On June 17, the U.S. Environmental Protection Agency (EPA) and Health Canada issued Drinking Water Health Advisories (DWHA) for microcystins and cylindrospermopsin for short-term (10 day) exposures. Microcystins are more common than cylindrospermopsin; at least three other algal toxins found occur with lower frequencies. Algal toxins are produced during some algal blooms, which often occur with noticeable and distinctive taste and odor. Algal blooms occur by a confluence of factors that involve excess nitrogen and phosphorus nutrients from fertilizer runoff, sunlight, and slow-moving shallow clear surface water. Specific circumstances leading to toxin production when it occurs are not well understood.

The DWHAs were based on conservative assessments from animal feeding studies: microcystins in rats and cylindrospermopsin in mice. The microcystins are considered liver toxins, and cylindrospermopsin produced kidney weight increases in the mouse study.

The U.S. EPA 10-day DWHAs:


  • 0.3 µg/L — infants and children less than age six
  • 1.6  µg/L — age six and above


  • 0.7  µg/L — infants and children less than age six
  • 3 µg/L — age six and above

Health Canada will retain its existing 1.5 µg/L for the general population, but based upon the rat study, pending external review, and rather than providing a numerical value for infants and young children, it recommends alternative or bottled water for powdered infant formula during an algal bloom.

The new DWHA calculations differ substantially from the World Health Organization’s (WHO) existing drinking water guideline of 1 µg/L for Microcystin-LR. The DWHA used a different study and a different animal model (rat vs mouse), and applies for 10-day exposure versus a lifetime for the general population, and it includes aggregated microcystins versus Microcystin-LR, which is considered most toxic and most frequently detected. WHO will review its guideline over the next few years.

DWHAs are not drinking water standards, but rather informal technical guidance and not legally enforceable by U.S. EPA. Most states will probably adopt them as virtual standards. A well-operated conventional surface water system should be able to meet the adult guidance values, perhaps augmented with powdered activated carbon and/or potassium permanganate. Meeting the child guidance value could be difficult in some locations.

See also: Algal Toxins in Drinking Water. Later articles will provide more information on the algal toxin DWHA issue.