Yes, There Is a “Safe Concentration” of Pesticides

When O Estado de São Paulo ran the headline in August 2019 — “There Are No Safe Doses of Pesticides” — the immediate public reaction was far less explosive than the words suggested. But soon after, public prosecutors were citing the article as grounds for legal action. And it’s easy to see why: if such a sweeping claim were backed by solid science, it would demand nothing less than a worldwide overhaul of agriculture — and of toxicology itself, as a scientific discipline.

But is the science solid? In truth, nobody knows. The researcher behind the claim, immunologist Monica Lopes-Ferreira of São Paulo’s Butantan Institute, never submitted her results to peer review, nor did she publish them in full for scrutiny by experts — the gold standard of science. She also refused requests from other scientists (including the author of this article) to share her raw data for independent analysis.

While no one is strictly obliged to share data, withholding it raises red flags. Many journals require transparency as a condition of publication. And based on the limited information available — combined with what toxicology already knows — any strong conclusions drawn from her work would be, to put it politely, highly questionable.

Worse, Lopes-Ferreira chose to announce her findings in the media, to a lay audience, before presenting them to experts for review. Historically, that’s a hallmark of pseudoscience. Immanuel Velikovsky’s claim that Venus was a fragment spit out by Jupiter bypassed astronomers and went straight into a popular book. In 1989, chemists Martin Fleischmann and Stanley Pons held a press conference to tout “cold fusion” before subjecting their methods to peer review. The Dose Makes the Poison

Around the time Portuguese sailors first reached Brazil, Swiss physician Paracelsus laid down a principle that still guides toxicology today: “All things are poison, and nothing is without poison; it is the dose that makes the poison.”

Some substances are deadly in minuscule amounts — botulinum toxin, for example. Yet, in tiny doses, that same toxin is safely used in medicine.

So the blanket claim that no safe dose exists for an entire class of molecules — especially molecules “coincidentally” grouped as synthetic pesticides — is implausible at best, and dangerous at worst. Dangerous because people and policymakers take scientists seriously. And that’s good: public trust in science has given us vaccination campaigns, sanitation systems, and environmental protections. Misusing that trust with catastrophic but unsupported claims can fuel needless fear, drive bad policy, and, over time, weaken science’s ability to raise legitimate alarms when they matter most. The Study in Question

According to Estadão, Lopes-Ferreira tested ten pesticides commonly used in Brazil on zebrafish embryos (Danio rerio). This tiny tropical fish — known locally as paulistinha — is often used to study reproductive and developmental toxicity.

She reported using concentrations recommended by Brazil’s health regulator, Anvisa, yet said that even these supposedly “harmless” doses killed every embryo. For glyphosate, malathion, and pyriproxyfen, she cited a “minimum concentration” of 22 mg/L.

The problem? Published studies show zebrafish embryos begin to die at 10 mg/L, half the concentration she claimed was safe. Researchers at the University of Exeter noted even that was an unrealistically high dose, rarely, if ever, found in natural conditions. In other words, her “low dose” was already double what experts consider lethal. How Toxicologists Measure Safety

Toxicologists use two main benchmarks:

LD50 (lethal dose for 50%): the single-dose amount that kills half of test animals.

Acceptable Daily Intake (ADI): the maximum amount that can be ingested daily over a lifetime without harm.

For perspective: acutely, coffee is more toxic than glyphosate. Table salt and paracetamol are more dangerous than many pesticides if taken in a single gulp. That doesn’t mean coffee or salt are “poisons” in practice — only that dose matters.

For chronic exposure, regulators rely on ADI, which builds in a 100-fold safety margin from the highest dose that caused no effect in lab animals.

On top of that, regulators set Maximum Residue Limits (MRLs) in food and safe levels in drinking water. For glyphosate, Brazil’s drinking water limit is 0.5 mg/L — far below the concentrations used in Lopes-Ferreira’s zebrafish tanks. Expert Reactions

Agronomist Caio Carbonari of São Paulo State University put it bluntly: “From a scientific point of view, the result is irrelevant. 22 mg/L is absurdly high. Under real-world use, such concentrations would never occur.”

Toxicologist Flavio Zambrone agreed: “The dosages are extremely high, and zebrafish are not the standard for human toxicology. Without a proper dose-response curve — from no effect to severe effect — you can’t define safety.”

Angelo Trapé, a retired professor of toxicology at Unicamp, added: “If she had used table salt at those concentrations, the fish would have died too. This isn’t persecution. It’s simply bad science.”

Even reports from the World Health Organization show typical environmental glyphosate concentrations are around 0.01 mg/L — a thousand times lower than the levels tested in her lab. Conclusion

Until the Butantan study is published, peer-reviewed, and its data made transparent, its claims can’t be taken seriously. What it really shows — if anything — is that you can’t raise fish in tanks saturated with pesticides. Hardly a revelation worth rewriting toxicology for.

As Embrapa researcher Décio Gazzoni quipped: “The only real conclusion is that you can’t farm fish in pesticide spray tanks.”

Science thrives on evidence, transparency, and review. Without those, sweeping claims like “there is no safe dose of pesticides” are not just scientifically flimsy — they risk misleading the public and undermining the credibility of science itself.

✍️ Natalia Pasternak is a researcher at the Institute of Biomedical Sciences (ICB) at the University of São Paulo, national coordinator of Pint of Science Brazil, and president of the Instituto Questão de Ciência.