Air pollution is a growing concern throughout the world, both from an ecocentric standpoint and from one pertaining to human health. As it stands, the world health organisation estimated an astonishing 6.5 million deaths from illnesses related to air pollution (2012), with at least 40,000 of those occurring in the UK. In fact, in London (2015), the level of harmful particulate matter smaller than 2.5 micrometres (PM 2.5) – the most dangerous kind – was six times higher than the safe limit. Even more worrying still, London alone has just surpassed the UK’s entire air pollution limit for the year.

Air pollution doesn’t usually garner much attention, most likely because of its subtlety. Rarely do we notice a shift in the quality of the air we breathe, so we tend to focus on the most obvious hurdles when it comes to environmental remediation. This is a huge issue though – never has the phrase ‘silent killer’ had a more appropriate extrapolative pathway.

The air pollutants in question are varied and abundant. They range from Volatile Organic Compounds (VOCs) such as benzene, formaldehyde, and trichloroethylene to gases like carbon monoxide and nitrogen oxides. These chemicals are being disseminated phenomenally quickly, each with different ensuing health conditions associated with exposure. Luckily, vegetation provides a much-needed amelioration point through active chemical subsuming.

We usually look to trees for carbon dioxide intake and oxygen expulsion, but they’re far more useful than that. Whilst the uptake of carbon dioxide is invaluable, and some trees such as the Norwegian Maple do it extremely well, they’re also able to take in a number of other pollutants. For example, elm trees, the common ash, and oak trees are all noted in primary literature for their ability to assimilate benzene, nitrogen oxides, dioxins and other pollutants. It doesn’t necessarily have to be trees either. There are a multitude of outdoor plants acknowledged for their pollution fighting properties - a common one being English Ivy. This plant, whilst also used for decorative purposes, is fantastic at ridding the air of mould, formaldehyde, faecal particles, benzene, carbon monoxide and trichloroethylene.

Developments, especially in recent years, have recognised the benefits of urban trees and their subsequent impact on air quality and energy expenditure. Trees have the ability to stabilise temperature in their immediate vicinity, either through evapotranspiration inciting a cooling trend, or by shielding buildings from harsh winds that’d usually steal heat through convection. There are now guidelines specifying the optimum distance/orientation of trees relative to development sites. These guidelines are in place so that developers can profit from the microclimate regulation and clean air whilst mitigating any potential for limited solar exposure.
There are negatives to urban vegetation though - human-induced negatives - but negatives nonetheless. Whilst trees are useful for ridding our air of pollutants and volatile organic compounds, some species actually excrete them. For example, oak trees are among the highest for standardised isoprene emission rates, meaning that these trees (relative to others) have some of the most profound impacts on ground level ozone emissions. This ‘bad’ ozone is created in the presence of sunlight and anthropogenic nitrogen oxides, where isoprene breaks down on contact into tropospheric ozone. Other tree species associated with elevated isoprene release include sycamores, willows, poplar and eucalyptus.
This is not to say that we should now start vilifying trees, the more appropriate solution to elevated ground-level ozone is to combat the prolific nitrogen oxide release that we’re responsible for. Limited atmospheric nitrogen oxides (a reagent) would inevitably lead to a limited reaction potential, solving the only real negative that vegetation brings to the air quality debate.

With that in mind, anyone who feels strongly about the cleanliness of our air should take precautions to ensure that they’re not exacerbating the situation. It could be as simple as planting a tree, but could extend to investment in a catalytic converter and the use of exhaust gas recirculation. Even just making sure everything on your car is tuned up could lessen the amount of nitrous oxide you emit (e.g. maintaining the correct tyre pressures). These are small but vital steps in our ongoing journey to a cleaner world.


Photo: Derry Stock

Derry Stock BSc (Hons)
Environmental Consultant Intern

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