As governments across the globe enforced lockdowns and social distancing policies to limit the spread of the coronavirus, traffic noticeably decreased, aeroplanes were grounded, and wildlife began to appear in urban regions.

It is perhaps only natural to ask: how have these responses affected ambient air quality, and are these changes likely to be short- or long-term?

Air pollution deaths and what causes them

Before the pandemic gripped the world, the WHO estimated that exposure to air pollution was responsible for about 7,000,000 premature deaths globally. Around 500,000 of these premature deaths occur in Europe, with estimates for Malta being just shy of 600 per year. Air pollution is harmful to human health in a variety of ways. For example, fine particulates, once inhaled, may lead to inflammation of the lungs and the exacerbation of pre-existing conditions, such as asthma and emphysema, as well as increasing the risk of cardiovascular disease.

From an environmental point of view, our increased combustion of carbon-based fossil fuels such as oil and coal has led to massive emissions of carbon dioxide (CO2) and nitrogen oxides (NOx). CO2 is a greenhouse gas which contributes to climate change and all its undesirable ramifications, while NOx are responsible for the formation of smog and acid rain. The best-studied of these NOx gases is nitrogen dioxide, NO2. Iincreased exposure to it has been linked with an increased risk of pulmonary disease.

Decreases in atmospheric NO2 levels across China during coronavirus lockdowns. Photo: NASA Earth Observatory

What effects have lockdowns had on air pollution?

It is clear that CO2 and NO2 have negative impacts on both human health and the natural environment.

However, given that the primary source of these pollutants is the combustion of oil, coal, and other fossil fuels, what has the impact of the coronavirus lockdowns been on their abundance in the atmosphere?

Simply put, atmospheric pollution has decreased overall. A recently released study by an international team of climate scientists showed that daily CO2 emissions for the year 2020 would be 4% lower than those for 2019 if pre-pandemic conditions returned by June, or 7% lower if restrictions lasted until the end of the year.

The reduction in NO2 concentrations was arguably even more extreme. In China, where the novel coronavirus is thought to have originated, major cities placed on lockdown experienced massive drops in atmospheric NO2 concentrations. These reductions were made famous by maps created using data from ESA’s Sentinel-5 satellite and NASA’s Aura satellite. Similar observations have been made in European countries affected by the coronavirus: during the height of the European lockdowns in March and April 2020, NO2 concentrations had plummeted by 47% in Milan, 48% in Madrid, 49% in Rome, and 54% in Paris compared to recorded levels in 2019.

Why has air pollution decreased during lockdown?

A natural question to ask is what is the main driver of this decrease in air contaminants? A recent study revealed that most of it is due to the drastic decline in pollution from transportation, mostly from cars and trucks. The strict enforcement of lockdown rules meant that, in many countries, people were only permitted to leave their homes for a handful of reasons, such as shopping for necessities or periodic exercise. Such measures resulted in drastic changes to the transportation sectors of various nations, with road traffic decreasing significantly and aeroplane fleets largely grounded.

Furthermore, the lockdowns have resulted in an unprecedented decrease in energy consumption, including the demand for electricity. Due to this, many coal-burning power plants are playing second fiddle to those making use of renewable sources of energy, which have much lower running costs. The lower demand for coal meant that less of it was combusted, and thus there were fewer emissions of CO2, NOx, and carcinogenic particulates. Great Britain, for instance, has now gone two months without burning any coal for its energy needs.

Traffic has returned to New Delhi after coronavirus restrictions were eased.. Photo: AFP

What does the future hold in store? 

It seems that the consequences of the coronavirus lockdowns on air quality have been largely positive, with the Centre for Research on Energy and Clean Air (CREA) estimating that the net decrease in pollution has actually saved around 11,000 lives and will result in 6,000 fewer cases of asthma in children.

However, these positive effects are likely to be only short-term and have been described as a “drop in the ocean” compared to the required measures needed to avert the potentially disastrous effects of climate change.

Moreover, with many nations in the process of emerging from their lockdowns, it is difficult to judge what the future holds in store. Certainly, government policy will dictate the way forward.

In Milan, one of Europe’s most polluted cities, an ambitious plan has been tabled which would see 35 km of roads converted to cycling and pedestrian zones over the summer months. Such a proposal would greatly reduce car use and, by extension, pollution from traffic. Parisian mayor Anne Hidalgo has also hinted that the French capital will propose plans to greatly reduce the presence of cars in the city, citing the mixture of air pollution and COVID-19 to be “a particularly dangerous cocktail”.

In contrast, as restrictions have been gradually lifted across Asia, pre-pandemic levels of traffic (and pollution) have returned to the streets. By late April 2020, NO2 levels in China had exceeded those for April 2019, while congestion in the streets of the Indian capital New Delhi had spiked.

Our global ambient air quality will also rely on a transition to more renewable sources of energy. Although the decrease in electricity demand brought about by the lockdowns has financially crippled the coal industry, there are some fears that it may linger on into the 2030s, particularly since China has pledged a 20% increase in the size of its coal sector and India has made certain aspects of its own coal industry eligible for financial assistance in a coronavirus stimulus package. Donald Trump has also repeatedly pledged to support the American coal industry, although his efforts have been largely unsuccessful.

Therefore, without government policies aimed at decarbonising both the transportation and energy sectors of major cities and nations, it is unlikely that any of the benefits of improved air quality that we have witnessed throughout these lockdowns will last for anything more than a couple of months.

Duncan V. Mifsud is a doctoral candidate at the Centre for Astrophysics and Planetary Science at the University of Kent and a Fellow of the Royal Astronomical Society. 

Benji Fenech-Salerno is a doctoral candidate at the Department of Chemistry, Imperial College London.