Cyanobacterial Toxins -- Microcystin-LR

Classification and Assessment

It is difficult to assess the risk to human health from microcystins in drinking water. Most of the relevant data have come either from reported human injury related to the consumption of drinking water from a source containing cyanobacteria or from limited work in experimental animals. There is evidence of liver injury in people who drank water contaminated by toxic cyano-bacteria and evidence for tumour promotion by Microcystis or microcystins in animals. With micro-cystins, the underlying mechanism is thought to be protein phosphatase inhibition. Taking the information on genotoxicity into consideration, microcystin-LR could possibly be carcinogenic to humans, and for this reason it has been placed in Group IIIB (inadequate data in humans, limited evidence in experimental animals). It is therefore considered appropriate to use a LOAEL or NOAEL from the most suitable chronic or subchronic study, divided by appropriate uncertainty factors, to derive a tolerable daily intake (TDI) for microcystin-LR, the only microcystin for which there is sufficient information available with which to derive a guideline value.

For microcystin-LR, the TDI is derived as follows:

where:

• 40 µg/kg bw per day is the NOAEL for liver changes derived from the 13-week mouse study conducted at the WRc in the United Kingdom⁹⁴
  
• 1000 is the uncertainty factor (×10 for intraspecies variation, ×10 for interspecies variation and ×10 for the less-than-lifetime study).

An additional uncertainty factor for limited evidence of carcinogenicity in animals was not considered necessary.

Rationale

The maximum acceptable concentration (MAC) for microcystin-LR is calculated from the TDI, as follows:

• 0.04 µg/kg bw per day is the TDI, as derived above
  
• 70 kg bw is the average body weight of an adult
  
• 0.80 is the proportion of total intake considered to be ingested in drinking water
  
• 1.5 L/d is the average daily consumption of drinking water for an adult.

The MAC of 1.5 µg/L for microcystin-LR is believed to be protective against exposure to other micro-cystins (total microcystins, i.e., free plus cell bound) that may also be present. It is a conservative value, as it is derived on the basis of daily consumption of microcystin-LR over a full year. However, as there are spatial and temporal variations in the levels of micro-cystins within supplies and as there are also likely to be other microcystins present that could go undetected, this value is considered appropriate.

Also, as a precautionary note, dialysis centres should be aware or informed if the source water from their local treatment plant is prone to blue-green algal blooms so that they may undertake to provide additional treatment of the water, if necessary. This treatment can range from GAC filtration followed by reverse osmosis to much more complex membrane filtration systems (e.g., ultrafiltration). The extent of additional treatment will depend entirely on the quality of the municipal water supply. Continuous monitoring of performance and equipment will be required to ensure adequate quality of the water. As well, it is important that all manufacturers' specifications be evaluated for the local conditions.