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.


Responsibilities for risk assessments

6.1 Assessments of the risks to human and animal health from cyanobacterial blooms are likely to be made for different purposes - and with different degrees of formality - by different "stakeholders" including:

  • owners and employers, including Scottish Water; for their duties in law to employees, customers and to other members of the public;
  • regulatory bodies; LAs, for the assessment of risk to public health, and SEPA, for determination of sources of pollutants, their impact and regulation and their duty under the Bathing Water Directive;
  • individuals; for their interests as employees or parents, or in connection with their recreational interests.

6.2 The roles and responsibilities of these and other stakeholders are outlined in Section 9.

6.3 The responsibility for risk assessments by the owners of waters and by employers relates to their responsibilities to protect employees and others under the Health and Safety at Work etc Act 1974. ( and related legislation, and, for Scottish Water, their customers.

6.4 Risk assessments by LAs and SEPA are likely to be directed in part to determining inspection and monitoring priorities and schedules (Section 5). The interest of local authorities is primarily in determining the existence, or otherwise, in their areas of nuisances or hazards to health, and this should include the risks associate with private water supplies. That of SEPA is primarily in determining the state of the environment and in the regulation and control of sources of pollutants, or in its duty under the Bathing Water Directive. A shared approach should be possible on specific waters where these interests require similar information.

Types of risk assessment

6.5 Risk assessments in respect of cyanobacterial hazards in inland and inshore Scottish waters can be considered under three general headings: (i) generic assessments of the risk for the whole of each of the areas covered by the LAPs, (ii) pro-active assessments for individual waters, similar to and including those required under the Bathing Water Directive and (iii) reactive assessments in response to identified occurrence or consequences of cyanobacterial blooms.

6.6 Each LAP should include an overall assessment of cyanobacterial problems for the whole of the NHS Board area. This should provide a general summary of cyanobacterial problems for the area and identify the waters that present the greatest risks because of their history of algal blooms and/or their use.

6.7 Proactive individual assessments for all inland and inshore waters would be neither practical nor cost effective. These should normally be confined to those waters identified in the LAP as presenting the greatest risks. However, the responsibilities identified in Paragraph 6.1 and Section 9 (for example, those for employers) are not affected by any classification of waters in the LAP.

6.8 Stakeholders receiving reports on occurrences or consequences of algal blooms for waters relating to their interests should consider the need for a reactive risk assessment. This should aim to identify the magnitude and nature of the risks and define any interventions that might be required to lessen these risks.

Content of risk assessments for individual waters

6.9 Proactive risk assessments should include consideration of inspection and monitoring programmes and the need for appropriate action (such as erection of signs). They should take account, in structured ways, of information relating to any previous algal blooms (Table 5.1) and to the nature and intensity of use for the water in question.

6.10 Subsequent interventions would normally depend on the results of inspections and/or monitoring or the outcomes of any reactive risk assessments.

6.11 Reactive risk assessments (made in reaction to reports from the public of the appearance or consequence of algal blooms, suspected health incidents, or to inspections or monitoring) should consider how the water in question is used (for example as a source for drinking water) and the resultant risk to human and animal health (not the probability of occurrence of a bloom).

6.12 For either proactive or reactive risk assessments, therefore, assignment of waters to high, medium and low risk categories according to their use can help to prioritise the needs for inspection and/or monitoring and action. Table 6.1 gives outline descriptions of the features of waters falling into each of these categories.

6.13 A reactive risk assessment should define clearly the need for, and nature and comparative merits of any interventions to mitigate the risks identified and for further inspection and/or monitoring.

Table 6.1: Categories of risk related to the use of waters (and hence the probability and extent of exposure)

Risk category Nature and intensity of use
High Waters that are either consumed by people or animals or used for activities involving immersion or appreciable skin contact.
Medium Waters for which the risk of ingestion of cyanobacterial material or of cyanotoxins is small and appreciable skin contact with blooms is unlikely. Standing Waters that are used for spray irrigation of crops.
Low Waters that are inaccessible or not used, or are used only for angling, or other non-contact activities.

Defining categories for frequency of occurrence or risks for cyanobacterial blooms

6.14 It is essential that different views about criteria for the assignment of waters to each of the categories defined in Tables 5.1 and 6.1 are discussed by bodies seeking co-operative action on monitoring.

Templates for risk assessments

6.15 Examples of templates for proactive and reactive risk assessments are provided in Annex F. Stakeholders might prefer to use their own formats but, if so, the information contained in the risk assessment should be at least that indicated in Annex F.

Specific considerations for public drinking water supplies

6.16 There are a number of Scottish public water supply reservoirs where problems associated with cyanobacterial growth recur. At these, the water treatment systems in place have been designed to cope, or the water authority is able to use an alternative water source. Should circumstances arise where neither of these provisions applies, appropriate alternative measures would be put in place to ensure the safety of the water supply. If an incident occurred where there was an actual or potential risk of contaminated water entering a supply, the provisions of the multi-agency Scottish Waterborne Hazard Plan (SWHP) should be used to coordinate a public health response and ensure that the local NHS Board and local authorities are involved in the health risk assessment and decisions on safe-guarding public health

6.17 Scottish Water is required to comply with the Security and Emergency Measures (Scottish Water) (Scotland). Direction 2002 (SEMD). This requires it, amongst other things, to have emergency plans in place to cover such eventualities as loss of water supply (Paragraph 3.8) and to have routine liaison meetings about emergency issues with LAs and Health Boards. These emergency plans are subject to an annual audit and certification by an independent consultant approved by the Scottish Government. An audit report is submitted to the Scottish Government by 31st March each year.

6.18 Consultation with Scottish Water in the production of LAPs for management of cyanobacteria should ensure that these are compatible with any emergency plans drawn up under the SEMD, and should also be consistent with the provisions of the SWHP.

6.19 About 3 % of the population of Scotland are on a private water supply. However, it is very unlikely that any of these will be affected by cyanobacteria due to the nature of the source waters for those supplies. Most (99%) of private water supplies come from wells or springs or running surface water (small burns etc.) which should not be affected, and only about 1% are sourced from lochs which possibly could be affected by cyanobacteria. Local authorities are responsible for monitoring the quality of private water supplies, and they should include cyanobacteria in their monitoring provisions if they consider it to be a risk.


Email: Janet Sneddon

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