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

Extract on exposure guidelines from the WHO Document Annex G

Guidelines for Safe Recreational-water Environments Draft for Consultation
Vol. 1: Coastal and Fresh-waters October 1998

Guidelines Derivation

Approaches to bathing water safety should address the occurrence of cyanobacteria as such, because it is as of 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 7.2. Additionally, the particular hazard of liver damage by microcystins may 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 major 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 7.4).

Relatively mild and/or low probabilities 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 per ml (corresponding to 10 g/ l of chlorophyll 'a' 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 per cent of the individuals exposed. At this cyanobacterial density, 2-4 g/ l of microcystin may be expected if microcystin-producing cyanobacteria are dominant, with 10 g/ l being possible with highly toxic blooms (regional differences of microcystin-content of the cells may be substantial). This level is close to the WHO provisional drinking-water Guideline Value of 1 g/ l for microcystin-LR (WHO, 1998) which is intended to be safe for life-long consumption. Thus, health outcomes due to microcystin are unlikely, and providing information for visitors to bathing sites with this low-level risk is considered to be sufficient. Additionally, it is recommended that the authorities are informed in order to initiate further surveillance of the site.

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 bathing, and the harm from ingestion of bathing water will be comparable with that from a drinking water supply with the same toxin content. A swimmer can expect to ingest 100-200 ml of water in one session, sailboard riders and water skiers probably more.

Table G.1:
Guidelines for safe-practice in managing recreational waters

* actual action taken should be determined in light of extent of use and public health assessment of hazard

A level of 100,000 cyanobacterial cells per ml (which is equivalent to approximately 50 g/ l of chlorophyll-a if cyanobacteria dominate), represents a Guideline Value for a moderate health alert in recreational waters. At this level, 20 g/ l of microcystins are likely, if the bloom consists of Microcystis and has an average toxin content per cell of 0.2 pg, or 0.4 g microcystin per g chlorophyll-a, (up to 50 g/ l of microcystin are possible). Levels may be approximately double if Planktothrix agardhii dominates. This level is equivalent to twenty times the WHO provisional Guideline Value concentration for microcystin-LR in drinking water, but would result in consumption of an amount close to the Tolerable Daily Intake (TDI) for an adult of 60 kg consuming 100 ml of water while swimming (rather than 2 l of drinking-water). However, a child of 15 kg consuming 250 ml of water during extensive playing could be exposed to ten times the TDI. The health risk will be increased if the person exposed is particularly susceptible, e.g. because of chronic hepatitis B. Therefore, cyanobacterial levels likely to cause microcystin concentrations of 20 g/ l should trigger further action.

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

An additional reason for increased alert at 100,000 cells/ml is the potential of some frequently occurring cyanobacterial species (particularly Microcystis spp. and Anabaena spp.) to form scum. These scums may increase local cell density and thus toxin concentration by a factor of one thousand 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 of one or two 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 50 g/ l chlorophyll-a (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 main options.

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

High risk 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 by mouth, numerous animals have been killed by consuming water with cyanobacterial scum material (section 7.1.1). This discrepancy can be explained by the fact that animals will drink higher volumes of scum-containing water in relation to their body weight, whereas accidental ingestion of scums by humans during bathing 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 a Microcystis scum for a protracted period and ingesting a significant volume could receive a lethal exposure, although no reports indicate that this has occurred in practice. Based on evidence that a lethal oral dose of microcystin-LR in mice is 5,000-11,600 g/kg body weight, for a child of 10 kg the ingestion of 2 mg of microcystin or less could be expected to cause acute liver injury. Concentrations of up to 24 mg/l of microcystins have been published from scum material. 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 bathers who come into contact with the scums. Immediate action to control scum contact is recommended for such situations.

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 shore lines ( Edwards et al. 1992). Some marine beaches report widespread problems due to a benthic marine cyanobacterium, Lyngbya majuscula, growing on rocks in tropical seas and causing severe blistering when trapped under the bathing suits of swimmers after a storm ( Grauer, 1961). This response may be due to acute toxicity, as in the case of Lyngbya which can produce irritant toxins. Measures of cyanobacterial cell density will not detect these hazards. Instead, this cyanotoxin hazard calls for critical and well-informed observation of bathing sites, coupled with a flexible response.

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