Does air pollution exposure increase the risk of severe illness or death from Covid-19? New studies appear every month, all seemingly indicating a link, but a definitive answer is yet to emerge. Envirotec reviewed some of the studies conducted so far and what we appear to be learning about the connection between air pollution and the illness associated with the SARS-Cov-2 virus
It seems natural to assume a link between dirty air and a new kind of respiratory illness given what we already know about the health impacts of air pollution. And the most recent data from the ONS appears to show that around 35% of deaths involving Covid-19 in England up to late June involved co-morbidities with respiratory and cardiovascular disease. Of course, medical opinion from official sources holds that the principal risk of catching Covid-19 comes from contact with an infected person. But there are three ways that air pollution might have a bearing on this seemingly new form of illness and its likely progression, in individuals and the world at large.
One is the fact that long-term exposure to air pollution might weaken hearts and lungs to the extent that a serious health outcome is more likely. There is also the fact of the lung inflammation caused by pollutants, which might be supposed to make a person more susceptible to catching the virus. Then, there is also the fact that some pollutants might transport small quantities of the virus through the air and potentially over long distances – the latter effect having been observed in studies with similar viruses.
Floating threat?
One early March study from China, “Longitudinal survey of microbiome associated with particulate matter in a megacity”, concluded that: “Airborne particulate matter accommodates rich and dynamic microbial communities, including a range of microbial elements that are associated with potential health consequences.” Published in Genome
Biology, the study said air pollution is likely to have carried the viruses causing bird flu, measles and foot-and-mouth disease over considerable distances.
A gene highly specific to Covid-19 was found in air samples collected by an April study conducted at the University of Bologna in Italy. It suggested that higher levels of particulate pollution could explain the seemingly higher rates of infection in Northern Italy before lockdown. And this region has some of the worst particulate pollution in Europe. However, the group said the work is preliminary, and doesn’t yet confirm whether or not the virus remains viable on pollution particles, and in sufficient quantity to cause disease
Pollution and health risk
But whether you catch it or not, is there anything to support the theory that hearts and lungs weakened by long-term pollution exposure will be more likely to develop severe symptoms? A number of studies this year appear to show such an effect.
In April a study of 324 cities in China reported a correlation, and findings suggesting that places with modestly higher levels of NO2 pollution (10µg/m3) in the five years preceding the pandemic had 22% more Covid-19 cases. Higher levels of PM2.5 pollution, on the other hand, saw a 15% rise.
An April study from Harvard University looked at the effects of PM2.5 pollution, and reported an observed 8% increase in Coronavirus deaths for a single-unit rise in this kind of pollution. The results were drawn from an examination of 3000 counties across the US. The results suggest that a small increase in pollution levels produces quite a large rise in death rate frowm Covid-19.
As with many of the studies reported thus far, this one had yet to complete peer review and full publication. In comments made to The Guardian lead author Professor Francesca Dominici suggested the slowness of peer-review was one of the reasons air quality was not receiving the attention it deserved.
Finer grained dataset
A widely-lauded, July-published study on the Netherlands was praised for the fact that it controls for a multitude of other factors that might affect people’s apparent susceptibility to Covid-19. Conducted by a team led by Prof Matthew Cole at the University of Birmingham, the paper also stands out in that it has been through peer review and published in an academic journal, Environmental and Resource Economics.
The study suggested an even more dramatic correlation, with a single-point increase in particulate pollution being associated with a 10% increase in hospital admissions and infections, and a 15% increase in deaths.
Compared to the Harvard study, the Dutch one used a finer resolution dataset, with the 355 Dutch municipalities being 95km2 in area compared to the 3,130km2 of a US county, as The Guardian noted. This analysis also had access to data all the way up to 5 June, unlike the April US study which looked only at the earlier wave of the pandemic.
One recent study announcement came from the UK Office of National Statistics (ONS) in early August, and suggested a less dramatic correlation than the other studies so far. It examined 46,000 Coronavirus deaths in England and seemed to show that a single-unit increase in people’s exposure to small-particle (PM2.5) pollution over the preceding decade may increase the death rate by up to 6%, while a single-unit increase in NO2 appeared to produce a 2% increase in death rates.
