If you’ve ever walked past a council worker spraying weeds on a hot summer afternoon,
…you’ve probably noticed the mist hanging in the air a little longer than you’d like. Maybe you’ve wondered: what happens to all that spray when the sun is beating down, the air is still, and we’re all already struggling with the heat?
Most of us think of glyphosate — or more accurately, glyphosate-based weedkillers like Roundup® — in terms of how much is used or where it ends up: in our food, in our waterways, or in our parks. But new research suggests another factor matters just as much: when it’s sprayed. It turns out that temperature — something as simple as whether it’s 19 °C in the morning or 28 °C in the afternoon — can change the way these herbicides interact with living systems.
And this doesn’t just affect frogs and insects. Heat changes the way humans absorb and respond to chemicals too. Higher temperatures can open our pores, increase sweat, and put extra strain on our bodies’ detox systems — all of which may make glyphosate-based weedkillers more harmful to those applying them or caught downwind in spray drift.
The lesson is clear: glyphosate formulations like Roundup® aren’t just a chemical risk — they’re a chemical risk that grows with the heat.
Why Safety Studies Don’t Tell the Whole Story
When regulators talk about glyphosate being “safe at approved levels,” they’re relying on data supplied by the very companies that manufacture and profit from it. These so-called safety studies are carried out under laboratory conditions: stable temperatures, filtered air, controlled exposure. It’s the scientific equivalent of testing a car only on a smooth racetrack, then declaring it safe for every pothole-filled road in the real world.
What’s more, many of these studies test glyphosate in isolation — the single “active ingredient.” But that’s not what gets sprayed on crops, roadsides, or playground verges. What’s actually used are glyphosate-based herbicides like Roundup®, chemical cocktails that include surfactants and solvents designed to help glyphosate stick, spread, and penetrate. These added ingredients are rarely tested in safety trials, even though research shows they often make the formulation far more toxic than glyphosate alone.
And then there’s the missing variable of temperature. The newt study shows that even a small rise in heat — just four degrees warmer — was enough to tip glyphosate’s impact from manageable to neurotoxic. That detail never appears in the official paperwork.
So when producers say glyphosate is safe “when used as directed,” the question we should be asking is: safe for whom, and under what conditions? Because real-world spraying in summer heat with a full commercial mix doesn’t look anything like the carefully controlled environment of a laboratory.
The Science of Heat and Glyphosate-Based Weedkillers
So why does heat matter so much? The answer lies in how living systems cope with stress.
Every organism — from frogs to humans — runs on delicate biochemical balances. When temperatures rise, those systems are already working harder. Cells produce more oxidative stress, bodies lose more water, and natural defenses like antioxidants get stretched thin. Add glyphosate-based weedkillers like Roundup® into the mix, and the strain increases.
In the Serbian study on newt larvae, an environmentally realistic dose of a glyphosate-based product (22.5 µg/L as glyphosate) was tested at 19 °C vs 23 °C for 14 days. At 19 °C, larvae showed increased protein carbonylation (a marker of protein damage). At 23 °C, antioxidant defenses (CAT, GST, GR and GSH) were induced — which appeared to limit oxidative damage to lipids and proteins — but acetylcholinesterase (AChE), an enzyme essential for nerve signaling, was inhibited. In plain terms: warmer temperatures triggered neurotoxic effects that weren’t seen at cooler conditions .
This isn’t just about amphibians. On hot days, human bodies change too. Pores open, sweat increases, and blood circulation shifts. All of this can make it easier for chemicals to enter the body — and harder for our natural detox systems to cope. What might look like a “safe” level of exposure on paper can become harmful in real life, especially for those spraying or standing downwind.
The takeaway? Glyphosate-based weedkillers don’t behave the same way in all conditions. They become more dangerous as the temperature rises.
A Summer Scene Close to Home
Picture this: it’s January, the sun is high, and the air shimmers with heat rising off the pavement. A council contractor in a high-vis vest walks the verge with a spray wand, misting weeds along the roadside. A cyclist rides past. A mum pushes a pram on the footpath. A group of kids kick a ball nearby.
This is how glyphosate-based weedkillers like Roundup® are actually used in our communities — not in a controlled lab, but in the height of summer, when heat and exposure risks peak together. The fine spray doesn’t just vanish on contact; in still, hot air, it lingers longer, drifting where it doesn’t belong. For those applying it, the combination of heavy PPE gear (if worn), heat stress, and chemical exposure is a health hazard. For those passing by, it’s an involuntary inhalation. For insects and amphibians sheltering in roadside ditches or drainage channels, it’s another layer of chemical stress on top of rising summer temperatures.
Yet none of this is considered in the official “safe use” guidelines. On paper, glyphosate is safe if applied correctly. In practice, a hot summer afternoon in New Zealand looks a lot more like the conditions in the newt study — the very ones where glyphosate formulations tipped into hidden neurotoxic effects.
Practical Risk Factors That Increase With Heat
- Midday spraying → hot, still air means drift lingers longer.
