When taking a deep breath you draw a range of gases into your lungs from oxygen and nitrogen to carbon dioxide and argon along with traces of water vapour. But that same breath could also contain microscopic amounts of copper, iron, zinc and even chromium. Despite making up a tiny fraction of the pollutants in the air, transition metals have some of the most damaging impacts on our health.
A study has now assessed the abundance of various airborne transition metals in urban areas across the US. While the overall particulate pollution has decreased over the past two decades, particularly in urban areas, some cities have, however, seen a rise in the amount of air-borne transition metals. By studying the trends, researchers are beginning to pinpoint what the likely sources of various metals are and how their emissions can be better controlled.
Diluting concentrations
Clean air is a staggeringly good investment. Since 1990, the US has spent an estimated $65bn on implementing the 1990 Clean Air Act but gained $2 trillion in benefits. According to the Environmental Protection Agency, this year alone the act has prevented around 230 000 early deaths, avoided 120 000 emergency room visits, and stopped 5.4 million sick-days in schools and 17 million sick-days at work.
Particulate pollution is responsible for some of the worst health and economic impacts of air pollution with transition metals believed to be more damaging than other compounds. That is because the metals act as a catalyst and help to produce oxidants, which can lead to oxidative stress. “Oxidative stress has been linked with the genesis and progression of many different diseases – it’s why there is so much research and marketing for foods that contain antioxidants,” says Christopher Hennigan, an environmental engineer at the University of Maryland, Baltimore County.
There is a strong body of scientific research that shows that a transition to more sustainable energy sources will have co-benefits in air quality
Christopher Hennigan
Hennigan and colleagues analysed seven different transition metals over the period 2001 to 2016, across ten different urban areas — Atlanta, Baltimore, Chicago, Dallas, Denver, Los Angeles, New York City, Seattle, St Louis and Phoenix. They found that around a decade ago concentrations of nickel and vanadium in port cities were around five times higher than non-port cities but that the difference has now all but disappeared. “The reductions in port-cities were most likely from regulations on marine fuel sulphur content,” explains Hennigan. Concentration of small air pollution particles has risen by more than one-third
The strong downward trend in vanadium across all urban areas clearly matched the introduction of diesel-fuel regulations in 2006. Yet copper, meanwhile, has stayed stubbornly constant in most areas. “Our results suggest that vehicle brake-lining dust is a major source of copper,” says Hennigan. The team also found higher concentrations of iron in western cities (by around a factor of two) than cities in the east, most likely because soil and dust are major sources of iron and prevailing winds cross more land and carry more dust to western cities.
One puzzle, however, was chromium, which increased in cities in the east and midwest, with a distinct spike in 2013. “We don’t have a good explanation for this which indicates a gap in our understanding of chromium sources and their magnitude,” says Hennigan.
The findings confirm how beneficial air-quality legislation has been for the US. It also makes a strong case for continuing to improve air quality with Hennigan believing there are still big gains to be make. “There is a strong body of scientific research that shows that a transition to more sustainable energy sources will have co-benefits in air quality,” he says.
The research is published in Environmental Research Letters.
