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.”



You could take the view that it is the fortunate classes breathing cleaner air who have the luxury of focusing on climate change regulation
Lisa Ramsay WSP South Africa

Conflicting agendas

It is a logical and effective approach. Applying it citywide is challenging, however, not least because, when there are so many potential sources of pollution, it is tempting to make unsupported assumptions about who or what are the biggest polluters. “Traffic is often seen as a major culprit, partly because you can actually see pollution exiting tail pipes, and because we all travel by car, we all feel responsible. But it has to be put in context. In India as a whole traffic only just makes it into the top five sources of pollution — coal burning is much worse.”

It is important not to generalize about the traffic itself, she adds. “Petrol cars are not the big emitters they were. They still emit NOx and carbon monoxide (CO), but in tiny amounts compared with the levels that existed before catalytic converters, and also compared with most diesels. Big diesels, like truck and bus engines, are by far the biggest particulate emitters on the roads.”

Getting people out of their cars may still be desirable. It will help with congestion, and from a global warming perspective, it will reduce a city’s carbon footprint. As a way to reduce NOx and particulate pollution, however, there might be easier wins and better targets, such as regulating bus engines.

As Ramsay says, it is a question of seeing the problem clearly: “Just as there is confusion about global warming and the ozone layer, there is a vast amount of confusion about pollution and global warming — about, for instance, addressing carbon dioxide (CO2) and CO emissions.”

For the record then, CO2 is a significant greenhouse gas, but not a significant pollutant, in that it has no adverse health effects in itself. CO, produced by machines such as vehicles and gas boilers, is a pollutant and does impact on health.

The Iceberg apartment complex in Aarhus, Denmark, by JDS Architects, CEBRA, SeARCH and Louis Paillard. Photo by Mikkel Frost

“It’s not uncommon for greenhouse gas reduction and air pollution agendas to come into conflict,” explains Ramsay. “Stack abatement filtration systems reduce particulates but they can be energy-intensive. Alternatively, burning methane from landfill to create energy is good from a global warming perspective, but there are emissions of health-impacting air pollutants such as sulfur dioxide (SO2) and NOx during the power generation process.”

A meteorologist by training, Ramsay stresses that global warming absolutely needs to be addressed. But she adds: “If you live in a very polluted city, as millions of people do, especially in developing countries, then pollution is seriously impacting your health right now. You could take the view that it is the fortunate classes breathing cleaner air who have the luxury of looking to the future and focusing on climate change regulation.” In many countries and jurisdictions, greenhouse gas budgets and pollution budgets are often managed separately. “But it’s vital that these issues are managed together and that we find solutions that work for both.”

Recent history is littered with examples of policies intended to reduce CO2 emissions having a dire effect on air quality. Most notorious is the EU’s response to the 1997 Kyoto climate change agreement. Motorists were encouraged to switch from petrol to diesel cars because the greater miles per gallon they offered would mean a reduction in CO2 emissions. This was true, but it failed to take into account how much more polluting diesel engines are. As John Gallagher, an air quality expert at Trinity College Dublin attests: “A lot of decisions are made without thinking through the consequences. There was a big push in Ireland to use diesel during the financial crisis — even though there was no doubt that diesel was more polluting. It resulted in a big spike in NOx emissions.”

The recent "dieselgate" scandal, in which Volkswagen was found to have deliberately misrepresented real-world diesel emissions from its engines, has only revealed the impact of the policy to have been even worse. While the pollutant output of compliant diesels fitted with filters is now comparable with petrol engines, it has been estimated that most diesel cars on European roads still produce more particulates and on average 30% more NOx than equivalent petrol models.

