This blog draws on a recent Wales Centre for Public Policy report, Air quality strategies and technologies: A rapid review of the international evidence, co-authored by Sarah Quarmby, Georgina Santos and Megan Mathias and explores what we know about different ways of cleaning up the air we breathe.
Air quality is measured according to the relative levels of pollution in the air. Most air pollution in urban areas is caused by road traffic. Heavy industry, agriculture and household pollution all play a part too, but these tend to be getting better or staying the same over time, whereas road traffic pollution is generally getting worse.
There are five main pollutants that are most harmful to human health: particulate matter (PM2.5 and PM10), nitrogen oxides (NOx), sulphur dioxide (SO2), carbon monoxide (CO) and volatile organic compounds (VOCs). High proportions of these substances in the air cause a range of health issues, and for this reason their levels are regulated by international guidelines, such as from the World Health Organisation, the European Union, as well as individual countries’ legislation. It is worth noting that the greenhouse gases which cause global warming are different from the pollutants listed above, but burning fossil fuels is a cause of both air pollution and global warming. In this way, global warming and air quality are separate but interrelated issues.
Governments around the world are getting creative with ways to improve the air that their citizens breathe. There are two main ways of approaching air quality: either try to stop pollutants being produced, or remove them once they’re in the air. Attempts at the latter tend to be more outlandish, with examples including a giant chimney in Xi’an, China that filters the air that passes through it, water cannons in Delhi that wash pollution from the air, and paint that reacts with nitrogen dioxide, removing it from the air. Recently, air filtration devices have also been fitted to buses in Southampton. There isn’t much evidence to suggest that these kind of initiatives work, not least because they only ever come into contact with a very small proportion of the air. As Professor Alastair Lewis wrote recently, “it is far, far easier to come up with technologies and schemes that stop harmful emissions at source, rather than to try to capture the resulting pollution once it’s free and in the air.”
Since most urban air pollution comes from traffic, one way of preventing pollutants from being produced is to swap individuals’ use of fossil fuel cars with alternative forms of transport. Electric vehicles, in contrast with fossil fuel powered vehicles, don’t produce emissions whilst they are being driven. Since the electricity that goes into powering them has to be produced somehow, electric vehicles still produce particulate matter from wear to brakes and tyres, but they are far less polluting than their fossil-fuelled counterparts.
Some other effective methods of improving air quality may be far more modest, despite China’s smog-sucking tower and electric cars making for better headlines. There’s evidence in favour of these initiatives:
Low emission zones, in which vehicle use is restricted are shown to have localised positive impacts on air quality, although there is the risk that people circumvent the area rather than not using their car altogether, which would mean that pollution was simply moved elsewhere.
Lowering speed limits is an effective measure mainly because it causes drivers to do less stopping and starting, rather than being as a result of slower driving itself. Lower speed limits also have an almost immediate impact on air quality, making them a useful short-term measure.
Physical barriers along roads can stop pollution from spreading to surrounding areas, depending on their design and the local weather conditions. Even better are barriers which are partly made of vegetation.
Active travel, including walking and cycling, also doesn’t produce any pollution. For cycling to be an attractive way of getting about, there need to be suitable infrastructure in place, such as a network of dedicated cycle lanes.
Shared forms of travel such as public transport are also less polluting, but people often require encouragement to make the move from private car use. One way of bringing about this behavioural change is in operation in the capital of Estonia, Tallinn, where public transport has been free at the point of use since 2013.
Finally, there’s a limited amount of evidence to suggest that “urban greening” initiatives such as vertical gardens and green walls have a beneficial effect on air quality, particularly in built-up environments. Vegetation can act as a natural air filter, as some pollutants are absorbed by their foliage. On the other hand, on streets with tall buildings, trees might actually trap pollution at street level, making air quality worse- they’re most beneficial in less built-up areas. (There are even studies that have been done on the best tree species for improving air quality).
This all goes to show that the most effective air quality initiatives may not be the most glamorous. Technologies which remove pollution from the air largely remain unproven, especially at the scale that would be needed to make any noticeable impact. Although the future of air quality measures might well include new technologies such as electric vehicles, it could also be based on the humble speed limit, roadside barrier and bicycle path.