Cyanobacteria (Blue-Green Algae) in Inland and Inshore Waters: Assessment and Minimisation of Risks to Public Health

Guidance to Directors of Public

Health, to Heads of Environmental Health in Local Authorities (LAs), and to others in

Scotland, on possible risks to public health of cyanobacteria (blue-green algae) in inland

and inshore waters. It updates previous guidance under the same title that was published

by the Scottish Government Health Directorate previously in 2002, and revised in 2007.



Guidelines for Safe Recreational-water Environments 2003

Vol. 1: Coastal and Fresh-waters October 1998

8.7 Guideline values

As discussed above, approaches to recreational water safety should address the occurrence of cyanobacteria as such, because it is as yet unclear whether all important cyanotoxins have been identified, and the health outcomes observed after recreational exposure-particularly irritation of the skin and mucous membranes-are probably related to cyanobacterial substances other than the well known toxins listed in Table 8.1. Additionally, the particular hazard of liver damage by microcystins should be considered. In face of the difficulty of representative quantitative sampling due to the heterogeneous distribution of cyanobacteria in time and space, particularly with respect to scum formation and scum location, approaches should further include addressing the capacity of a water body to sustain large cyanobacterial populations. Health impairments from cyanobacteria in recreational waters must be differentiated between the chiefly irritative symptoms caused by unknown cyanobacterial substances and the potentially more severe hazard of exposure to high concentrations of known cyanotoxins, particularly microcystins. A single guideline value therefore is not appropriate. Rather, a series of guideline values associated with incremental severity and probability of health effects is defined at three levels (Table 8.3).

8.7.1 Relatively low probability of adverse health effects

For protection from health outcomes not due to cyanotoxin toxicity, but rather to the irritative or allergenic effects of other cyanobacterial compounds, a guideline level of 20,000 cyanobacterial cells/ml (corresponding to 10μg chlorophyll-a/litre under conditions of cyanobacterial dominance) can be derived from the prospective epidemiological study by Pilotto et al. (1997). Whereas the health outcomes reported in this study were related to cyanobacterial density and duration of exposure, they affected less than 30% of the individuals exposed. At this cyanobacterial density, 2- 4μg microcystin/litre may be expected if microcystin-producing cyanobacteria are dominant, with 10μg/litre being possible with highly toxic blooms. This level is close to the WHO provisional drinking-water guideline value of 1μg/litre for microcystin- LR (now WHO, 2011), which is intended to be safe for lifelong consumption. Thus, health outcomes due to microcystin are unlikely, and providing information for visitors to swimming areas with this low-level risk is considered to be sufficient. Additionally, it is recommended that the authorities be informed in order to initiate further surveillance of the site. The results of the epidemiological study (Pilotto et al., 1997) reported some mild irritative effects at 5,000 cells but the level of health effect and the small number of people affected were not considered to be a basis to justify action.

8.7.2 Moderate probability of adverse health effects

At higher concentrations of cyanobacterial cells, the probability of irritative symptoms is elevated. Additionally, cyanotoxins (usually cell-bound) may reach concentrations with potential health impact. To assess risk under these circumstances, the data used for the drinking-water provisional guideline value for microcystin-LR may be applied. Swimmers involuntarily swallow some water while swimming, and the harm from ingestion of recreational water will be comparable to the harm from ingestion of water from a drinking-water supply with the same toxin content. For recreational water users with whole-body contact (see chapter 1), a swimmer can expect to ingest 100-200 ml of water in one session, sailboard riders and waterskiers probably more.

A level of 100,000 cyanobacterial cells/ml (which is equivalent to approximately 50μg chlorophyll-a/litre if cyanobacteria dominate) represents a guideline value for a moderate health alert in recreational waters. At this level, a concentration of 20μg microcystin/litre is likely if the bloom consists of Microcystis and has an average toxin content of 0.2 pg/cell, or 0.4μg microcystin/μg chlorophyll-a. Levels may be approximately double if Planktothrix agardhii dominates. With very high cellular microcystin content, 50-100μg microcystin/litre would be possible.

The level of 20μg microcystin/litre is equivalent to 20 times the WHO provisional guideline value concentration for microcystin-LR in drinking-water (WHO, 1998) and would result in consumption of an amount close to the tolerable daily intake (TDI) for a 60-kg adult consuming 100 ml of water while swimming (rather than 2 litres of drinking-water). However, a 15-kg child consuming 250 ml of water during extensive playing could be exposed to 10 times the TDI. The health risk will be increased if the person exposed is particularly susceptible because of, for example, chronic hepatitis B. Therefore, cyanobacterial levels likely to cause microcystin concentrations of 20μg/litre should trigger further action.