- Human heat stress → already strains the body, may reduce tolerance to additional exposures.
- Warm, shallow water → higher concentrations for amphibians and aquatic insects.
- Formulation effects → surfactants and additives can magnify absorption and toxicity.
It’s Never Just Glyphosate
We’ve been using the shorthand “glyphosate” throughout this article, but it’s important to pause and remember: what’s being sprayed on our crops, parks, and road verges is never just glyphosate. It’s glyphosate-based weedkillers like Roundup® — full formulations that include surfactants, solvents, and other additives designed to make glyphosate work more effectively.
And those additives matter. Research shows that the complete formulations are often far more toxic than glyphosate alone. Surfactants such as POEA (polyethoxylated tallow amine) increase glyphosate’s ability to penetrate living cells — but also bring their own toxicity, especially to aquatic life and human cells. These “inert” ingredients can change the way glyphosate behaves in the body and the environment, creating risks that the original lab studies on glyphosate never accounted for .
We’ve covered this in more depth before — see our article Roundup vs Glyphosate Toxicity — but it’s worth restating here. Because when you combine these more toxic formulations with the added strain of summer heat, you end up with a chemical threat that’s even harder to dismiss.
Implications for New Zealand
For New Zealand, the message is clear: we can’t afford to keep treating glyphosate-based weedkillers as though their risks are fixed, predictable, and fully understood.
Councils and contractors are spraying these formulations in the height of summer, often around schools, parks, and footpaths where children and families are exposed. The newt study shows that temperature makes a measurable difference in how these chemicals behave — yet our “safe use” guidelines ignore time of day, season, or climate. Spraying at midday in January may be far more dangerous than spraying in the cool of winter, but our rules don’t make that distinction.
Food safety regulators also rely on outdated assumptions. Maximum Residue Levels (MRLs) are set without considering how heat can amplify toxicity, or the fact that real-world formulations are more harmful than glyphosate alone. A slice of bread or a bowl of cereal might meet official residue limits — but those limits were calculated under idealised conditions, not the reality of summer spraying, drift, and cumulative exposure.
Ecosystems face the same blind spot. Frogs, insects, and fish in our waterways are already under pressure from warmer summers and shrinking habitats. Adding glyphosate-based herbicides into the mix, especially in hot conditions, creates risks we’re not measuring — let alone regulating.
In short, New Zealand’s entire risk framework for glyphosate is built on narrow, producer-driven studies that don’t reflect the way these chemicals are actually used here. And that means we’re flying blind when it comes to protecting public health, food safety, and the environment in a warming climate.
Looking Ahead
The science is telling us something simple but profound: glyphosate-based weedkillers don’t act the same way under all conditions. Heat amplifies their risks. What looks safe in a cool lab can become harmful in the real world of hot summer spraying, lingering drift, and vulnerable ecosystems.
And yet, none of this is reflected in New Zealand’s regulations. Councils still spray Roundup® around our neighbourhoods at the hottest times of year. Food safety limits are still calculated on outdated assumptions. Ecosystems already strained by rising temperatures are left to carry an extra chemical burden that no one is monitoring.
If we are serious about protecting our health, our food, and our environment, we can’t ignore this blind spot. Time of day, season, and climate all matter — just as much as the chemical itself.
It’s time for New Zealand to ask harder questions: not just whether glyphosate is “safe,” but whether any level of exposure can be considered acceptable when heat and formulations make its risks so unpredictable.
Because the evidence is clear — in the summer sun, glyphosate-based weedkillers are even more dangerous than we’ve been told.
Resources & References
Behind every reassuring “safe when used as directed” claim lies a different story in the scientific literature. These resources reveal how temperature, formulations, and overlooked additives change the picture — and why official safety limits can’t be taken at face value.
Effects of glyphosate-based herbicide on oxidative stress and neurotoxicity parameters in newt larvae under temperature changes – 22.5 µg/L (as glyphosate), 14-day exposure, 19 °C vs 23 °C; antioxidant defenses induced at 23 °C; AChE inhibition at 23 °C.
Read the abstract on PubMed
Roundup vs Glyphosate Toxicity – Our breakdown of why real-world formulations like Roundup® are more toxic than glyphosate alone, and why regulators often overlook this critical difference.
Read the article on NoMoreGlyphosate.nz
Glyphosate-based herbicides are more toxic than glyphosate alone – Review showing surfactants amplify toxicity and raise risks for ecosystems and human health.
Read the review on Environmental Health
Polyethoxylated tallow amine (POEA) toxicity – A closer look at one of the most common Roundup® surfactants and its harms to aquatic life and human cells.
Read the POEA entry on Wikipedia
Taken together, these studies make one thing clear: glyphosate-based weedkillers don’t operate in a vacuum. Heat, co-formulants, and real-world conditions amplify their risks in ways regulators haven’t accounted for. The evidence is out there — the only question is whether we’re willing to act on it.
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