The study adds to mounting evidence showing huge amounts of lead remain in the soil in urban areas – a legacy from decades of adding the toxic metal to paint and petrol. “These findings do not come as a surprise,” said RMIT Associate Professor Suzie Reichman, who led the study. “Given it’s all in urban Melbourne, and a lot of the sites in central Melbourne are older and used to be industrialised, it does not surprise us that we got this result.” Associate Professor Reichman’s team tested soil from 136 vegetable gardens across Melbourne, and found that 21 per cent had soil lead levels that exceed 300 milligrams per kilogram. Older houses that may have been painted with lead paint in the past, houses in industrial areas, and houses near roads tended to have soils with the highest lead levels.

She acknowledged that the small sample size, and the fact the samples were all self-submitted rather than randomly obtained, had the potential to produce bias in the experiment. A similar study in 2017 found 40 per cent of home gardens sampled in Sydney exceeded 300 milligrams per kilogram of soil. Government regulations say soil lead above this level needs “further investigation”. Professor Mark Taylor, the first study’s co-author and Australia's preeminent lead expert, has franker advice: don’t garden in it. “If it exceeds 300, you should think about reducing exposure to those soils. “Gardeners need to be aware of where they are living. If they want clean, green gardens, they might want to look at how old their place is and what sort of contaminants might be in their yard.”

Lead can be taken up by vegetables grown in the soil, particularly root veggies such as onions and potatoes. Young children who may try to eat the soil are particularly at risk. Professor Taylor’s citizen-science program VegeSafe has sampled the soil sent in by thousands of backyard gardeners across Australia, allowing him to build a remarkably clear picture of lead contamination. “We have shown this contaminant is extremely persistent. It’s present in soils, dusts, veggie gardens, and it presents a potential risk of harm if it’s inhaled or eaten.” he says. Last year, in a study on bees, his Macquarie University team proved that lead can travel through an ecosystem; they discovered bees were picking up lead from flowers and taking it back to their hives. Honey from highly exposed hives had lead concentrations up to 295 micrograms per kilo.

The World Health Organisation says there is no known safe blood lead concentration. Even extremely low levels may be associated with decreased intelligence. Extrapolating from American data on blood lead levels in the population, Professor Taylor believes as many as 100,000 Australian children may have levels high enough to affect intelligence. In the lead-smelting town of Port Pirie, testing shows a small but persistent reduction in IQ among local children. Lead has also been linked to crime; in one hypothesis, children exposed to lead have poorer impulse control, making them more likely to commit violent crimes as adults. A study led by Professor Taylor in 2016 found airborne lead levels in areas of NSW strongly predicted violent crime levels 21 years later. “We’re not saying causation, we’re saying association,” he cautions.

“But we need to consider the impact of these chemicals we use in the long-term. The initial problem may go away, but you’re often left with this legacy.” What you should do: The Environment Protection Authority advises anyone with concerns to get their soil tested through Macquarie University’s VegeSafe program. If soil is contaminated, gardens can still be made in raised vegetable beds filled with imported clean soil. Vegetables known to accumulate lead including lettuce, amaranth and caraway and root vegetables such as carrots, onions, turnips and radishes should not be cultivated.