An immediate ban has been imposed on the herbicide dactal. What are the health risks?

Last week, the Australian government revoked the registration of all products containing chlorothal-dimethyl, a herbicide commonly known as dactal.

There is no phase-out period. The cancellation is immediate, due to the risks it poses to human health, especially 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 dactal exposure – and how long will we know about them?

What is dactal?

Dacthal and chlorothal-dimethyl are alternative names for dimethyltetrachloroterephthalate or DCPA. This is a herbicide registered for the control of weeds in both agricultural and non-agricultural environments.

Dacthal works by inhibiting auxin, a growth hormone in plants 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 the weeds emerge, often 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 hold products until they are recalled, but are not allowed to use them. The government says it will soon provide information about the product recall.

What are the health risks?

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

However, there is a health risk to unborn babies whose mothers are directly exposed. This can be done by mixing the chemical, loading and applying it, or by residue on treated crops up to five days after the first application.

The chemical has been linked to low birth weight and lifelong consequences, including impaired brain development and motor skills.

The government has advised pregnant farm workers who are concerned to speak to their doctor.

What has changed?

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

In 2013, the U.S. Environmental Protection Agency called for new safety data to investigate dacthal’s effects on thyroid hormones.

Fast forward to 2022. At that time, the company producing dacthal had failed to produce the required research. As a result, the U.S. Environmental Protection Agency issued a notice that it would suspend technical-grade products containing dacthal.

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

The US government determined that this did not change the recommendations for adults. However, Dacthal may affect a fetus’s thyroid function at lower doses than those harmful to adults.

What has the research in rats yielded?

Dacthal was shown to inhibit two thyroid hormones in rat pups whose mothers had been exposed during pregnancy.

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 exposure from their mother.

A decrease in these two hormones is associated with risks for unborn children, including low birth weight and reduced brain development, IQ and motor skills.

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

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

However, the Australian Pesticides and Veterinary Medicines Authority considers this research relevant to humans. The regulatory changes are based on the potential harm if unborn babies are exposed to the disease 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 thoughtal/kg body weight/day – was considered appropriate to reduce the risk to the unborn child (and was not expected to harm adults).

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

Even under strict safety conditions, potential harm to an unborn child could not be excluded. For this reason, the US stopped the sale of dacthal via an emergency order on August 6, 2024. Australia has since followed its own ban.

How long have we known about this?

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

This is not easy, because the pesticide is used under a variety of conditions, including:

• mixing and preparing the pesticide using personal protective equipment
• downstream sterilization 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 occur via inhalation and/or skin contact. All this must be taken into account and these calculations take time.

Should I be concerned?

If you were not pregnant and wore personal protective clothing while using or applying dactal herbicides, there is no need to worry. Your exposure is below the maximum limit.

But if you were pregnant when you used dactal pesticides, consider consulting your child’s pediatrician.

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