An immediate ban has been issued for the herbicide dacthal. What are the health risks?

Last week the Australian government cancelled the registration of all products containing chlorthal dimethyl, a weedkiller commonly known as dacthal.

No phase out period applies. The cancellation is immediate, due to the risks it poses to human health – primarily unborn babies.

This means using dacthal as a chemical agricultural product “is now illegal”, according the Australian Pesticides and Veterinary Medicines Authority.

So what has changed? What are the health risks of being exposed to dacthal – and how long have we known about them?

What is dacthal?

Dacthal and chlorthal dimethyl are alternative names for dimethyl tetrachloroterephthalate, or DCPA. This is a herbicide registered to control weeds in both agricultural and non-agricultural settings.

Dacthal works by inhibiting auxin , a growth hormone in plants which promotes the development of buds, roots and lengthening cells.

It is used to selectively kill annual grasses and many other common weeds, without killing turf grasses, flowers, fruits and vegetables. Dacthal is applied before weeds emerge, often when still in their seed stage.

In Australia it is used in twelve herbicide products. All have been cancelled as of October 10 2024.

Farmers and retailers are allowed to hold products until they’re recalled, but must not use them. The government says it will provide information about product recall shortly.

What are the health risks?

As dacthal targets a hormone found only in plants, for adult humans and mammals the chemical has limited acute and subchronic toxicity . This means brief exposure to high levels of dacthal, or longer-term exposure to modestly high levels, have no effect.

However there is a health risk for unborn babies whose mothers have been directly exposed. This could be through mixing the chemical, loading and applying it, or from residue on treated crops – for up to five days after first applied.

The chemical has been linked to low birth weight, and life-long impacts , which can include impaired brain development and motor skills.

The government has advised pregnant agricultural workers who are concerned to speak to their clinician.

What changed?

Safety data for chemicals such as pesticides are periodically reevaluated. This is to see if any new risks have become apparent with advances in technology and our understanding of biology.

In 2013, the United States Environmental Protection Agency called for fresh safety data to look at effects of dacthal on thyroid hormones.

Fast forward to 2022. In that time, the company producing dacthal had failed to produce the required study. So the US Environmental Protection Agency issued a notice it would suspend technical-grade products containing dacthal.

In response, the company submitted a thyroid study performed in rats. This study showed dacthal could affect thyroid function at doses lower than previously known.

The US government determined this did not change recommendations for adults. However dacthal may affect thyroid function of a fetus at lower doses than those those that harm adults.

What did the study in rats find?

Dacthal was found to inhibit two thyroid hormones in rat pups whose mother had been exposed while pregnant.

There was a 35-53% decrease in the hormone triiodothyronine, known as T3. And for thyroxine (T4), rat pups experienced a 29-66% decrease after their mother’s exposure.

Decreases in these two hormones are associated with risks to unborn children including low birth weight and impaired brain development, IQ and motor skills.

Of particular concern was the effects occurred at much lower levels than previously thought. The decreases in T3 and T4 occurred in rat pups exposed to levels of dacthal ten times lower than the safe threshold for their mothers. This means pregnant rats exposed to dacthal at those levels had no adverse effects, but their unborn babies did.

Exactly how the chemical caused decreases in T3 and T4 in rat pups is not clear.

However the Australian Pesticides and Veterinary Medicines Authority considers this study relevant to humans. The changes in regulation are based on the potential harms if unborn babies are exposed via their mothers.

What exposure is safe?

The rat study was used to calculate maximum levels of exposure for pregnant workers. This maximum – 0.001 mg dachtal/kg body weight/day – was considered appropriate to reduce risk to the unborn child (and was not expected to harm adults).

However, the maximum acceptable level was exceeded in all estimates of exposure to dachtal. This was the case even when the person was wearing protective clothing, gloves, and using a respirator.

Even under stringent safety conditions, potential harms to an unborn child could not be ruled out. For this reason the US stopped sale of dacthal via an Emergency Order on 6 August 2024. Australia has since followed suit with its own ban.

How long have we known about this?

The US government only received the thyroid information in 2022. It then had to determine whether the levels of exposure under real world conditions would equate to risk in humans.

This is not straightforward, as the pesticide is used under a variety of conditions, including:

• mixing and preparing the pesticide using personal protective equipment
• downstream spay drift
• treatment of lawns and exposure to the lawn after treatment.

Each of these scenarios requires careful analysis of potential risks.

In addition, exposure can be through inhalation and/or skin contact. All this must be taken into account and these calculations take time.

Should I be worried?

If you were not pregnant and using personal protective clothing while using or applying dacthal herbicides, this is little cause for worry. Your exposure is below the maximum limit.

But if you were pregnant when using dacthal pesticides, please consider consulting your child’s paediatrician.

Ian Musgrave has received funding from the National health and Medical Research Council to study contaminants in herbal medicines. He has received ARC funding for studying Alzheimer's disease in the recent past. He is a member of the Science Communicators South Australian Branch.