By Joe Schwarcz —
Pesticides are designed to kill insects, fungi, and weeds. So, it is reasonable to wonder whether they can harm us as well. Recent reports of pesticide residues in berries exceeding Canadian limits sound alarming, but exceeding a regulatory limit doesn't necessarily mean exceeding a safety threshold. How much risk is really involved? Time to crunch some numbers!
I've had several people ask me if I have second thoughts about my berry consumption in light of a swarm of news stories about some pesticide residues in Driscoll berries “exceeding Canada’s safety standards.” I’ve often talked about my breakfast of a mix of blueberries, strawberries, raspberries and bananas smothered in unflavoured Greek yogurt and topped with Fiber One cereal, so the question about my being concerned was not unreasonable.
To cut to the chase, I’m not concerned. I am quite aware of how “Maximum Residue Levels” or “MRLs” are determined and am equally aware of studies that place berries comfortably in the “healthy food” category. I think the benefits of eating berries outweigh any theoretical risk posed by pesticide residues, even in the rare instances when these exceed the Maximum Residue Levels.
I reach this conclusion based on what I’ll call an educated guess, because the trial that would give us a conclusive answer can't be done. That would require an experimental group and a control group matched in every possible way, with the only difference being the types of berries consumed. One group would consume berries with pesticide residues that exceed the MRLs to the extent occasionally noted, and the other group would eat the same amount of berries with pesticide residue levels always below the MRLs.
Furthermore, the two groups would have to be large enough for the results to be statistically significant. They would have to be followed for decades since nobody thinks that berries with pesticides that exceed the MRL are going to cause disease today, tomorrow, or next week. Such studies are administratively and economically impossible to carry out.
The current issue about “unsafe berries” was sparked by a “whistleblower” who alleges that he was fired by the Driscoll company after raising concerns about the company knowingly shipping berries to Canada with pesticide residues in excess of the MRL. Driscoll disputes his allegations.
First, the MRL measures whether a pesticide has been used appropriately and is not a determinant of safety! It does have a safety component, but it certainly is not a “safety limit” that, when exceeded, presents a risk. The MRL is based in part on the amount of pesticide residue that consumers could ingest in a realistic diet relative to the “Acceptable Daily Intake (ADI),” which is the amount of pesticide residue that can be consumed every day over a lifetime without appreciable risk of harm. The ADI is based on animal studies that determine the maximum dose that causes no detectable harm. This is called the No Observed Adverse Effect Level, or NOAEL. The ADI is then typically determined by dividing the NOAEL by a factor of 100 to account for differences between animals and humans and differences in sensitivity among people.
Next, regulators calculate the MRL. First, the amount of residue that remains on a crop after the pesticide has been applied according to approved label directions is quantified. Then, accounting for realistic dietary exposure, regulators determine whether the resulting exposure remains below toxicological reference values, including the ADI for long-term exposure and the Acute Reference Dose (ARfD) for short-term exposure.
The MRL can then be established. But remember that the ADI is based on the maximum dose that does not cause an adverse effect in an animal and typically includes a 100-fold safety factor! So, exceeding the MRL cannot be equated with compromising safety.
For example, the highest residue level found in Canadian surveys was for the fungicide captan on blackberries, at a level 34 times greater than the MRL. Even at that concentration, however, a typical consumer's exposure would remain well below the ADI. A 70 kg adult would have to eat about 2 kg of these berries a day to exceed the ADI.
Still, if MRLs are exceeded, it does mean that further scrutiny is appropriate because the pesticide may not have been used in the approved fashion. However, exceeding MRLs is rare, with Canadian monitoring programs generally reporting compliance rates in the high 90% range. The Driscoll case did involve shipments between 2022 and 2024 in which Canadian authorities found residues exceeding applicable MRLs. There was no recall because while the MRLs were exceeded, the residues were not judged to pose a health risk.
Now for the other side of the coin. Berries contain numerous bioactive compounds, particularly polyphenols that downregulate inflammatory pathways, reduce oxidative stress, improve the flexibility of arteries, and reduce the risk of type 2 diabetes. Berries also provide a good dose of fiber. And we have epidemiological studies for evidence of benefit! For example, the U.S. National Health and Nutrition Examination Survey (NHANES) of 37,232 adults found that people reporting regular berry consumption had about a 21% lower rate of all-cause mortality than non-consumers after adjustment for multiple factors.
There you have it. MRLs aren't set at random; they are based on sound science, include large safety margins in the underlying risk assessment, and are rarely exceeded. So, I will keep eating my well-washed berries happily.
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Joe Schwarcz
Dr. Joe Schwarcz is Director of McGill University's “Office for Science and Society” which has the mission of separating sense from nonsense. He is well known for his informative and entertaining public lectures on topics ranging from the chemistry of food to the connection between the body and the mind.
