Clear blue skies over the world’s megacities have become a visible mark of the pandemic. It’s a welcome change for many, not least because outdoor air pollution kills around 4.2 million people worldwide each year.

But when it comes to Covid-19, these risks to respiratory health are not the sum of their parts; researchers in the US are building a case that suggests air pollution has significantly worsened the Covid-19 outbreak and led to more deaths than if pollution-free skies were the norm. As well as predisposing the people who have lived with polluted air for decades, scientists have also suggested that air pollution particles may be acting as vehicles for viral transmission.

These new findings could have a significant impact on how governments choose to ease lockdowns in the coming months, as scientists say that improving air quality could play an important role in overcoming the pandemic.

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One recent study found that even small increases in fine particulate matter, known as PM2.5, have had an outsized effect in the US. An increase of just 1 microgram per cubic metre corresponded to a 15% increase in Covid-19 deaths, according to the researchers, led by Xiao Wu and Rachel Nethery at the at the Harvard University T.H. Chan School of Public Health.