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GUEST AUTHOR: Alison Bernstein, PhD is a neuroscientist, who studies the role of epigenetics and environmental exposures in Parkinson’s disease. Follow her on Facebook and G+, where she writes as “Mommy PhD”, and on Twitter @mommyphd2.

While I love to see that my explainer essay “Glyphosate Vs. Caffeine: Acute and Chronic Toxicity Assessments Explained” has been widely shared and read since it was published, I have seen it used to support the very arguments that it was meant to critique. Because of this observation, I’d like to clarify a few points.

• This piece began from a frustration at the confusion over chronic and acute toxicity metrics being used interchangeably. When consumers express concern over chronic toxicity, science advocates often cite acute toxicity metrics. On the other side, when science advocates talk about chronic toxicity, anti-chemical activists challenge them to drink a glass of [insert chemical here], which would be an experiment in acute toxicity, but tells us nothing about chronic toxicity.

• The main purpose of this piece was to define measures of acute and chronic toxicity, explain how those are calculated and used, and explain how regulatory agencies currently use these metrics to determine “safe” levels. (Safe is in quotes because safety and risk are always relative – “safe” being commonly meant to refer to relatively low risk.) It is not an exploration of what level of risk should trigger regulation, which is a related, but separate, issue.

• There are some major differences between glyphosate and caffeine that make comparing their toxicity an apples-to-oranges comparison. The adverse effects for caffeine include things that some would classify as desired effects (you take caffeine to stay awake and thus report sleeplessness). A wider range of very subtle effects are used, which drives these metrics down for caffeine. Generally, when you drink caffeine, you WANT these effects but here they are considered to be toxic effects. These types of effects are not part of the regulatory assessments for pesticides (which mainly consider carcinogenicity and neurodevelopmental effects).

• The purpose of the comparing glyphosate to caffeine in that essay was to explore how these toxicity metrics are calculated and to illuminate how we perceive risk differently for chemicals that we are completely comfortable.

• An important take home from that piece was that our risk tolerance is often affected by things unrelated to actual risks. Is exposure to the hazard within our control? Does the risk involve our children? Our perception of risk from things we cannot control and from things that involve our children is greatly exaggerated.

• Another important take home is that toxicity metrics (acute or chronic) are not measures of risk. The are a benchmark of what level of exposure is deemed to be minimally risky. Risk assessment involves comparing actual (or potential) exposures to those benchmarks to determine if a risk exists.

• For more on hazards, risks and risk perception, I’d encourage you to read the Risk in Perspective series that I co-wrote with Iida Ruishalme. You can find it on SciMoms and Thoughtscapism.