COVID-19 lockdowns had a smaller impact on air quality than first expected — with INCREASE in ozone levels counteracting the health benefits of NO2 reductions

  • Previous studies reported dramatic decreases in nitrogen dioxide levels in cities
  • But these did not account for weather, seasonal trends and clean air policies
  • UK researchers performed a new analysis based on 11 cities including London
  • They found coronavirus lockdowns did lower airborne NO2 and PM2.5 levels 
  • But, traffic reduction also lowered nitric oxide emissions that break down ozone

The beneficial impact of the initial set of COVID-19 lockdowns on urban air quality was lower than was previously suggested, a study has concluded.

Researchers from Birmingham analysed the changes in air pollution level in 11 global cities — including London, New York and Wuhan.

They found that, as millions stayed at home, concentrations of nitrogen dioxide (NO2) in the air fell by between 10–50 per cent.

This pollutant — which is emitted by road traffic — has been linked to the development of both cancer and respiratory infections. 

However, lockdowns also saw ozone concentrations increase in all the cities — by up to 30 per cent — the team found after accounting for weather and seasonal trends.

Ozone can cause chest pain in humans, damage lung tissue and worsen asthma.

The beneficial impact of the initial set of COVID-19 lockdowns on urban air quality was lower than was previously suggested, a study has concluded. Pictured, air pollution over New York

Road traffic emissions normally remove ozone from the air — but with fewer vehicles on the road, there were less emissions and less ozone reduction, lead author Zongbo Shi of the University of Birmingham told the Thomson Reuters Foundation.

Traffic gives off nitric oxide (NO), which helps break down surface ozone into nitrogen dioxide and oxygen — hence the complicated effects of the lockdowns.

‘We found increases in ozone levels due to lockdown in all the cities studied,’ added paper author and atmospheric scientist William Bloss, also of Birmingham.

‘This is what we expect from the air chemistry, but this will counteract at least some of the health benefit from nitrogen dioxide reductions.’

Air pollution is the single largest environmental risk to human health globally — killing an estimated 7 million people each year — the World Health Organization (WHO) has reported.

In their study, Professor Shi and colleagues analysed air pollution levels in 11 cities that underwent extensive lockdowns due to COVID-19 — Beijing, Berlin, London, Los Angeles, Madrid, Milan, New Delhi, New York, Paris, Rome and Wuhan.

Previous studies has reported much more dramatic falls in nitrogen dioxide levels — including one from the Chinese Academy of Science that said that Wuhan, where COVID-19 was first detected — saw a 93 per cent decrease at the outbreak’s start.

According to the researchers, however, a meaningful analysis needed to go beyond simply comparing the air quality before and after the lockdown restrictions began, or against pollution levels over the same periods in previous years.

Instead, they used machine learning to strip out the impacts of weather on air quality and analysed data from 2015 to May 2020, taking into account how cities with clean air policies would inherently see emission reductions over time. 

The researchers found that, as millions stayed at home, concentrations of nitrogen dioxide (NO2) in the air fell by between 10–50 per cent. However, lockdowns also saw ozone concentrations increase in all the cities — by up to 30 per cent — the team found after accounting for weather and seasonal trends

‘Emission changes associated with the early lockdown restrictions led to abrupt changes in air pollutant levels but their impacts on air quality were more complex than we thought, and smaller than we expected,’ said Professor Shi.

‘Weather changes can mask changes in emissions on air quality,’ he added.

Importantly, our study has provided a new framework for assessing air pollution interventions, by separating the effects of weather and season from the effects of emission changes.’

The team also considered so-called PM2.5 — particulate matter like soot and smoke — which is emitted by cars and industry and can lodge in the lungs and enter the bloodstream, causing fatal lung and heart diseases.

They found that PM2.5 concentrations decreased in all cities except London and Paris — but that the declines were nowhere near enough to meet World Health Organisation guidelines.

The full findings of the study were published in the journal Science Advances.

Revealed: MailOnline dissects the impact greenhouse gases have on the planet – and what is being done to stop air pollution


Carbon dioxide

Carbon dioxide (CO2) is one of the biggest contributors to global warming. After the gas is released into the atmosphere it stays there, making it difficult for heat to escape – and warming up the planet in the process. 

It is primarily released from burning fossil fuels such as coal, oil and gas, as well as cement production. 

The average monthly concentration of CO2 in the Earth’s atmosphere, as of April 2019, is 413 parts per million (ppm). Before the Industrial Revolution, the concentration was just 280 ppm. 

CO2 concentration has fluctuated over the last 800,000 years between 180 to 280ppm, but has been vastly accelerated by pollution caused by humans. 

Nitrogen dioxide 

The gas nitrogen dioxide (NO2) comes from burning fossil fuels, car exhaust emissions and the use of nitrogen-based fertilisers used in agriculture.

Although there is far less NO2 in the atmosphere than CO2, it is between 200 and 300 times more effective at trapping heat.

