This article was originally published in August 2019 in WSP’s magazine The Possible
Airborne contaminants have been the scourge of built-up areas for so long that our towns and cities don’t just suffer from air pollution — they have been actively shaped by it.
As the philosopher Friedrich Engels noted back in 1845, for example, wealthier suburbs are more usually built on the western side of English cities to avoid pollution drifting east on the prevailing wind. Similarly, well-to-do “folks who live on the hill” are there because they can afford to breathe air that is fresher than in the valley below.
In Engels’ time there was little in the way of scientific analysis of air, the contaminants it contained and the harm they could do: coughing, breathlessness and short lives were enough to persuade people that dirty air was worth avoiding.
Today, though, we do have the data, and it is not pretty. According to the World Health Organization, an astonishing 91% of the global population breathes air that fails its air quality targets. For city dwellers, the situation is even worse, with pollution in developing cities up to 15 times higher than guideline levels. Poor air quality is blamed for 4.2 million premature deaths annually, from health conditions such as heart disease, strokes, respiratory illnesses and cancer.
Pollution comes from many sources and takes many forms (see “What’s the problem?” below). The most commonly measured are NOx gases — nitrogen oxides formed by combustion — and PM2.5, particulates with a diameter of less than 2.5 microns, also formed by combustion. In essence, the solution would seem simple: stop the emitters from emitting. In practice, things are more complicated. Discovering which pollutants are emitted by what, for example, is seldom straightforward.
“Even when you are measuring pollution at an individual chimney stack, you have to know what you are looking for — it is not always obvious,” says Lisa Ramsay, air quality specialist at WSP and research fellow at the University of KwaZulu-Natal in South Africa. “So you look at the processes involved, deduce what pollutants could be produced, and then measure for them. Once you know what you are dealing with, you can advise on modifying the process to produce less pollutants, or suggest mitigation measures such as filters for particulates or scrubbers for unwanted gases.”
WHAT'S THE PROBLEM? THE MAIN POLLUTANTS, THEIR SOURCES AND EFFECTS