Behind closed doors 

Other examples are depressingly commonplace. In Europe, for example, wood-fired stoves have been marketed as an environmentally friendly form of domestic heating because burning wood is (arguably) carbon neutral. Their burgeoning popularity has, however, led to wood smoke becoming the major source of particulate pollution in many residential areas — a fact lamented by Barny Evans, head of WSP’s sustainable places, energy and waste team in London. “These issues also arise with wood-fired boilers for commercial property. Similarly, in the UK, we have for years been encouraged to use local combined heat and power (CHP) plants for large developments because they produce less CO2 than heating and powering large buildings conventionally. But CHP produces five to ten times the NOx emissions of conventional gas heating. There is a reason why power stations were moved out of cities. It seems perverse to bring them back to where their pollution can affect the greatest number of people.” BRE, the UK’s building science research centre, now seems to agree — up to a point — and has recently downgraded the credits awarded to developments for incorporating CHP under the BREEAM sustainability rating scheme.

There are always going to be cities that find it tougher than others, because of their particular infrastructure or atmospheric conditions
Alice Lovegrove WSP USA

Evans says that this is symptomatic of a tendency to focus on buildings when seeking to mitigate CO2, and on traffic with regards to pollution — when, in reality, both are responsible for both. “Astonishingly, if you read almost any air quality strategy, they focus almost exclusively on vehicles,” he says. “Of course it is important to address traffic-related issues, but there are limits. At the moment there’s no realistic alternative to letting diesel lorries into your city to deliver goods. And look at any pie chart of city NOx emissions: as a rule of thumb 30% of NOx is actually coming from gas boilers for commercial and residential heating and hot water.”

He argues that while diesel use will have to be reduced gradually, as alternatives slowly become available, it would be possible to dictate how new buildings are heated almost immediately. “In London, we build 40,000 homes a year and almost all are gas-heated. With heat pump technology, there is no need, with the added benefit that heat pumps can act as air-conditioning when temperatures rise.” Housebuilders may quarrel with Evans’ assertion that banning gas boilers could be immediate, but it does seem that for cities such as London we might be missing an easy win.

The Palacio de Congresos Europa in Vitoria-Gasteiz, northern Spain. A green belt of parks and natural areas around the city centre has contributed to a sharp fall in levels of air pollution. Photo by Marek Stepan/Alamy Stock Photo

Crisis cities

For some places, no opportunity for improvement can be squandered. Delhi has made huge efforts to improve air quality, but still struggles to shake off the unenviable title of “world’s most polluted capital”. In the 1990s, tuk-tuks (motor rickshaws) were forced to switch from diesel to compressed natural gas (CNG). So were buses. In recent years a new metro system has come on stream with 200km of track, 60 stations, and more planned. Delhi has even banned the sale of fireworks — to no avail. “I grew up in Delhi, and pollution is even worse now than it was 20 years ago,” says Snigdha Jain from WSP’s sustainable city analytics team in London. “You would have thought the metro would have really helped, but though the trains are full, it made almost no difference.”

The experience of Delhi illustrates the fact that cities are fast-changing dynamic entities over which, try as they might, mayors and urban planners can never exercise complete control. This applies to first-world cities as well as those in developing countries. Riverside County in California, for example, enjoys all the benefits of the state’s environmentally conscious government and effective regulations — yet routinely fails to meet US air quality standards, partly due to air currents that flow inland from the busy ports of Los Angeles and Long Beach.

“There are always going to be cities that find it tougher than others, because of their particular industrial infrastructure or local atmospheric conditions,” says Alice Lovegrove, national director of air quality for the US at WSP. “For those cities, it is even more important to understand exactly where pollution is coming from. You need first-class monitoring and modelling to fully understand how pollution is affecting people.”

Like Ramsay, Lovegrove stresses the importance of strategies that are specific and informed: “There’s no one-size-fits-all solution. If ozone is your problem, then you might want to concentrate on tackling NOx, as this encourages its formation. If it’s particulates, then maybe look at construction sites, the dust they make and the diesels they are using.”

Cities that are struggling to get their pollution issues under control should not become disheartened. Though Delhi’s metro did not solve its pollution problem, the situation would undoubtedly be worse still without it. And, as Lovegrove points out, there is always more that can be done: “When you have picked the lower hanging fruit, keep looking. With the right information you can always identify what you need to do next — that intervention which is going to really help — and take your air quality strategy to the next level."


 This article was originally published in August 2019 in WSP’s magazine The Possible

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