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



General requirements

7.1 Actions intended to reduce the probability of acute or delayed effects of cyanotoxins on people or animals are directed at reducing the probability of:

i. skin contact with or ingestion of algae in, or on the shore of inland waters;

ii. ingestion of drinking water containing algae or cyanotoxins, and exposure to such water during bathing and showering;

iii. exposure to toxins by eating fish or shellfish from algae-rich waters; and

iv. delivery of contaminated water to patients undergoing haemodialysis.

7.2 Of these, 7.1(i) is the most likely. Acute effects from ingestion of publicly supplied drinking water containing cyanotoxins are considered unlikely in Scotland due to the effects of volume dilution and also the removal and degradation of the toxins during normal water treatment processes. Scottish Water will take appropriate action to ensure the safety of supplies where cyanobacterial blooms are identified. The risk of longer-term exposure to cyanotoxins from contaminated private supplies can be reduced by practical measures discussed later in this section or, if necessary, by substitution of an alternative supply or bottled water. Action may however also be required to provide substitute supplies in situations where taste or odour problems render drinking water unacceptable, even in the absence of risk from toxins.

7.3 Particular attention should also be paid to the health risks for patients undergoing haemodialysis. In normal circumstances, cyanotoxins are effectively excluded by the reverse osmosis units that are used to treat the water supply to dialysis units in Scotland. However, the possible consequences of exposure to cyanotoxins (and indeed to other pollutants) due to system failure should be addressed. Local NHS Boards should ensure appropriate resilience for this threat.

7.4 It is prudent to consider possible health concerns about ingestion of cyanotoxins in affected foods. This includes consideration of: (i) whether fish from heavily affected waters should be eaten, (ii) the possibility of accumulation of microcystins on or in plants irrigated with water from sources containing cyanobacterial blooms and their toxins, and (iii) the potential for accumulation of the toxins on the external surfaces of edible plant material, for example, on salad plants.

7.5 Any proposed restrictions on the use of water because of the presence of a cyanobacterial bloom should be based on a careful assessment of the resulting benefits and detriments. This assessment should, among other matters, take account of the circumstances of use and of the relevant guidance documents for drinking water and for recreational waters referred to in Paragraphs 7.6, 7.7, and 7.8, as well as the requirements of the Bathing Water Directive 2006/7/EC.

Triggers for action

7.6 The WHO (2011) Guidelines for Drinking Water Quality. Fourth Edition. (WHO, Geneva, Switzerland, ISBN: 978 92 4 154815 1) define a provisional value of 1μg/l of microcystin-LR (one of the commonly found cyanobacterial hepatotoxins, in drinking water) for drinking water that is intended for lifelong consumption.

7.7 In addition to the WHO lifetime exposure guideline value, short term exposure values for toxins have been proposed by the UK Water Industry Research's Water Research Centre (UKWIR/WRc) for several toxins. These 'Suggested No Adverse Response Levels' (SNARLs)* have been included in the HPS document "Short-Term Health Risk ActionValues for Drinking-Water in Scotland (SHRAVS)" ( ). These may be of assistance in assessing the risk of acute adverse effects (as opposed to chronic effects) from contaminated drinking water supplies.

Cyanotoxins - UKWIR/WRc SNARL levels

Toxin Units 24 hour 7 day Health Effects
Anatoxin-a μg/L 1.0 Neurotoxin. Neuromuscular blocker and nicotinic agonist. Causes respiratory muscle paralysis and asphyxiation.
Cylindrospermopsin μg/L 9.0 4.5 Hepatotoxin and genotoxin.
Microcystin-LR μg/L 12.0 6.0 Hepatotoxin; microcystin-LR is one of the most toxic of the microcystin.

* These SNARLs values are included with permission from UKWIR/WRc. Before using them as a basis for a specific risk assessment it would be prudent to check with HPS or with Scottish Water whether these have been updated.

7.8 The equivalent WHO guidance document for recreational water is the 2003 Guidelines for safe recreational-water environments. Volume 1: Coastal and fresh-waters. WHO Geneva, Switzerland. ISBN 92 4 154580 1 ( ).

7.9 The 2003 WHO guidelines for recreational waters state that "Health impairments from cyanobacteria in recreational waters must be differentiated between chiefly irritative symptoms caused by unknown cyanobacterial substances and the potentially more serious hazard of exposure to high concentrations of known cyanotoxins, particularly microcystins. A single guideline value therefore is not appropriate." For recreational waters, therefore, the document recommends "a series of guideline values associated with incremental severity and probability of health effects" and these values are then defined for low, moderate or high probabilities of adverse health effects.

