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Air Pollution Science

Fewer Cars on the Road Doesn’t Mean There’s No Pollution

Picture of Dr Gabriela Adler Katz
By Dr Gabriela Adler Katz on April, 14 2020

Overall air quality improved in many urban centers under national COVID-19 lockdowns but this doesn’t mean air pollution totally went away. Pollutants come from a wide range of different sources, both natural and man-made. This reality means air pollution is around us, even when there are less cars on the road.

Ozone Pollution & ‘The Weekend Effect’

Ground level Ozone does not get emitted directly, instead it is formed in the atmosphere from other primary pollutants in the presence of solar radiation (i.e. sun). Intuitively, you’d think any reduction in the concentration levels of primary pollutants that Ozone is created from - (Nitrogen Oxides (NOx), Volatile Organic Compounds (VOCs) and Carbon Monoxide (CO)) - would automatically result in reductions in levels of Ozone. 

However this is not always the case. It also depends on the ratio levels of Nitrogen Oxides to Volatile Organic Compounds. As NOx is a primary pollutant emitted by vehicle emissions, when there are less cars on the road, there may be less NOx in the air.

The effect of this on overall ratio levels means that some urban centers actually experience higher levels of Ozone during quiet periods such as the weekend or during a lockdown. This phenomena has been termed by some scientists as the ‘weekend effect.’

Rural Doesn’t Always Mean Clean 

During lockdown, the city of Paris saw significantly reduced levels of NO2 in the air - however there were still significant levels of PM2.5 detected in the area.




Some believe the use of fertilizer on crops near to the city which contain ammonia (NH3) is contributing to the creation of PM2.5 in the atmosphere as a secondary pollutant.

Far Away Sources 

Sometimes there are few direct sources of air pollution at a particular location, but high levels of air pollution are still detected. Air pollution presents a complex and dynamic reality, which means air pollution in your area could have originated from a different, far away location. In fact, this effect has been seen in a number of locations during their lockdown period.

For example, as Kings College London stated this week, London is due to see high levels of air pollution as a result of pollutants imported from the European mainland over the coming days.

Increased Healthcare Burden as Pollen Season Kicks In

High Pollen Levels-min

BreezoMeter Pollen Map

During pollen season, allergic asthma poses a significant risk as unmanaged exposure contributes to the likelihood of a serious asthma attack. Asthma sufferers - already currently at higher risk from COVID-19 - should take particular care to keep up to date with the pollen levels and manage their symptoms if they also suffer from seasonal allergies.

Allergy season also risks placing additional strain on already burdened healthcare systems - across the US and UK, there have been reports of people confusing allergy symptoms with early COVID-19 symptoms, filling up waiting rooms and placing more demand on testing.

One family physician in South Texas stated that 

“Patients who mistakenly believe they have the virus are a significant problem due to limited resources for testing and treatment of COVID-19, especially as the virus continues to spread”

Officials say No to Barbecues 

In 2020, the dangers posed by smoke inhalation prompted some policy makers in the UK to advise residents against lighting bonfires or barbecues out of of respect for their more vulnerable neighbors. 




As temperatures increased, there was of course the increased seasonal risk of wildfires and smoke-related pollution. As parts of the world move into their seasonal fire seasons, it’s likely they'll face a number of logistical questions around shortages of incident management teams and keeping firefighters socially distanced.

When we wrote this piece, the Ukraine was grappling with a particularly severe blaze, prompting fears that the fire could cause another nuclear disaster for residents living nearby.

Sun Comes Out to Play, Dust Won’t Go Away?

Warmer weather also means there’s less rain to wash away our common air pollutants, meaning we’re more exposed to the harmful health effects of dust storms.  In general, wind can be a friend or foe, helping to move air pollution away from us or closer.

At the end of February 2020, clouds of red sand from the Sahara completely overwhelmed the Canary Islands, severely reducing visibility and prompting alerts for residents with respiratory problems to take care.

In 2019, during Easter, the same sub saharan African dust reached as far as Scotland, coming down as ‘blood rain’ and leaving red deposits on everything it fell on.

In Summary 

For vulnerable groups and respiratory disease sufferers in particular, it’s important to stay vigilant. Air pollution continues to be a serious breathing hazard and it's also changing around us on a regular basis. Especially during the COVID-19 pandemic, as many of us as possible should be taking steps to manage our own personal exposure through access to real-time air quality information at our location.

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