Assessing the Risk of Exposure to Cyanotoxins through Drinking-Water and Short-Term Responses to Occurrence

A modern water treatment plant equipped with an effective filtration system for physical removal of cells as well as the removal of dissolved toxins should remove cyanotoxins to below hazardous levels, provided it is operated with attention to avoid disruption of cyanobacterial cells and release of dissolved toxin (see Chapter 10). However, this requires it to be validated for meeting this target. Also, many of the world’s drinking-water supply systems and treatment plants are more rudimentary, and large populations may depend upon such vulnerable water supplies or on untreated surface waters for drinking and preparing food.

For exposure assessment, particularly for MCs and CYNs, it is important to differentiate between daily exposure for significant parts of a lifetime and short-term episodic exposure. If concentrations exceed the values intended for lifetime daily consumption of drinking-water, but are below the short-term guideline values (GVs) given in Table 5.1 (or nationally derived standards; Table 5.3), use of the water supply for drinking may continue, and action may first focus on assessing which measures are locally most appropriate to ensure better control of the cyanotoxin concentrations. These might include addressing the cause for waterbody conditions leading to

Table 5.3 Standards, guideline values, maximum acceptable concentrations or maximum values set by a number of countries for cyanotoxins in drinking-water

Cyanotoxin

Type of value

Numerical value

Country

Microcystins

Guideline value

1.3 pg/LMC-LR toxicity equivalents

Australia

Standard

1 pg/L MCs

Brazil

Maximum acceptable concentration

1.5 pg/LMC-LR

Canada

Standard

1 pg/L MC-LR

Czech Republic

Standard

1 pg/L sum of MCs

France

Provisional maximum value

1.3 Mg/L MC-LR equiv.

New Zealand

Restrictions on water use

>1.0 pg/L sum of MCs

Finland

Ban on water use

>10.0 pg/L sum of MCs

Finland

Standard

1 pg/L MC-LR

Singapore

Standard

1 pg/L sum of MCs

Spain

Standard

1 pg/L MC-LR

Uruguay

Standard

1 pg/L sum of MCs

Turkey

Guideline value

1 pg/L MC-LR

South Africa

Provisional maximum value

1 pg/L

New Zealand

Nodularin

Cylindrospermopsin

Health Alert Level

1 pg/L

Australia

Guideline value

1 pg/L

Brazil

Provisional maximum value

1 pg/L

New Zealand

Health Alert Level

3 pg/L

Australia

Saxitoxins

(as saxitoxin toxicity equivalents)

Guideline value

3 pg/L

Brazil

Provisional maximum value

3 pg/L

New Zealand

Provisional maximum acceptable concentration

3.7 pg/L

Canada

Anatoxin-a

Provisional maximum value (valid also for homoanatoxin-a)

1 pg/L

New Zealand

Provisional maximum value

1 pg/L

New Zealand

Source: Data from Ibelings etal.(20!4).

blooms (which may or may not be feasible in the short term; see Chapters 7 and 8), shifting the raw water offtake to avoid blooms (Chapter 9) or implementing additional treatment steps (Chapter 10). Allowing such flexibility for the locally most effective response if a GV intended for lifetime daily exposure is exceeded is particularly pertinent to short-lived bloom situations if past experience shows that they are likely to disperse within a few days, thus no longer causing elevated cyanotoxin concentrations. The shortterm GVs are intended for periods of about 2 weeks and are not intended to endorse repeated seasonal exceedances of the lifetime GV. Where water with concentrations ranging up to these values is distributed, it is important to inform the population about this situation so that specifically vulnerable groups may take specific measures, such as using bottled water. This may be relevant, for example, for hepatitis patients in the case of hepatotoxins and is particularly important for those responsible for bottle-fed infants because the short-term drinking-water GV is based on exposure of adults. Since infants and children can ingest a significantly larger volume of water per body weight (e.g., up to 5 times more drinking-water/kg bw for bottle- fed infants compared to an adult), as a precautionary measure WHO recommends that alternative water sources such as bottled water are provided for bottle-fed infants and small children when MC concentrations are greater than 3 pg/L for short periods (WHO, 2020).

Defining National or Regional Cyanotoxin Levels Requiring Action

As discussed at the beginning of this chapter, when setting national standards or defining threshold concentrations that should trigger specific action, it is important to consider whether the WHO GVs given in Table

5.1 and used in the Alert Levels Framework (ALF) below are locally or nationally appropriate, or whether they would better be adapted to local or national circumstances. Besides differences in the ratios between toxin concentration and the indicators used to trigger the alert, such circumstances may include the amount of drinking-water consumed and the fraction of cyanotoxin allocated to uptake through drinking-water in relation to other exposure pathways (see sections 5.2-5.5). Further considerations include the extent and duration of cyanotoxin exposure in relation to other hazards: where public health impacts from exposure to other hazards (in particular pathogens) are substantial and toxic cyanobacterial blooms are short-lived events, a decision might be to tolerate somewhat higher concentrations (possibly only as an interim solution) in order to focus available capacity and resources on controlling exposure first to those hazards which are causing the highest risks for health. Such considerations are particularly important when setting national or local water quality regulations, because where other quality issues are likely to have a higher public health impact, enforcing a low cyanotoxin standard may distract funding from investments needed to remediate the more pressing public health problems.

A number of countries have implemented concentrations triggering action for a range of cyanotoxins (see examples in Table 5.3). Particularly for cya- notoxins other than MCs, they have typically not been set as standards in the legal sense of values that all water suppliers in the country need to meet in order to be in compliance with regulations but rather guideline values (GVs) or “Health Alert Levels” that are used to trigger a notification to the health authority, further assessment of the situation and/or other management responses.

 
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