The World Health Organization (WHO) has set a preliminary guideline for microcystin concentrations in drinking water of 1µg per liter. This guideline is based on concentration in whole-water as ingested and assumes an adult consumes 2 liters per day. The WHO guideline is for microcystin-LR equivalents. Microcystin-LR is one of the most toxic strains of microcystin. Unfortunately, microcystin-LR is also one of the most common forms found in the Great Lakes.
For recreational contact, the WHO recommends a series of guideline values associated with the probability of health effects at three levels.
Low probability of adverse health effects: 2-4µg per liter of microcystin. This guideline is based on protection against irritative and allergenic effects. Recreational contact with microcystin at or below this concentration is unlikely to pose a health risk to an average person. However, individual sensitivities to allergens vary greatly making it difficult to determine 'safe' concentrations.
Moderate probability of adverse health effects: 20 µg per liter of microcystin-LR. This guideline is based on protection against hepatotoxic effects due to accidental ingestion. It is based on the tolerable daily intake for an adult (60kg) accidentally ingesting 100mL of water while swimming. Risk increases for children (due to lower body weight and greater likelihood of accidental ingestion) and to individuals with compromised liver function.
High probability of adverse health effects: Scums can represent thousand-fold concentrations of microcystin. Accidental ingestion of small volumes could cause serious harm. Immediate action to avoid contact with visible scums is advised.
These WHO recreational guidelines are originally expressed as total cell concentrations (number of cells per volume). WHO extrapolates these cell numbers to the range of microcystin concentrations above, assuming all cells are Microcystis. However, they note that if species composition was different, then the toxin concentration range could be quite different. These recreational guidelines are based on epidemiological case reports of exposures to water characterized on a total cell density basis and WHO notes that the effects could be associated with a variety of components in the bloom, not limited to microcystins.
Australia and other countries have recommended a recreational contact standard of 20 times the drinking water guideline (20µg per liter) for 'high risk' activities such as swimming to 100 times the drinking water guideline (100µg per liter) for 'low risk' activities such as fishing or boating.
Microcystin is not significantly absorbed through the skin; skin contact may cause a rash or allergic reaction but is extremely unlikely to provoke a hepatotoxic reaction. Inhalation risk of aerosol particles (such as spray created by jet skis) has not been adequately studied, but the above recreational guidelines are believed to be sufficient to cover this risk as well.