Assessing risks and the system's performance in controlling them

Prioritising risks: cyanotoxins in the context of risks from other hazards

How dangerous are cyanotoxins compared to other hazards in water? The answer to this question is highly specific for a given setting. It will depend on the other hazards that occur, on cyanobacterial proliferation in the water-body as well as on barriers protecting people from exposure. Finding answers thus requires analysing hazards comprehensively and assessing the risk they pose in relation to the risks from other hazards.

Ranking your cyanotoxin risk in relation to the health risks from other hazards in the water will help you assess which priority measures to control the cyanotoxin risks should take in relation to measures that control risks from other hazards. Note that many measures will serve to control multiple hazards, although in this decision support tool they are discussed only in relation to controlling cyanotoxins.

Simple risk matrices comparing the likelihood of a hazard to occur against the severity of its public health impact are a useful tool for such rankings. They typically use between 3 and 5 categories for ranking the likelihood of a hazardous event to occur and the severity of its consequences, as shown in the left-hand matrix. The right-hand examples show a ranking for a recreational setting and for a drinking-water supply, in these examples using only 3 categories.

è For assessing the risk to human health from exposure to cyanotoxins in your setting in relation to risks estimated from exposure to other hazards (after having gone through assessing cyanotoxin risk) enter your results in the worksheet for your entries.

è For examples and further discussion of ranking risks relative to each other, read the text below.

Examples

Risk assessment for recreational site X

For recreational site X, the water safety plan team ranked the public health risks from drowning clearly above those for exposure even to high microcystin concentrations, as the one obviously is lethal while the other is uncertain. Among the hazards causing less severe impact, because of the potential long-term consequence of skin cancer the risks from heavy and frequent sunburn were ranked above those from moderate microcystin exposure.

Risk assessment for drinking-water system Y

In the example of the drinking-water supply system Y, the water safety plan team ranked risks from Cryptosporidia and Hepatitis-a higher than those from microcystin exposure to concentrations up to 5 times the Guideline value, because in this setting the former are more likely to occur and they implicate a certain death rate, while the health impact of microcystin exposure to these levels is uncertain. The health risk from Legionella received an equally high ranking in this setting, although Legionella are assessed as less likely to occur, simply because their occurrence also implicates a certain death rate.

In this setting, iron often causes the water to appear brownish, and this is not ranked in the “green” area because its unpleasant taste and colour leads users to turn to other – often less safe – water sources. Nitrate levels occur in concentrations up to 100 mg/L, and while in theory this could be a more serious hazard, in this setting it was not ranked in the “red” area because infants are well protected through widespread information of parents as well as breast-feeding.

Generic aspects of ranking cyanotoxins in relation to other hazards

Microcystin-LR is one of 94 substances for which WHO provides toxicologically based Drinking-water Guideline Values (see WHO 2004, GDWQ). As the two examples above illustrate, generally, quite in contrast to widespread public perception in most industrialized countries of health being threatened by chemicals, health risks are usually substantially higher from pathogens such as bacteria, viruses and parasites that get into drinking-water from human excreta through poor sanitation and cause infectious disease. Unsafe drinking-water still kills 3 million people annually on the earth, and in industrialized countries also, demonstrated water-borne outbreaks of illness are generally due to pathogens.

Illness from chemical exposure via water is harder to demonstrate, and if it occurs, this would almost always be chronic, i.e. through long-term exposure. However, humans are exposed to toxic chemicals largely through food and air (including dust). Only a few toxins occur more widely in drinking-water in health relevant concentrations. The few demonstrated examples include health impairments from naturally occurring elevated levels of arsenic or fluoride in boreholes in some regions, and in others, lead and copper are an issue due to inadequate use of these materials in plumbing. Extremely high levels of nitrate may cause the “blue baby syndrome” in bottle-fed infants, though usually only in conjunction with diarrhoeae. Most other chemicals from human pollution can occur in water in hazardous concentrations in local “hot spots” (e.g. due to spills), but are not a widespread problem.

Likewise, for swimming and other recreational water uses, chemicals in water are by no means the major health hazard. Rather, illness results primarily from accidents such as drowning or back injury through unsafe diving (often in conjunction with excessive alcohol consumption), sunburn, and – again – infectious disease from pathogens in the water.

How do toxic cyanobacteria fit into this overall exposure assessment? Among the chemicals, in the bigger picture cyanotoxins are likely to be among the top 5 or top 10, judging from the frequency at which they occur in health-relevant concentrations. However, from illness demonstrated to have been caused by water, they range far behind infectious disease. This is reflected e.g. in the European Union’s new Bathing-Water Directive, which concentrates on regulating bathing-water surveillance for pathogen indicators and refrains from regulating chemicals, with the exception of article 8 which addresses toxic cyanobacteria (EC 2006).

This is about as far as generic statements can go. The bottom line is: risk assessment requires site-specific assessments, beginning with identifying the hazards that may potentially occur, and then ranking them according to their likelihood of occurrence in relation to the severity of their public health impact.

Your next step is to define and document your control measures and their operational monitoring.

è Go back to START for systematically going through your control measures.