The Hidden Rise in Toxic Air Pollution during the COVID-19 Pandemic: is our response worsening the respiratory burden of particulate matter in the UK?

Recent research has highlighted the concerning paradigm that reducing ambient concentrations of legislated air pollution, in the form of PM2.5, may lead to a decline in air quality by increasing the concentration of more harmful ultrafine particles (UFP; diameter less than 100nm). It has been shown that PM2.5 plays a crucial role in suppression of UFP numbers, with larger PM2.5 particles acting to scavenge the more toxic UFP from the air. Consequently, removal of PM2.5 as a UFP sink may result in enhanced ambient UFP numbers, particularly if UFP emissions and gaseous precursor concentrations remain unchecked. With the outbreak of the COVID-19 pandemic, anthropogenic activity around the globe has decreased in a vast and unprecedented manner, to such an extent that concentrations of commonly measured primary air pollutants have decreased dramatically. However, levels of secondary pollutants (which often correlate with finest UFP fractions) have already begun to rise. It follows that this sudden, global drop in anthropogenic activity and ambient PM2.5 has the potential to drive an unexpected and potentially significant increase in UFP concentrations and associated detrimental health effects. If we are now inadvertently adding an unlooked-for cardio-respiratory pollutant into the air at a time when many are vulnerable to a virus that impacts the respiratory system, we risk widening the COVID-19 'window of vulnerability', where increased UFP exposure acts to exacerbate underlying conditions for individuals with pre-existing co-morbidities, who may otherwise be considered 'lower risk', and worsen the impact of the virus for those already considered as 'at risk'.