Coronavirus has been detected on particles of air pollution by scientists investigating whether this could enable it to be carried over longer distances and increase the number of people infected.

The work is preliminary and it is not yet known if the virus remains viable on pollution particles and in sufficient quantity to cause disease.

The Italian scientists used standard techniques to collect outdoor air pollution samples at one urban and one industrial site in Bergamo province and identified a gene highly specific to Covid-19 in multiple samples. The detection was confirmed by blind testing at an independent laboratory.

Leonardo Setti at the University of Bologna in Italy, who led the work, said it was important to investigate if the virus could be carried more widely by air pollution.

“I am a scientist and I am worried when I don’t know,” he said. “If we know, we can find a solution. But if we don’t know, we can only suffer the consequences.”

Two other research groups have suggested air pollution particles could help coronavirus travel further in the air.

A statistical analysis by Setti’s team suggests higher levels of particle pollution could explain higher rates of infection in parts of northern Italy before a lockdown was imposed, an idea supported by another preliminary analysis. The region is one of the most polluted in Europe.

Neither of the studies by Setti’s team have been peer-reviewed and therefore have not been endorsed by independent scientists. But experts agree their proposal is plausible and requires investigation.

Previous studies have shown that air pollution particles do harbour microbes and that pollution is likely to have carried the viruses causing bird flu, measles and foot-and-mouth disease over considerable distances.

The potential role of air pollution particles is linked to the broader question of how the coronavirus is transmitted. Large virus-laden droplets from infected people’s coughs and sneezes fall to the ground within a metre or two. But much smaller droplets, less than 5 microns in diameter, can remain in the air for minutes to hours and travel further.

Experts are not sure whether these tiny airborne droplets can cause coronavirus infections, though they know the 2003 Sars coronavirus was spread in the air and that the new virus can remain viable for hours in tiny droplets.

But researchers say the importance of potential airborne transmission, and the possible boosting role of pollution particles, mean it must not be ruled out without evidence.

Quick guide Will there be a second wave of coronavirus?
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Epidemics of infectious diseases behave in different ways but the 1918 influenza pandemic that killed more than 50 million people is regarded as a key example of a pandemic that occurred in multiple waves, with the latter more severe than the first. It has been replicated – albeit more mildly – in subsequent flu pandemics.

How and why multiple-wave outbreaks occur, and how subsequent waves of infection can be prevented, has become a staple of epidemiological modelling studies and pandemic preparation, which have looked at everything from social behaviour and health policy to vaccination and the buildup of community immunity, also known as herd immunity.

Is there evidence of coronavirus coming back elsewhere?

This is being watched very carefully. Without a vaccine, and with no widespread immunity to the new disease, one alarm is being sounded by the experience of Singapore, which has seen a sudden resurgence in infections despite being lauded for its early handling of the outbreak.

Although Singapore instituted a strong contact tracing system for its general population, the disease re-emerged in cramped dormitory accommodation used by thousands of foreign workers with inadequate hygiene facilities and shared canteens.

Singapore’s experience, although very specific, has demonstrated the ability of the disease to come back strongly in places where people are in close proximity and its ability to exploit any weakness in public health regimes set up to counter it.

What are experts worried about?

Conventional wisdom among scientists suggests second waves of resistant infections occur after the capacity for treatment and isolation becomes exhausted. In this case the concern is that the social and political consensus supporting lockdowns is being overtaken by public frustration and the urgent need to reopen economies.

The threat declines when susceptibility of the population to the disease falls below a certain threshold or when widespread vaccination becomes available.

In general terms the ratio of susceptible and immune individuals in a population at the end of one wave determines the potential magnitude of a subsequent wave. The worry right now is that with a vaccine still months away, and the real rate of infection only being guessed at, populations worldwide remain highly vulnerable to both resurgence and subsequent waves.

Peter Beaumont

Prof Jonathan Reid at Bristol University in the UK is researching airborne transmission of coronavirus. “It is perhaps not surprising that while suspended in air, the small droplets could combine with background urban particles and be carried around.”

He said the virus had been detected in tiny droplets collected indoors in China.

Setti said tiny droplets between 0.1 and 1 micron may travel further when coalesced with pollution particles up to 10 microns than on their own. This is because the combined particle is larger and less dense than the droplet and can remain buoyed by the air for longer.

“The pollution particle is like a micro-airplane and the passengers are the droplets,” said Sett. Reid is more cautious: “I think the very small change in the size of the [combined] particles is unlikely to play much of a role.”

Prof Frank Kelly at Imperial College London said the idea of pollution particles carrying the virus further afield was an interesting one. “It is possible, but I would like to see this work repeated by two or three groups.”

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Another expert, Prof John Sodeau at University College Cork, in the Republic of Ireland, said: “The work seems plausible. But that is the bottom line at the moment, and plausible [particle] interactions are not always biologically viable and may have no effect in the atmosphere.” He said the normal course of scientific research might take two or three years to confirm such findings.

Other research has indicated correlations between increased Covid-19 deaths and higher levels of air pollution before the pandemic. Long-term exposure to dirty air is known to damage lung health, which could make people more vulnerable to Covid-19.

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