Sulfur dioxide 

Sulfur dioxide (SO2) also primarily comes from fossil fuel burning, but can also be released from car exhausts.

SO2 can react with water, oxygen and other chemicals in the atmosphere to cause acid rain. 

Carbon monoxide 

Carbon monoxide (CO) is an indirect greenhouse gas as it reacts with hydroxyl radicals, removing them. Hydroxyl radicals reduce the lifetime of carbon dioxide and other greenhouse gases. 


What is particulate matter?

Particulate matter refers to tiny parts of solids or liquid materials in the air. 

Some are visible, such as dust, whereas others cannot be seen by the naked eye. 

Materials such as metals, microplastics, soil and chemicals can be in particulate matter.

Particulate matter (or PM) is described in micrometres. The two main ones mentioned in reports and studies are PM10 (less than 10 micrometres) and PM2.5 (less than 2.5 micrometres).

Air pollution comes from burning fossil fuels, cars, cement making and agriculture 

Scientists measure the rate of particulates in the air by cubic metre.

Particulate matter is sent into the air by a number of processes including burning fossil fuels, driving cars and steel making.

Why are particulates dangerous?

Particulates are dangerous because those less than 10 micrometres in diameter can get deep into your lungs, or even pass into your bloodstream. Particulates are found in higher concentrations in urban areas, particularly along main roads. 

Health impact

What sort of health problems can pollution cause?

According to the World Health Organization, a third of deaths from stroke, lung cancer and heart disease can be linked to air pollution. 

Some of the effects of air pollution on the body are not understood, but pollution may increase inflammation which narrows the arteries leading to heart attacks or strokes. 

As well as this, almost one in 10 lung cancer cases in the UK are caused by air pollution. 

Particulates find their way into the lungs and get lodged there, causing inflammation and damage. As well as this, some chemicals in particulates that make their way into the body can cause cancer. 

Deaths from pollution 

Around seven million people die prematurely because of air pollution every year. Pollution can cause a number of issues including asthma attacks, strokes, various cancers and cardiovascular problems. 


Asthma triggers

Air pollution can cause problems for asthma sufferers for a number of reasons. Pollutants in traffic fumes can irritate the airways, and particulates can get into your lungs and throat and make these areas inflamed. 

Problems in pregnancy 

Women exposed to air pollution before getting pregnant are nearly 20 per cent more likely to have babies with birth defects, research suggested in January 2018.

Living within 3.1 miles (5km) of a highly-polluted area one month before conceiving makes women more likely to give birth to babies with defects such as cleft palates or lips, a study by University of Cincinnati found.

For every 0.01mg/m3 increase in fine air particles, birth defects rise by 19 per cent, the research adds. 

Previous research suggests this causes birth defects as a result of women suffering inflammation and ‘internal stress’. 

What is being done to tackle air pollution? 

Paris agreement on climate change

The Paris Agreement, which was first signed in 2015, is an international agreement to control and limit climate change. 

It hopes to hold the increase in the global average temperature to below 2°C (3.6ºF) ‘and to pursue efforts to limit the temperature increase to 1.5°C (2.7°F)’.

Carbon neutral by 2050 

The UK government has announced plans to make the country carbon neutral by 2050. 

They plan to do this by planting more trees and by installing ‘carbon capture’ technology at the source of the pollution.

Some critics are worried that this first option will be used by the government to export its carbon offsetting to other countries.

International carbon credits let nations continue emitting carbon while paying for trees to be planted elsewhere, balancing out their emissions.

No new petrol or diesel vehicles by 2040

In 2017, the UK government announced the sale of new petrol and diesel cars would be banned by 2040.  

However,  MPs on the climate change committee have urged the government to bring the ban forward to 2030, as by then they will have an equivalent range and price.

The Paris Agreement, which was first signed in 2015, is an international agreement to control and limit climate change. Pictured: air pollution over Paris in 2019.

Norway’s electric car subsidies

The speedy electrification of Norway’s automotive fleet is attributed mainly to generous state subsidies. Electric cars are almost entirely exempt from the heavy taxes imposed on petrol and diesel cars, which makes them competitively priced.

A VW Golf with a standard combustion engine costs nearly 334,000 kroner (34,500 euros, $38,600), while its electric cousin the e-Golf costs 326,000 kroner thanks to a lower tax quotient. 

Criticisms of inaction on climate change

The Committee on Climate Change (CCC) has said there is a ‘shocking’ lack of Government preparation for the risks to the country from climate change. 

The committee assessed 33 areas where the risks of climate change had to be addressed – from flood resilience of properties to impacts on farmland and supply chains – and found no real progress in any of them.

The UK is not prepared for 2°C of warming, the level at which countries have pledged to curb temperature rises, let alone a 4°C rise, which is possible if greenhouse gases are not cut globally, the committee said.

It added that cities need more green spaces to stop the urban ‘heat island’ effect, and to prevent floods by soaking up heavy rainfall. 

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