are more likely to accidentally swallow some water).

Watersports, boating, and even fishing are usually safe unless there is a massive bloom.

Pay attention to local algae advisories as many local governments monitor the water quality in the summer when they know algae blooms are common.

Shower with fresh water immediately after potential exposure.

While most green algae are perfectly safe for humans, you can easily confuse it with blue-green algae, a type of bacteria that looks similar to harmless algae but releases dangerous cyanotoxins in the water.

When in doubt, stay away from an algae bloom.

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