Almost half of London’s lead-polluting air comes from leaded gasoline that fueled cars more than half a century ago, a study found. Although leaded gasoline was banned in 1999, the toxic metal in the form of leaded dust has remained on the streets of the British capital, blown up by the wind and traffic.
Lead emissions have now fallen below the legal limit, but there is still around 50 times more lead in the air than the natural background value – a situation study director Dominik Weiss from Imperial College London compares it to secondhand smoke. Neither amount is likely to be completely safe. “We have to live with this long-term exposure,” he says.
Like many other lead compounds, tetraethyl lead is highly toxic – chemists already knew this at the beginning of the 20th century. Its lipophilicity allows it to get into the brain, making it immune to chelation therapy that can remove other heavy metals from the body. High levels of lead trigger acute poisoning; Long-term exposure causes memory loss, neurological problems, and a decrease in cognitive function, especially in children.
However, none of this prevented tetraethyl lead from becoming the fuel additive of choice for many decades. Introduced by General Motors in 1921, it was a desperate move after years of looking for a compound that would reduce engine knock. At the time, it was classified as safe by the US Public Health Service. Tetraethyl lead and the inorganic lead salts, which leave car exhaust fumes as a fine aerosol, became the main source of environmental lead pollution in the course of the 20th century. It would be until the 1990s and early 2000s for most countries to finally ban tetraethyl lead as a fuel additive.
In the UK, air levels of lead dropped dramatically after the ban. In 1990, 74% of lead emissions came from leaded petrol; from 2000 it was only 1%. But that decline has slowed, perhaps more so than expected. “We are now at half the average we found at the beginning of the century, so it was a slow decline,” says Eléonore Resongles of the University of Montpellier, France, who co-led the study with Weiss. ‘It has been stationary since 2010.’
One of the reasons is that the lead from the cars of the 60s and 70s just got stuck in the road dust and topsoil. In their study, the Weiss and Resongles team shows that more than 40% of the lead found in the London air today comes from historic leaded gasoline. The remaining 60% likely comes from either brake and tire wear – auto parts may contain lead as a by-product – or other historical non-gasoline sources, such as coal burning. “Even if the concentrations drop, what was there in the past is basically recycled,” explains Weiss.
From 2014 to 2018, the team collected suspended lead particles on Marylebone Road and on the roof of Imperial College in central London. By comparing the isotope ratios of these metal particles with historical data, the researchers discovered the tell-tale signature of leaded gasoline.
Lead from gasoline is less radioactive than lead from other sources. While natural lead is responsible for each of the four stable isotopes (204Pb, 206Pb, 207Pb and 208Pb), radiogenic lead is the end product of radioactive decay chains in uranium and thorium ores. In contrast to the UK natural background levels or coal-derived lead, the leaded gasoline used in the UK had a low a 206Pb /207Pb, and thus high 208Pb /206Pb explains Resongles. ‘It also had lower ones 206Pb /204Pb and 208Pb /204Pb ratios. This is because the lead ore used to make lead additives comes from geologically ancient lead deposits and was mainly imported from Australia or Canada. ‘
The researchers estimate that between 45 and 800 kg of lead stored in the ground and road dust circulates in London’s air each year. The reason for this wide range is that the emission factors of vehicles that throw up road dust while driving are associated with great uncertainties.
Stefan Reis, Head of Atmospheric Chemistry and Effects Research at the UK Center for Ecology and Hydrology, says the study is the missing link to explain why measurements and models sometimes mismatch. “A look at the evolution of emissions over time, along with the concentrations, shows that emissions and concentrations drop for many pollutants. For some pollutants, however, it does not decrease as much as you would expect from the decreasing emissions. Here I find this type of study extremely useful in finding out what might be the reason for this.
No safe limit?
Whether the remaining lead is a cause for concern is unclear as little is known about long-term low exposures. Weiss explains that lead is likely a non-threshold metal, meaning there are no levels below which it could be considered non-toxic. “Many people believe that lead is no longer an issue because lead is no longer emitted,” says Reis. “As research progresses, we are finding more and more evidence of health effects in general with lower air pollution.” For this reason, the World Health Organization is currently considering lowering the safety limits for particulate matter, he emphasizes.
“In the next 50 years we can imagine that [lead] the values just stay as they are if we don’t take any additional measures, ”says Resongles. She says it is now time to examine Londoners’ blood lead levels and identify possible routes of exposure, which could include old houses with lead in paint and pipes. “Then maybe we can come up with strategies and measures to reduce that exposure,” she says.
Reis says removing lead nationwide may not be feasible, but street cleaning and washing could reduce concentrations in densely populated areas. Another option, explains Resongles, is to cover floors in lead hotspots with a layer of unpolluted soil.
But the most effective policy could simply be to reduce emissions of air pollutants in general. “We know that even with fully electric vehicles, tire and brake wear and abrasion of the road surface etc. still occur, which creates particles laden with metals,” says Reis. ‘The question is [to find] Measures to reduce emissions in the first place, because once you have them in the environment it is really difficult to get them out. ‘