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We know forests absorb carbon dioxide, but, like a sponge, they also soak up years of pollutants from human activity. When bushfires strike, these pollutants are re-released into the air with smoke and ash.

air samples to "fingerprint" the pollution sources.

The data show that the source of the lead ranges from natural origins derived from the weathering of rocks to those from leaded petrol emissions.

Leaded petrol started being phased out due to environmental and health concerns, and hasn't been used in vehicles since 2002. Much smaller amounts are still used in AVGAS—the fuel used to power small piston aircraft engines.

As a result, lead levels in Sydney's air decreased dramatically from 1984 to 2004. At the same time, the lead isotopes in the air changed.

The lead used in NSW predominantly came from the mines at Broken Hill. Broken Hill lead has a very different isotopic signature to the lead found in Sydney's main bedrock, Hawkesbury Sandstone. This corresponds to previous research showing ash from contained Broken Hill lead.

In 1994, lead in Sydney's air was closer to the Broken Hill lead signature. By 2004, the lead isotopes in air resembled natural Sydney rocks. But during bushfires in 2001-2002 and 2004, the lead that was released started to look more like Broken Hill lead again.

This shows that the forests had absorbed leaded petrol emissions over the 70 years it was used and stored them. When the forests went up in flames, the lead was remobilised along with smoke and other bushfire particles.

What does that mean for our health?

Breathing in bushfire smoke is a serious . Bushfire smoke resulted in more than 400 excess deaths during the devastating 2019-2020 bushfires.

Recently, the focus of air quality and health research has shifted to very fine particles: "PM2.5". These are particles 2.5 microns or smaller that can penetrate deep into our lungs. During the Black Summer bushfires of 2019-2020, PM2.5 levels reached , more than three times the Australian air quality criteria of 25 micrograms per cubic meter.

While our study shows that potentially toxic metals were more elevated in the atmosphere during bushfires, the concentrations were not likely to be a health risk. The main risk is from the total concentration of fine particles in the air, rather than what they are made of.

The concentrations of the trace metals measured during the four major bushfires in our study were below and criteria. The period of increased exposure was also very limited, further reducing risk.

Nevertheless, it's important to minimize exposure to all chemical contaminants. This is because many, such as lead, have no safe lower exposure limit and the effects are often proportionately greater at the first and .

A lingering legacy

It's not just Australian forests that have a lingering toxic legacy. In Ukraine and Belarus, from Chernobyl have been released during bushfires.

And as global knowledge of the of pesticides grew, we stopped using them. Yet we far from civilisation in the frozen Arctic, waiting to be released when the ice melts.

Metals such as lead, copper, manganese and uranium continue to be mined and processed in Australia. The most significant are felt by the immediately surrounding communities, , as on surfaces and are later ingested.

Globally, the continues to contaminate communities and environments, particularly those in .

Yes, our modern lifestyles these metals and other toxic chemicals. So, we must mine, use and dispose of them with great care, because once in the environment, they go away.

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