7.10 A copy of the relevant section from this WHO document [Section 8.7 Guideline Values] is appended here as Annex G. The guidance levels recommended by the WHO are summarised in Column 1 of Table 7.1. However, the advice given in Column 2 of Table 7.1 differs from that in the WHO Guidance document (Annex G) by recommending that, as an additional precaution, all four of the "typical actions" defined by the WHO for 100,000 cells cyanobacteria/ml be adopted at the lower level of 20,000 cells cyanobacteria/ml. (It should be noted here that the general equivalence implied in the final row of Table 7.1 between cell numbers and chlorophyll-a concentration (1μg chlorophyll-a per 2,000 algal cells) actually depends on cell type. Also, for some types, such as filamentous cyanobacteria, individual cells are not easily identified or counted. These issues are considered in more detail in Annex E)

Table 7.1: Guidance levels and related "typical actions" derived from current WHO guidance

Guidance level or situation Typical Actions
Cyanobacterial scum
formation in bathing areas
  1. Immediate action to control contact with scums; possible prohibition of swimming and other water-contact activities
  2. Public health follow-up investigation
  3. Inform public and relevant authorities
20,000 cells cyanobacteria/ml
10 µg chlorophyll-a/ l with dominance of cyanobacteria
  1. Watch for scums or conditions conducive to scums
  2. Discourage bathing and further investigate hazard
  3. Post on-site risk advisory signs
  4. Inform relevant authorities

Specific actions in response to a cyanobacterial bloom

7.11 Column 2 of Table 7.1 summarises the "typical actions" that should be taken to protect people who might come into contact with recreational waters affected at the extent indicated in Column 1. The WHO guidance also notes that "actual action taken should be determined in light of extent of use and public health assessment of hazard". Section 4 of this document refers to the need for LAPs to make provisions for such public health (risk) assessments and Section 6 gives general requirements.

7.12 Actions defined in Table 7.1 relate mainly to the provision of information and advice and to discouraging or prohibiting water-contact activities. Responsibility for these actions will vary according to ownership and use of the waters in question and these responsibilities should be defined in LAPs and in pro-active risk assessments (Annex F).

7.13 Information and advice might be provided by leaflets, warning notices, letters to stakeholders or public announcements (for example on local radio and by local press notices). Leaflets can provide more information to water-users than is possible in a warning notice and might be particularly appropriate (in addition to warning notices) in circumstances where there is extensive recreational use of a waterbody. More detailed consideration of public information provisions is given in Section 8.

7.14 In addition to advice aimed at minimising public health risks, advice should also be given to dog-owners to protect dogs and other pets from ingestion of cyanobacterial material in the water or on the shoreline. Similar advice should be given to farmers to protect livestock.

7.15 A suggested text for a warning notice is given in Annex H.

Fish and Shellfish

7.16 LAPs and proactive risk assessments should also consider the need for advice to avoid eating freshwater shellfish.

7.17 Fish should not be consumed if fish mortalities, or behavioral abnormalities, are observed at waterbodies containing mass populations of cyanobacteria. In the event of cyanobacterial scum being present, or cyanobacterial cell numbers exceeding 20,000 per ml (Annex G), toxin analysis of fish intended for consumption should be carried out. Should toxins be detected by analysis, expert advice will be necessary on whether concentrations are sufficient to justify restrictions on the consumption of fish. The absence of taint and odour does not indicate the absence of cyanotoxins since there is no correlation between the production of compounds affecting taste and odour and the production of toxins by cyanobacteria. The liver and gut from fish caught in waters affected by cyanobacteria should not be fed to pets.

Crop Irrigation

7.18 LAPs and proactive risk assessments should consider the use of standing waters for irrigation of crops. While there is evidence of the possibility of accumulation of microcystins and other cyanotoxins by aquatic and terrestrial plants, there is no convincing evidence of related health effects from human consumption. A more significant source of concern is where sprayed irrigation water becomes trapped in the centres of, for example, salad plants or where cyanobacterial cells and cyanotoxinsare deposited on plant surfaces when sprayed water dries. Spray irrigation using water from sources containing cyanobacterial blooms and toxins also presents potential health hazards for workers or bystanders who might be exposed to cyanotoxins by skin contact or by inhalation of spray drift. Occupational exposure is subject to the provisions for risk assessment arising from the Management of Health and Safety at Work Regulation 1999 ( ) but general advice from the WHO is that exposure of workers, bystanders and animals to spray irrigation water containing cyanotoxins should be avoided. Advice should be given on precautions that are appropriate to local circumstances.

Drinking water supplies and recreational water bodies

7.19 Where water is used for potable supply, toxin analysis should be planned and carried out as appropriate as an aid to risk management.

7.20 Guidance levels for recreational waters are defined in terms of concentrations of the algae themselves rather than the toxins. However, cyanotoxin analysis for recreational waters should be considered, depending on individual circumstances, in conjunction with advice from relevant sources (Annex C). Where a bloom is highly localized or confined to one area, the risks of exposure to significant quantities of cyanotoxin are likely to be low except where there is immersion in or ingestion of water in close proximity to the bloom itself.

7.21 Pre-emptive action should also be considered if, from knowledge of the water and recent weather, the probability of bloom scum or mat formation is judged to be high.

