Sources of Exposure & Risk Factors

People can swallow contaminated water while they are swimming or playing in the water. Active water sports (like water-skiing) pose a higher risk of accidental ingestion ¹. Swimmers may swallow up to 16–200 mL of freshwater (the equivalent of 0.5 – 6.8 ounces of water) during one swim ².

Animal Exposures to Natural/Ambient Waters

Animals can become sick when they drink contaminated water, groom themselves after swimming in contaminated water, or eat toxic algae or contaminated fish and shellfish in the water. In fact, animals are at a higher risk of ingesting contaminated water because they do not avoid water that is discolored or smells bad ^3-6.

Food

Freshwater

People and animals can be exposed to HAB toxins by eating fish or shellfish from a freshwater body with a HAB. Freshwater fish can become contaminated with cyanotoxins when they eat toxin-producing cyanobacteria ⁷.

Marine water

People and animals can be exposed to HAB toxins by eating seafood from contaminated marine (salt) water bodies. Shellfish can become contaminated when they filter and concentrate water with a HAB, and reef fish can become contaminated through the food chain (eating other animals that have accumulated the toxin) ^8,9.

Drinking Water

People and pets might be exposed to HAB toxins through a contaminated tap water supply, though this is still an uncommon occurrence. Exposure to HAB toxins can occur in a healthcare setting, although this is also rare. This type of exposure can occur if patients receive a medical treatment, such as dialysis, that uses water that has not had HAB toxins removed. In 1974, 23 dialysis patients in Washington, DC, became ill when the local water source became contaminated with cyanotoxins, which are toxins from a type of phytoplankton called cyanobacteria ¹⁰. In 1996, 52 dialysis patients died and another 64 became ill in Brazil when the water used for dialysis treatment was contaminated with cyanotoxins ¹⁰.

The occurrence of cyanotoxins in drinking water can depend on the level of cyanotoxins in untreated or raw source water and the effectiveness of water treatment methods to remove them. Some public drinking water systems use surface water from lakes that could contain a HAB. A survey of 24 source water and treatment facilities between 1996-1998 in the United States and Canada found 80% of their 677 water samples contained detectable levels of microcystin, a type of cyanotoxin ^11,12. Of the 80% of water samples with detectable microcystin levels, 4% of the samples exceeded the World Health Organization (WHO) drinking water guideline limit of 1µg L ^-1 for microcystin in drinking water ¹³. However, of the 4% of samples that exceeded the WHO drinking water guidelines, only 2 water samples were finished or treated water samples (water ready for consumption). This means that the majority of sample water contained microcystin, but the facilities could adequately remove microcystin to levels below the WHO drinking water guideline.

Water treatment facilities have options to remove cyanobacteria and their toxins from water during treatment. Coagulation, sedimentation, and filtration have been shown to remove between 90 and 99.9% of algae ¹⁴. However, when algae begin to die, toxins contained in the algae can be released from the algal cells ^15,16. These toxins may not necessarily be removed through coagulation, sedimentation, or filtration of the water ¹⁹. However, the toxins released by algae can be treated through other processes, such as activated carbon filtration and oxidation ¹⁶. These methods are not always a part of water utility treatment processes.

The Environmental Protection Agency (EPA) suggests the risk of a HAB contaminating drinking water is increasing due to nutrient pollution of source waters ¹⁵ such as agricultural runoff of fertilizer containing phosphorus and nitrogen which promotes algae growth. You can find out more about your local drinking water on EPA’s website.

There are currently no federal regulations on the acceptable levels of cyanobacteria or cyanotoxins allowed in public drinking water ¹⁹. However, as of June 29, 2015, the US EPA issued new health advisories for algal toxins in drinking water. The EPA health advisory recommends limits for two types of cyanotoxins, microcystin and cylindrospermopsin. The advisory states that algal toxin levels in drinking water do not exceed 0.3 micrograms per liter for microcsystin and 0.7 micrograms per liter for cylindrospermopsin for children younger than school age (<5 years of age). For all other ages, the EPA recommends that algal toxins levels do not exceed 1.6 micrograms per liter for microcystin and 3.0 micrograms per liter for cylindrospermopsin. If algal toxins exceed these levels, the public should be notified that no one should drink or boil the water.

Nutritional supplements

Nutritional supplements that contain algae can also pose a risk for exposure to HAB toxins. When algae are harvested to produce supplements, a toxin-producing cyanobacteria (such as Microcystis) might accidentally be collected as well ^16,17.

In 1996 in Oregon, a HAB produced large amounts of the HAB toxin microcystin in Upper Klamath Lake ¹⁶. Oregon officials issued a precautionary advisory for supplements that used algae collected from the lake. The state established an allowable limit of 1 microgram of microcystin per gram of algal supplement ¹⁶. As a result of the advisory issued by state officials and consumer concern, 87 samples of algae supplements sourced from Upper Klamath Lake were tested, and 72% had levels of microcystin that exceeded this limit ¹⁶. New technologies have since been used for collecting algae to reduce the risk of taking toxic cyanobacteria from this lake ¹⁸.

References

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3. Work TM, Barr B, Allison MB, Fritz L, Quilliam MA, Wright JLC. Epidemiology of domoic acid poisoning in brown pelicans (Pelecanus occidentalis) and Brandt's cormorants (Phalacrocorax penicillatus) in California. J Zoo Wildl Med. 1993;24(1):54-62.
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15. EPA. Creating a cyanotoxin target list for the unregulated contaminant monitoring rule. 2011.
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18. Heussner AH, Mazija L, Fastner J, Dietrich DR. Toxin content and cytotoxicity of algal dietary supplements. Toxicol Appl Pharmacol. 2012;265(2):263-71.
19. Lopez CB, Jewett, EB, Dortch Q, Walton BT, Hudnell HK. Scientific assessment of freshwater harmful algal blooms. [PDF - 78 pages] 2008.