There have been many theories tabled with regard to why we are seeing hotspots of disease during the current COVID-19 pandemic. One theory that seems to be persisting is a link between poor air quality and an increased disease severity. Both Wuhan in China where the pandemic began and Northern Italy which has been hit hard, are areas renowned for poor air quality due to dense population and high levels of industrial activity. But is this a causative link or just a coincidence?
Is there a link to COVID-19 mortality on the worst affected areas?
With COVID-19 research flowing thick and fast, a study came out looking at the possible link between the poor air quality in Northern Italy and the increased COVID-19 morbidity and mortality rates in the region. The researchers present evidence that people living in areas with high levels of air pollutants are more prone to developing chronic respiratory conditions and chronic inflammation, an already well-established link. From this they suggest air pollution in Northern Italy should be considered an additional co-factor of the high COVID-19 mortality rate in the area but do not present any data to test this hypothesis.
Another preprint, which has therefore yet to be peer reviewed, has been released looking at a similar link in China, Italy and the US. This study did actually analyze air pollutant data from these locations using information from ground monitoring stations and Sentinel-5 satellite data. Whilst they control for population density in statistical analyses, they do not consider that the population density itself rather than the pollution in the areas may be a reason for the higher incidence of disease. They also fail to acknowledge the socio-economic factors that may also contribute to higher rates of disease and mortality that often go hand in hand with areas of poorer air quality. The researchers remark upon the starkness of correspondence between poor air quality, SARS-CoV-2 occurrence and its induced high mortality seen in Italy, but omit to mention that as a country with one of the oldest average populations in the world, it automatically puts them at much greater likelihood of an increased mortality rate.
A further study looking at cases taken from 66 administrative regions in Italy, Spain, France and Germany also used satellite data to look for an association between NO2 levels and COVID-19 fatalities. However, it too sufferers some of the limitations of the studies highlighted above. Taking data at the level of administrative areas too is problematic as there may be significant and relevant variations within those regions that are important to the analyses.
Does the data support the hypothesis in the UK?
Following these studies, a group of researchers from the University of Cambridge have released a preprint paper, which has therefore yet to be peer reviewed, exploring the correlation between three major air pollutants linked to fossil fuels and SARS-CoV-2 lethality in England. The study compared death rates with air pollution data from 120 monitoring sites and found that levels of some markers of poor air quality, including ozone and nitrogen oxides, correlated with increased COVID-19 mortality. This led them to conclude that the levels of some air pollutants are associated with COVID-19 morbidity.
Whilst on the surface, this might make sense, there are other possible explanations for these correlations. “COVID-19 around the world has affected areas with the greatest population densities to a much greater degree. The population density and more people to get infected could well explain their results rather than the direct effects of air pollution” commented Prof Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases, University of Nottingham.
Digging into the analyses used to reach their conclusion, further cracks appear. “The headline finding of this study, that air pollution exposure exacerbates the severity of COVID-19 disease is very plausible. Air pollutant exposure is known to harm lung development, reduce lung function and increase the prevalence of other lung diseases. It would therefore be expected to increase the impact of an infection which attacks the lungs” commented Prof Roy Harrison, Professor of Environmental Health, University of Birmingham. He continued “This, however, is a very superficial study which does little to advance knowledge. It is solely observational, so, as acknowledged by the authors, cannot prove cause and effect.”
Prof Anna Hansell, Professor in Environmental Epidemiology, University of Leicester also identified shortcomings in the methodology used in this study. “The authors use regional air pollution concentrations, but most air pollution researchers now use concentrations at home address to best reflect people’s exposure - levels of pollutants can vary a lot e.g. between someone living near a busy main road vs. someone living in the countryside. Using regional concentrations will result in a lot of uncertainty in the analysis.” She continued ““There are differences between regions other than air pollution that may explain the findings. The statistical analysis does not make any corrections for these differences - such as population density, ethnicity, deprivation and age distribution of the population. It’s important also to remember we do not know the true underlying coronavirus infection rate in the population - the testing regime in the UK to date has mainly been to confirm the diagnosis in people with more severe disease. Regions with higher deaths and cases may just reflect the fact that (i) the epidemic started earlier in those regions, (ii) they include densely populated areas where social distancing is more difficult so the disease is like to have spread more quickly (but we don’t have data on this) and (iii) they include very deprived areas where people have higher rates of chronic illness that predispose to more severe COVID-19 -so they would be more likely to be detected and to die from the disease.
Although the authors acknowledge the need for further analysis relating to some of the confounding factors in the concluding remarks, this goes to show the importance of the peer review process in identifying possible weaknesses in scientific research. Whilst the scientific community may be able to read between the lines when assessing information from preprint studies, topics like this that are of broad public interest are grabbed by the mainstream media or picked up by general public who may not be savvy to the limitations of such findings.
More work required
Prof Hansell concluded “We know that air pollution increases the risk of cardiovascular and respiratory diseases and that these diseases predispose to worse outcomes with COVID-19, which gives us an indirect link with air pollution. We need detailed research to explore whether there is a direct link between air pollution and COVID-19.” So, whilst there may indeed be a valid link between air pollution and COVID-19 frequency or severity, studies like this fail to prove that conclusively and authors should be mindful not to over-sell their findings, especially on such publicly emotive subjects.