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Microcystin-LR: How Toxic Is It? (Part I)

Posted by Stephanie A. Smith, Ph.D. on Mon, Jul 08, 2013 @ 06:00 AM

Harmful Algal Bloom How toxic does something have to be before we care about being exposed to it?  That's a really tough question, because such decisions are usually based on the risk that an individual is willing to take, and how they interpret the information that is available to them. Microcystin-LR is one of the most commonly encountered cyanobacterial toxins in harmful algal blooms (HABs), and therefore its potential toxicity is of interest because people can be exposed to it during recreational use of waters.  One way toxicologists assess the potency of a toxin is a measurement called LD50, or "lethal dose, 50%."   The LD50 is often expressed as the µg of a toxin per kg of body weight of an animal it is administered to by a specific route (e.g., oral, intramuscular, intravenous, etc.).  The 50% is telling us that when adminstered that dosage in µg agent/kg body weight, 50% of the animals exposed would likely die.  To kill animals twice as large of the same species, it would take twice as much toxin.  Such extrapolations cannot be legitimately made across species; mouse data can’t be extrapolated to humans by weight alone.   I previously assembled a table of LD50 values for Microcystin-LR , and it demonstrated the impact of animal and administrative route on the measured LD50.  In very general terms, though, the lower the LD50, the more “toxic” something is expected to be.

So, how does one compare the toxicity of Microcystin-LR against other nasty things, like sarin that was allegedly used by Syria recently, or saxitoxin, which has killed people that have eaten contaminated oysters?  In practice, human data are rarely available (thankfully!), and there are not studies that do a direct head-to-head comparison of, for example, sarin and Microcystin-LR.  As such, the best one can do is compare data from different studies that are similar with respect to their design, and make one’s best judgment as to what the data suggest.  When I did this for oral doses of Microcystin-LR and a handful of common “poisons,”  the data suggested that Microcystin-LR is very toxic relative to arsenic and cyanide (see table below), but less toxic than another biological toxin, saxitoxin, and the poisons sarin, dioxin, and strychnine.  However, the values shown here are acute toxicities, so they really don’t address longer-term health effects like cancer.  In my next blog, I will discuss how the carcinogenicity of Microcystin-LR is poorly understood, and how more data are needed to help regulators and safety officers assess exposure scenarios.



LD50 (µg/kg)


Arsenic Mouse 26,000-48,000 EPA AEGLs 2007
Sodium cyanide Rat 15,000 EPA AEGLs 2010
Microcystin-LR Mouse 5000 LD50 values for Microcystin-LR, compiled by Beagle Bioproducts
Sarin Rat 125 Scremin et al., 2003
Dioxin Mouse 114 National Library of Medicine, for emergency responders
Saxitoxin Mouse 10 Humpage EPA abstractFalconer EPA chapter
Strychnine(rat poison) Rat 5 Government of Queensland factsheet
1These are not original sources, but rather are readily accessible sources online that the readers can view without a subscription.  


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