One possible explanation for the observed smaller influence of air pollution compared to the earlier studies is the fact that the latter focused on the earlier stages of the pandemic, when the virus seemed to be confined to cities, mostly.
The ONS analysis seems to show that deaths from Covid-19 were more common in highly polluted areas, particularly early in the pandemic, and that this correlation between pollution and mortality seemed to tail off as the number of deaths rose and lockdown was introduced. And the fact that so much of the early mortality occurred in London, where pollution levels are the highest in the country, will have emphasised the correlation. In the period running up to the start of lockdown (27 March), 45% of Covid-19 deaths in England occurred in London. By 12 June (the point where the ONS analysis ends) this had fallen to 18%. As the report says, “As the virus spread across the country and deaths became more evenly distributed, the correlation between air pollution exposure and COVID-19 mortality decreased.”
The report also mentions that, while death rates have tended to be higher in more polluted areas, this does not in itself establish pollution exposure as a cause of Covid-19 deaths. To try to isolate air pollution’s impact, the group said they developed a statistical model to control for factors such as levels of deprivation, population density, public health (levels of smoking, for example), and pre-existing health conditions. And this provided a way to compare death rates among populations of similar health and from the same sort of area, with the main difference being long-term exposure to air pollution.
One factor that seems tricky to accommodate in such studies – and with an adequate level of control – is ethnicity. A July study of 400 hospital patients in Birmingham linked the apparently more severe impacts of Covid-19 observable in BAME patients with the fact that the latter more often live in highly polluted areas and in poor-quality housing. In an earlier study, minority ethnicities had been shown to account for 34% of critically ill Covid-19 patients in the UK, disproportionate to the 14% contribution they make to the population.
The ONS analysis seemed to be inconclusive on the issue of ethnicity, and the group conceded that this study was unable to pull apart the different factors of deprivation that might be at work here. “To disentangle factors such as pollution and ethnicity, and their contribution to COVID-19 mortality,” said the report, “we would need to identify detailed characteristics of the individuals who have died from COVID-19. While we hope to publish this kind of analysis in future, the data and modelling required take much longer to put together.”
A further limitation cited by the authors is the fact that, as with the other studies, they have only been able to examine air quality and COVID-19 at the population level, which requires making assumptions – for example, that air pollution exposure levels are consistent across a 1km grid square, whereas in reality it could vary from one end of a street to another. Also, an individual’s exposure is influenced not only by where they live but where they go to work.
Clearer skies ahead?
Given the already massive death toll attributed to air pollution – 7 million deaths per year, according to the WHO – there was much made of the fact that lockdown may have saved lives given the sudden drop in air pollution in China and elsewhere. But estimates of the air pollution drop that occurred during lockdown seem to vary widely. One study cited a 25% drop in pollutant levels during lockdown in China (23 January to 31 March), and said this might have prevented 24,000 to 36,000 deaths. However, a report appearing in late July, a collaboration between the University of Leeds and the Southern University of Science and Technology in China, suggested a more modest decline – NO2 concentrations fell by about 27% over the period, the largest drop being in Hubei province (50.5%).
But smaller reductions were seen with other pollutants – PM2.5, for example, fell by 11% across China, and didn’t fall at all in northeast China.
In Europe, the Centre for Research on Energy and Clean Air (CREA) reported in May that, in the month up to 24 April, around 11,000 deaths from air pollution had been avoided, including 1,700 in the UK – an effect attributed to drops in power generation from coal (37% drop) and oil consumption (one third drop).
Many air pollution observers seemed hopeful that the aftermath of the lockdowns will bring impetus and political will to make long-lasting changes, Many cities and regions have seen new initiatives to cut car use and boost cycling, including Milan, Paris, Berlin, Mexico City, and Oakland, California. While it’s too early to talk of silver linings it seems likely that the taste of cleaner air will have whetted people’s appetite for more.
