The global response to the COVID-19 pandemic has resulted in unprecedented reductions in economic activity. But lockdown events have reduced air pollution levels by approximately 20% across most of the countries in the world. The reduced air pollution levels come with a substantial health co-benefit in terms of avoided premature deaths and paediatric asthma cases that accompanied the COVID-19 containment measures.

In many developing nations economic growth has exacerbated air pollutant emissions with severe consequences for the environment and human health. Long term exposure to air pollution including fine particulate matter(PM) with a diameter less than 2.5µm (PM2.5) and ozone (O3) are estimated to cause 8.8 million excess deaths annually, while nitrogen dioxide (NO2) results in 4 million new paediatric asthma cases annually. Despite the apparent global air pollution “pandemic”, anthropogenic emissions remain on positive trajectories for most developing and some developed nations.  

The major ambient outdoor air pollution sources include power generation, industry, traffic, and  residential energy use. With the rapid emergence of the COVID-19 and in particular the government enforced lockdown measures aimed at containment, economic activity has come to a near-complete standstill in many countries. Lockdown measures have included partial or complete closure of international borders, schools, non-essential business and in some cases restricted citizen mobility. The associated reduction in traffic and industry has both socio-economic and environmental impacts which are yet to be quantified. In parallel to the societal consequences of the global response to COVID-19, there is an unprecedented opportunity to estimate the short-term effects of economic activity counterfactual to business as usual on global air pollution and its relation to human health. 

The hypothesis test revealed that reduced air pollution levels during Feb/Mar 2020 were related to the COVID-19 lockdown events. To test the hypothesis, satellite data are used to provide a global perspective over Feb/Mar. But to estimate exposure levels relevant to public health, we must observe derive ground-level measurements from more than ten thousand air quality stations for accounting meteorological variations. The air pollution anomalies during COVID-19 lockdown are then used to quantify mortality and paediatric asthma incidence that have been potentially avoided. Finally it’s perform a counterfactual projection of the public health burden assuming NO2, O3 and PM2.5 anomalies during lockdown are maintained for the remainder of 2020. In doing this it do not imply that lockdown economic activity is sustainable or desirable, however, it do intend to use the current situation as an intuitive means of highlighting the global air pollution health crisis. 

While satellites provide global data coverage, they do not necessarily reflect pollutant concentrations at ground-level that are relevant to human exposure and health. Therefore it’s needed to supplement satellite data with ground-level pollutant concentrations collected by over ten thousand air quality stations. In contrast to the satellite data we used, station data allowed us to calculate a more robust 3-year baseline measure of expected pollution levels for Feb/Mar. These data largely corroborate the satellite data in that it found the same spatial patterns and net directions of Feb/Mar 2020 pollutant anomalies. Specifically, NO2 declined by 22.9% (20% IQR)which equates to an absolute decline of 7.6 µg m-3 (9 µg m-3 IQR), O3 increased by 5.4% (18% IQR) whereas particulate matter (PM2.5) declined by 17.2% (30% IQR). The direction and magnitude of PM2.5 change near the surface is different to the Aerosol Optical Depth(AOD) measured by satellites. Thus it’s highlighting the importance of ground-level measurements to complement satellite-derived global trends.

Focusing on the ground-level trends is illustrative of the change at both global and country scales. Here, the deviation in NO2 and PM2.5 levels from three year average values increases significantly from mid-Jan onward. The timing of the initial deviation is potentially an effect of the dramatic air pollution reductions in China coincident with the rapid lockdown response in Wuhan province at the outset of COVID-19. Thereafter, the spread of COVID-19 led to lockdowns in various countries, associated with a greater negative deviation in NO2 and PM2.5 from three year baseline values. Some notable outliers include Australia and Mexico. Australia exhibited drastic declines in PM2.5 from January onward likely reflecting the tail-end of the recent wildfires. The rapid decline in NO2 over Mexican station is more difficult to explain along with Taiwan, Slovakia and Sweden. As since Taiwan, Slovakia and Sweden were one of the few countries not to enforce any national lockdown measures.