Action to reduce the development of cyanobacterial blooms

7.22 In waterbodies where persistently high cyanobacterial concentrations occur or regular blooms develop (Categories 1 and 2 in Table 5.1), and where the attendant risk is categorised as "High" (Table 6.1) the source(s) of the problem should be established, and where possible, appropriate action taken. These measures should be enacted in consultation with SEPA and Scottish Water where a public water supply is affected. In the unlikely event that a private water supply is affected, these measures should be enacted in consultation with SEPA and the relevant local authority.

Control of Nutrients

7.23 Algal blooms, including cyanobacterial blooms, can result from a combination of natural factors, including availability of nutrients and light, water temperature and wind conditions and can be increased due to human impacts e.g. excessive nutrient input to waterbodies or reduced water flow. Availability of nutrients is a principal concern, as they are essential for plant and algal growth and cyanobacterial blooms have generally become more widespread and intense due to raised nutrient levels in the environment. Typical nutrient sources from human activity in Scotland include discharges from sewage works, industry, and agriculture.

7.24 Information on reducing nutrient enrichment of surface waters from agricultural sources can be found in "Prevention of Environmental Pollution from Agricultural Activity Code of Good Practice (2005)" which is available from the Scottish Government Methods for preventing nutrient losses from urban areas are included in the Sustainable Urban Drainage Systems ( ).

Further information and guidance can also be found in the 2003 WHO Guidance document for recreational waters. It is likely that SEPA will take account of such matters under the EC Water Framework Directive. ( SEPA is developing ecological classification systems based on the condition of biological communities, including cyanobacteria, to help assess the ecological quality of surface waters in response to environmental pressures. Furthermore, SEPA regulates activities including abstraction, impoundment and engineering activities, as well as pollution, under the Water Environment (Controlled Activities) Regulations 2011 (

Control of Blooms

7.25 Reducing nutrient inputs from the catchment is part of the long-term strategy to reduce the development of algal blooms, but additional measures may be effective in reducing these events in the short-term. These measures include the use of barley straw, biomanipulation, increased flushing, forced circulation, and chemical control.

Barley straw - This method involves the use of (small) bales or nets of barley straw submerged at the inlet to the waterbody and at other suitable locations. However, it is variably effective and then usually only in small water bodies. Further details are available at:

Biomanipulation - This aim here is to make the aquatic ecosystem less conducive to the establishment and persistence of algal blooms. Approaches include interventions aimed at increasing the populations of zooplankton (which feed on algae) and of aquatic macrophytes (rooted or floating water plants) which compete with algae for light and nutrients. The first of these might involve manipulation of the fish community to reduce the rate of zooplankton predation by fish. Macrophytes, e.g. reeds, can be used in constructed wetlands which can act as filter-beds to reduce nutrient concentrations before water enters a lake. The second approach can involve active planting of native water plants, such as water lilies, in the loch.

Increased flushing - If the regulation of water entering or leaving the water body is controlled, it may be possible to optimise flushing to reduce nutrient concentrations and algal blooms. Blue green algae generally do not develop large populations where the loch is flushed through in less than 30 days.

Forced circulation - Water circulation can be forced by a wind- or electrically-driven turbine within the water body, or by sparging with compressed air. This ensures that the water body is evenly mixed to avoid stratification, and this helps to reduce internal release of phosphorus from the bottom sediments of the waterbody under anaerobic conditions. It also forces the algal cells to spend an increasing proportion of the day away from sunlight, reducing their population growth.

Chemical control methods - Precipitating agents have been used to encourage binding of phosphorus to sediments (reduce internal release); their application is not, however, recommended without expert guidance. Certain algaecides and herbicides have indicative approval for use on or near waters in the UK. However, the proposed use of any control chemicals on or near waters in Scotland must be notified to SEPA for approval where appropriate.

Further information on catchment management, biomanipulation and other control methods can be obtained from the Centre for Ecology & Hydrology (Annex C).

Waste Disposal

7.26 In exceptional circumstances, cyanobacterial scums and mats may have to be removed and disposed of. Disposal of waste that has certain hazardous properties ('Hazardous waste' as defined by Article 1(4) of the Hazardous Waste Directive (91/689/EEC) ( will have to comply with the requirements of the Hazardous/Special Waste Regulations. Further details and information is available from SEPA (

Public Health Follow-up

7.27 Table 7.1 also raises the possibility of "Public health follow-up investigation". This would be a matter for the local Director of Public Health but Section 9 of this Guidance gives some general indications. Where any follow-up investigations are conducted it would be helpful to have the findings reported to HPS via the SEISS surveillance system to which NHS Boards have electronic access.

7.28 The long-term adverse effects of cyanotoxins are not fully understood. In cases of exposure only to skin, long-term follow-up is probably not indicated. If toxic bloom or scum or cyanobacterial mat material have been ingested, medical or veterinary monitoring might be needed for adverse health effects and further advice should be sought (Annex C).