Two main risk factors are currently known to raise the chance of dying from the novel coronavirus that has brought the world to a halt: being old and having a weak immune system.
Air pollution makes the second of those more likely.
"If you live in a polluted area, your lungs are compromised like somebody who smokes, so you're more susceptible to the coronavirus," said Kofi Amegah, an epidemiologist and air pollution expert at the University of Cape Coast in Ghana.
Dirty air, which claims more than 7 million lives a year, could make COVID-19 more deadly by contributing to chronic health conditions that leave patients weak in the face of infection.
The European Public Health Alliance said last week that air pollution is likely to cut survival chances from COVID-19.
Research on previous outbreaks has also suggested bad air makes viruses more deadly and spread further. A study of SARS-CoV-1 victims in 2003 found that patients were twice as likely to die in regions where air pollution was high rather than low. Even in regions that were only moderately polluted, the risk of dying was 84% higher.
If a similar dynamic exists for COVID-19, it could add pressure on the critical care units of hospitals in smoggy cities with rapidly rising cases, such as Madrid, London and New York. It could also spell trouble for countries in the global south where most people burn wood, dung, kerosene or coal indoors to cook and heat their homes.
In northern Italy and the Chinese city of Wuhan, home to high levels of pollution and some of the most severe outbreaks to date, preliminary data suggests that particulate matter may already have played a role in overwhelming health care systems.
PM2.5 — particulate matter smaller than 2.5 microns in diameter, less than the width of a human hair — can penetrate the lung barrier and enter the bloodstream, raising the risk of developing heart and lung disease.
The case fatality rate in China was nine times higher for people with cardiovascular disease and six times higher for patients with diabetes, hypertension and respiratory disease than it was for people without underlying health conditions, a joint study by the World Health Organization and China found in February.
In Italy, health officials reported in March that 99% of a sample of patients who died from COVID-19 had an underlying illness — with almost half the deceased having suffered from three or more — though the sample was not drawn randomly and may not represent the population. The most common ailments were high blood pressure, heart disease and diabetes.
The WHO says the pandemic is too recent to draw a link between air pollution and the deadliness of COVID-19, but this shouldn't stop countries from acting.
"Whether or not we have this correlation between COVID-19 and air pollution, we need to reduce air pollution no matter what," Maria Neira, director of public and environmental health at the World Health Organization, told DW.
"Stop smoking and reduce the levels of air pollution — that is a recommendation we can make even without having more evidence."
Together with ozone pollution, PM2.5 particles shave almost three years off our lives, a study in the journal Cardiovascular Research found last month. The global loss of life from outdoor air pollution is 10 times greater than that of all forms of violence put together.
Moreover, about nine in 10 premature deaths caused by air pollution — including toxic gases NO2 and SO2 — hit people in low and middle-income countries. Even within rich cities in Europe and North America, working class, black and ethnic minority communities tend to breathe the dirtiest air.
Across sub-Saharan Africa, women are particularly exposed to pollutants from indoor cooking.
"For these women, their lung systems are compromised," said Amegah from Ghana's University of Cape Coast, adding that if COVID-19 spreads they will be especially vulnerable.
"We pray and keep our fingers crossed we don't see the levels [we're seeing] in northern Italy and China."
As well as weakening the body, airborne pollutants could even act as a carrier of the new coronavirus and allow it to survive in the air attached to particulates, a team of Italian researchers suggested in March.
High concentrations of particulate matter in parts of northern Italy in February may have "boosted" the spread of the epidemic this way, according to a position paper published by the Italian Society of Environmental Medicine that has not yet been peer-reviewed.
Other scientists have cast doubt on this, pointing out that there are no reported cases of this coronavirus spreading in the air and that people are the main vector of transmission.
"It's good to reduce air pollution to promote health, even to help decrease preconditions that could aggravate coronavirus, such as asthma, but I cannot see [air pollution] as an important contribution to the discussion about containment of the virus," said Jos Lelieveld, director of atmospheric chemistry at the Max Planck Institute and lead author of the study on deaths from air pollution.
As coronavirus cases rise exponentially around the world, lockdowns to stop its spread have reduced pollution levels.
Satellite images of China and Italy show striking drops in NO2, a toxic gas that inflames the airways, as governments closed factories and kept cars off the streets. The drop in air pollution in China may even have saved more lives than were lost from COVID-19, a study that has not yet been peer-reviewed suggested on Friday, though this comparison does not factor in the lives that would have been lost had the coronavirus spread unchecked.
Not all of the fall in air pollution seen from space can be attributed to lockdowns, either. Air pollution is higher in colder months anyway because people heat more and drive cars more often, so it tends to fall around this time of year, said Christian Retscher of the European Space Agency.
"Certainly, we see an effect of the coronavirus on NO2 … We see an additional effect [but] we don't know the precise number."
While lockdowns have helped clean the air, it is also uncertain how long they will keep pollution levels down.
"Once the crisis is over, and we see this in China, there's a temptation to compensate for the weeks and months lost," said Zoltan Massay-Kosubek, a policy expert for clean air and sustainable transport at the European Public Health Alliance.
Nonetheless, this shows that air pollution can be reduced and lives saved, said the WHO's Neira. "Now we need to maintain that — not the fact that we'll be confined, but reducing the air pollution levels outside."