The trends for O3 and PM2.5 are more heterogeneous over space and time relative to the ubiquitous declines in NO2. For instance, increases in O3 over southern China differ significantly from the decreases observed over the Wuhan province, the epicentre of COVID-19. We expect this to be a consequence of synoptic redistribution of O3 by atmospheric circulations. Similarly, the local decreases over parts of Spain are in contrast to increases observed over eastern Europe. This is not surprising given that O3 is affected by long-distance transport as well as non-linear chemical interactions with volatile organic compounds  (VOCs) and NOx, mediated by mesoscale and urban canopy weather patterns.  

To test primary hypothesis that pollution anomalies were directly associated with COVID-19 lockdown events, then it must be calculated on average ground-level concentrations for each country separately. Instead of averaging over Feb/Mar, one must focus on the two weeks after lockdowns were announced in each country. It first corrected for the effects of local and meso-scale weather patterns  (temperature, humidity, precipitation and wind speed) which can significantly affect ground-level pollutant concentrations and thereby compromise any observable effect of COVID-19 lockdowns. Using regression models, it’s estimated the lockdown-attributable anomaly as the difference between observed and expected pollutant concentrations given weather during lockdown. It was found that a net decline of about 20% (5% to 35% – 95% confidence interval) across all three pollutants in countries where significant anomalies were detected. There were significant declines in NO2 (29 %; 13% to 44%) and O3 (11%; 2% to 20%), however model estimates were not able to control for the confounding effects of weather enough to detect significant declines in PM2.5 (9% decline; -10 to 28%). Indeed, our meteorological-control models were able to explain less of the variance in PM2.5 compared to NO2 and O3. This suggests PM2.5 has a weaker coupling to land-transportation and small business activity declines during lockdown compared to NO2 and O3. In Many countries PM2.5 is more strongly  linked to residential energy use, power generation and agriculture. In addition, PM2.5 is significantly influenced by long-distance atmospheric transport of mineral dust and therefore the local effects of economic activity may be diluted or even overwhelmed. 

Using the two week post-lockdown anomalies in combination with published exposure-response functions for NO2, O3 and PM(2.5), it’s confirm  estimated changes in daily all-cause mortality burden and paediatric asthma incidence. During the two weeks post-lockdown, there were a total of 7400 deaths and 6600 paediatric asthma cases avoided across 27 countries with recorded COVID-19 mitigation measures. The number of PM2.5-related deaths avoided exceeded those related to NO2 and O3. While for paediatric asthma incidence, NO2 reductions contributed to more avoided cases compared to O3 and PM2.5. In China and India alone, the PM2.5-related reductions in mortality burden were 1400 and 5300, respectively. These are countries with both the highest baseline pollution levels and population densities, and therefore have the most to gain from pollutant declines.

Furthermore, as of performed a counterfactual projection of reduced health burden assuming ground-level air pollution, deviations experienced during lockdown are maintained for the remainder of 2020. The cumulative effect of the reduction in NO2, O3 and PM2.5 over the remainder of 2020 is that 0.78 million deaths and 1.6 million paediatric asthma cases could be avoided. Our findings suggest that, in spite of the modest response of PM2.5, countries would have much to gain in maintaining PM2.5 lockdown levels because that would prevent 0.6 million deaths and 1.1million paediatric asthma cases which is 3- and 5-fold higher than those from NO2 and 5- and 30-fold higher than those from O3. The bulk of the benefit gained would take place during the latter half of the year when air pollution levels are at their highest over countries with the largest air pollution health burden (i.e. India and China).

Reducing economic activity to levels equivalent to a lockdown state are impractical, yet maintaining business as usual clearly exacerbates global pollutant emissions and associated deaths. As study documents the dramatic short-term effect of global reductions in transport and economic activity on reducing ground-level NO2, with mixed effects on O3 and PM2.5 concentrations. Maintaining reductions in pollutant emissions corresponding to lockdown conditions can substantially reduce the global burden of disease. By no means imply that global pandemics such as the COVID-19, nor lockdown actions, are beneficial for public health. However, it suggest the current situation is a useful lens through which to view the global air pollution “pandemic”. Time will tell how significant the change in health burden has actually been. Nevertheless, the early evidence presented here suggests it is likely significant. Reduced premature mortality from air pollution thus appears as a co-benefit of the minimized number of deaths from the lockdown measures. Finding economically and socially sustainable alternatives to fossil fuel based transport and industry are another means of reaching the pollutant declines we have observed during the global response to COVID-19.  

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