Air pollution is linked to significantly higher rates of death in people with Covid-19, according to analysis.

The work shows that even a tiny, single-unit increase in particle pollution levels in the years before the pandemic is associated with a 15% increase in the death rate. The research, done in the US, calculates that slightly cleaner air in Manhattan in the past could have saved hundreds of lives.

Given the large differences in toxic air levels across countries, the research suggests people in polluted areas are far more likely to die from the coronavirus than those living in cleaner areas. The scientists said dirty air was already known to increase the risk of acute respiratory distress syndrome, which is extremely deadly and a cause of Covid-19-related deaths, as well as other respiratory and heart problems.

A separate report from scientists in Italy notes that the high death rates seen in the north of the country correlate with the highest levels of air pollution.

The scientists said their findings could be used to ensure that areas with high levels of air pollution take extra precautions to slow the spread of the virus and deploy extra resources to deal with the outbreak. Air pollution has already fallen because of widespread lockdowns, but the scientists said ensuring cleaner air in the future would help reduce Covid-19 deaths.

The study, by researchers at the Harvard TH Chan School of Public Health in Boston,analysed air pollution and Covid-19 deaths up to 4 April in 3,000 US counties, covering 98% of the population. “We found that an increase of only 1μg/m3 in PM2.5 [particles] is associated with a 15% increase in the Covid-19 death rate,” the team concluded.

A small increase in exposure to particle pollution over 15-20 years was already known to increase the risk of death from all causes, but the new work shows this increase is 20 times higher for Covid-19 deaths.

“The results are statistically significant and robust,” they said. The study took account of a range of factors, including poverty levels, smoking, obesity, and the number of Covid-19 tests and hospital beds available. They also assessed the effect of removing from the analysis both New York City, which has had many cases, and counties with fewer than 10 confirmed Covid-19 cases.

“Previous work showed that air pollution exposure dramatically increased the risk of death from [the] Sars [coronavirus] during the 2003 outbreak,” said Rachel Nethery, one of the Harvard team. “So we think our results here are consistent with those findings.”

Xiao Wu, a fellow team member, said: “This information can help us prepare by encouraging populations [with high pollution exposure] to take extra precautions and allocate extra resources to reduce the risk of poor outcomes from Covid-19. It is likely that Covid-19 will be a part of our lives for quite a long time, despite our hope for a vaccine or treatment. In light of this, we should consider additional measures to protect ourselves from pollution exposure to reduce the Covid-19 death toll.”

The authors said the results highlighted the need to keep enforcing existing air pollution regulations, and that failure to do so could potentially increase the Covid-19 death toll. They noted that the US Environmental Protection Agency suspended its enforcement of environmental laws on 26 March.

The study is being fast-tracked for publication in a major medical journal.

Prof JonathanGrigg, from Queen Mary University of London, said the study was methodologically sound and plausible, but had some limitations, for example, important factors such as smoking were not measured at the individual level.

“Clearly, we urgently need more studies, since locally generated particle pollution will bounce back once the lockdown is eased,” he said.

The US has the third highest death toll to date, after Italy and Spain. A second study focusing on Italy, published in the journal Environmental Pollution, said: “We conclude that the high level of pollution in northern Italy should be considered an additional co-factor of the high level of lethality recorded in that area.”

It noted that northern Italy was one of Europe’s most polluted areas and that the death rate reported up to 21 March in the northern Lombardy and Emilia-Romagna regions was about 12%, compared with 4.5% in the rest of Italy.

“It is well known that pollution impairs the first line of defence of upper airways, namely cilia, thus a subject living in an area with high levels of pollutant is more prone to develop chronic respiratory conditions and [is more vulnerable] to any infective agent,” it said.

Medical scientists warned in mid-March that air pollution exposure could make Covid-19 worse. Early research on Covid-19 had suggested that the weakened lungs of smokers and former smokers made them more susceptible to the virus.

While lockdowns have caused air pollution to fall dramatically, a comprehensive global review published in 2019 found that over long periods air pollution may be damaging every organ and virtually every cell in the human body.

• This article was amended on 8 April 2020 to correct a misspelling of the surname of Jonathan Grigg.