Guidance level or
How guidance level derived Health risks Typical actionsb
Relatively low probability of adverse health effects
20,000 cyanobacterial cells/ml
10μg chlorophyll-a/litre with dominance of cyanobacteria
  • From human bathing epidemiological study
  • Short-term adverse health outcomes, e.g., skin irritations, gastrointestinal illness
  • Post on-site risk advisory signs
  • Inform relevant authorities
Moderate probability of adverse health effects
100,000 cyanobacterial cells/ml
50μg chlorophyll-a/litre with dominance of cyanobacteria
  • From provisional drinking-water guideline value for microcystin-LRc and data concerning other cyanotoxins
  • Potential for long-term illness with some cyanobacterial species
  • Short-term adverse health outcomes, e.g., skin irritations, gastrointestinal illness
  • Watch for scums or conditions conducive to scums
  • Discourage swimming and further investigate hazard
  • Post on-site risk advisory signs
  • Inform relevant authorities
High probability of adverse health effects
Cyanobacterial scum formation in areas where whole-body contact and/or risk of ingestion/aspiration occur
  • Inference from oral animal lethal poisoning
  • Actual human illness occur histories.
  • Potential for acute poisoning
  • Potential for long-term illness with some cyanobacterial species
  • Short-term adverse health outcomes, e.g., skin irritations, gastrointestinal illness
  • Immediate action to control contact with scums; possible prohibition of swimming and other water contact activities.
  • Public health follow-up investigation.
  • Inform public and relevant authorities

a Derived from Chorus & Bartram, 1999.
b Actual action taken should be determined in light of extent of use and public health assessment of hazard.
c The provisional drinking-water guideline value for microcystin-LR is 1μg/litre (WHO, 1998).

Non-scum-forming species of cyanobacteria such as Planktothrix agardhii have been observed to reach cell densities corresponding to 250μg chlorophyll-a/litre or even more in shallow water bodies. Transparency in such situations will be less than 0.5 m measured with a Secchi disc. Planktothrix agardhii has been shown to contain very high cell levels of microcystin (1-2μg microcystin/μg chlorophyll-a), and therefore toxin concentrations of 200-400μg/litre can occur without scum formation.

An additional reason for increased alert at 100,000 cells/ml is the potential for some frequently occurring cyanobacterial species (particularly Microcystis spp. and Anabaena spp.) to form scums. These scums may increase local cell density and thus toxin concentration by a factor of 1,000 or more in a few hours, thus rapidly changing the risk from moderate to high for bathers and others involved in body-contact water sports. Cyanobacterial scum formation presents a unique problem for routine monitoring at the usual time intervals (e.g., 1 or 2 weeks) because such monitoring intervals are unlikely to pick up hazardous maximum levels. Because of the potential for rapid scum formation at a cyanobacterial density of 100,000 cells/ml or 50μg chlorophyll-a/litre (from scum-forming cyanobacterial taxa), intensification of surveillance and protective measures are appropriate at these levels. Daily inspection for scum formation (if scum-forming taxa are present) and measures to prevent exposures in areas prone to scum formation are the two principal actions important in these situations.

Intervention is recommended to trigger effective public information campaigns to educate people on avoidance of scum contact. Furthermore, in some cases (e.g., areas with frequent scum formation), restriction of water contact activities may be judged to be appropriate. An intensified monitoring programme should be implemented, particularly looking for scum accumulations. Health authorities should be notified immediately.

8.7.3 High probability of adverse health effects

Abundant evidence exists for potentially severe health outcomes associated with scums caused by toxic cyanobacteria. No human fatalities have been unequivocally associated with cyanotoxin ingestion during recreational water activities, although numerous animals have been killed by consuming water with cyanobacterial scum material. This discrepancy can be explained by the fact that animals will drink greater volumes of scum-containing water in relation to their body weight, whereas accidental ingestion of scums by humans during swimming will typically result in a lower dose.

Cyanobacterial scums can represent thousand-fold to million-fold concentrations of cyanobacterial cell populations. Calculations suggest that a child playing in Microcystis scums for a protracted period and ingesting a significant volume could receive a lethal dose, although no reports indicate that this has occurred. Based on evidence that a lethal oral dose of microcystin-LR in mice is 5,000-11,600μg/kg body weight and sensitivity between individuals may vary approximately 10-fold, the ingestion of 5-50 mg of microcystin could be expected to cause acute liver injury in a 10-kg child. Concentrations of up to 24 μg microcystin/litre from scum material have been published (Chorus & Fastner, 2001). Substantially higher enrichment of scums-up to gelatinous consistency-is occasionally observed, of which accidental ingestion of smaller volumes could cause serious harm. Anecdotal evidence indicates that children, and even adults, may be attracted to play in scums. The presence of scums caused by cyanobacteria is thus a readily detected indicator of a risk of potentially severe adverse health effects for those who come into contact with the scums. Immediate action to control scum contact is recommended for such situations.

8.7.4 Conclusions

The approach outlined in this section does not cover all conceivable situations. Swimmers may be in contact with benthic cyanobacteria after a storm breaks off clumps of filaments or cyanobacterial mats naturally detach from the sediment and are accumulated on shorelines (Edwards et al., 1992). Measures of cyanobacterial cell density will not detect these hazards. Instead, this cyanotoxin hazard calls for critical and well informed observation of swimming areas, coupled with a flexible response.

It is difficult to define "safe" concentrations of cyanobacteria in recreational water for allergenic effects or skin reactions, as individual sensitivities vary greatly. Aggravation of dermal reactions due to accumulation of cyanobacterial material and enhanced disruption of cells under bathing suits and wet suits may be a problem even at densities below the guideline levels described above.


Email: Janet Sneddon

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