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

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

This means the use of dacthal as an agricultural chemical “is now illegal”, according to the Australian Pesticides and Veterinary Medicines Authority.

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

What is dacthal?

Dacthal and dimethyl chlorthal are alternative names for dimethyl tetrachloroterephthalate, or DCPA. It is a herbicide approved to control weeds in agricultural and non-agricultural environments.

Dacthal works by inhibiting auxin, a plant growth hormone that promotes the development of buds, roots and elongating cells.

It is used to selectively kill annual grasses and many other common weeds, without killing lawn grasses, flowers, fruits and vegetables. Dacthal is applied before weeds emerge, often while they are still in the seed stage.

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

Farmers and retailers are allowed to keep their products until recalled, but must not use them. The government says it will provide information on product recalls soon.

What are the health risks?

Because dacthal targets a hormone found only in plants, for adult humans and mammals the chemical has limited acute and subchronic toxicity. This means that brief exposure to high levels of dacthal, or long-term exposure to slightly elevated levels, has no effect.

However, there is a risk to the health of unborn babies whose mothers have been directly exposed. This may be due to mixing of the chemical, loading and application, or residue on treated crops, up to five days after the first application.

This chemical has been linked to low birth weight and lifelong effects, which can include brain development and impaired motor skills.

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

What has changed?

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

In 2013, the United States Environmental Protection Agency requested new safety data to examine the effects of dacthal on thyroid hormones.

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

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

The U.S. government has determined that this does not change the recommendations for adults. However, dacthal may affect fetal thyroid function at doses lower than those that harm adults.

What did the rat study reveal?

Dacthal was found to inhibit two thyroid hormones in pups whose mothers were exposed during pregnancy.

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

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

What is particularly concerning is that the effects occurred at much lower levels than previously thought. The decreases in T3 and T4 occurred in pups exposed to dacthal levels ten times below the safe threshold for their mothers. This means that pregnant rats exposed to dacthal at these levels had no adverse effects, unlike their unborn babies.

It’s unclear exactly how the chemical caused a decrease in T3 and T4 levels in the pups.

However, the Australian Pesticides and Veterinary Medicines Authority considers this study relevant to humans. The regulatory changes are based on the potential harm if unborn babies are exposed through their mothers.

What exposure is safe?

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

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

Even under strict safety conditions, potential dangers for an unborn child cannot be excluded. It was for this reason that the United States stopped the sale of dacthal via an emergency order on August 6, 2024. Australia has since followed suit with its own ban.

How long have we known this?

The US government did not receive the thyroid information until 2022. It then had to determine whether exposure levels in real-world conditions would amount to a risk to humans.

This is not simple, as the pesticide is used in a variety of conditions, including:

• mix and prepare the pesticide using personal protective equipment
• downstream spray drift
• lawn treatment and exposure of the lawn after treatment.

Each of these scenarios requires careful analysis of potential risks.

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

Should I be worried?

If you were not pregnant and used personal protective clothing when using or applying dacthal herbicides, there is no cause for concern. Your exposure is below the maximum limit.

But if you were pregnant at the time you used dacthal pesticides, consider consulting your child’s pediatrician.

This article is republished from The Conversation under a Creative Commons license. Read the original article.