Air too burdened to breathe!

Earth was a beautiful landscape but man has ruthlessly exploited it for his greed, especially in the past century. With rapid industrialization and random urbanization environmental pollution has become a serious problem. Over exploitation of open spaces, ever-increasing number of automobiles and demographic pressure has further aggravated the problem. Pollution, the major problem in cities, is compounded by the fact that there is no exhaust for the polluted air to escape. The truth is that air pollution is not restricted to one city. It’s a global phenomenon. India and China are famous for making headlines for the deadliest air pollution, but while China has by and large managed to reduce PM2.5 levels by enacting a Clean Air Action Plan, pollution levels in India are still on the up. So is it just a gap in policy that limits India in tackling the airpocalyptic situation? The answer is no. In India, the public outrage over air pollution is still seasonal…

Living in the state capital of Uttar Pradesh, Kavita Kumar knows the personal and economic price of breathing some of the world’s most toxic air. At 32, she learned that pollution was the likely driver of the cancer growing inside in her lungs. She had never touched a cigarette. Her husband sold land to pay for her treatment. They borrowed money from family. Their savings slowly disappeared. “The government is thinking about the economic growth of the country, but people are dying of diseases or suffering from diseases,” she says. “How can you grow economically when, within your country, your citizens are facing economic problems because of the air pollution?” Similar are the views of Anjali Chauhan, mother of two, who is very upset by the fact that her younger daughter is developing signs of asthma. “This is when I have bought both my kids ‘pollution masks’ that are available in the market. But are they really useful? I doubt,” she claims while pointing out quite angrily “People here are just not concerned about the problem. You can see them burning waste, including the so called officially banned plastic, and no one can check them. We hesitate to correct our neighbours, look the other way when safai workers burn trash, raise no alarm when vehicles keep running their engines during traffic jams and cause vehicular pollution, not to mention the unnecessary honking that is taking place all the time at the crossings. And even those who do try to correct the wrong doers, no one comes to their support, not even the policeman whose duty is to ensure implementation of pollution control measures put in place by the government. When roadside trees are cut, who comes to their rescue? Everywhere you can see construction work taking place. More and more under-construction flyovers, buildings and metros are causing such a lot of din and loading the air with pollutants. Add to this private constructions and you have a death-like situation for those living in cities, especially metro cities. The so-called plantation along the metro lines is just an eyewash and the available green cover is falling a prey to developmental claims. It is the people who must come forward and fight for their fundamental right to clean air. When they are not bothered, the government is also taking advantage. The few protests that take place are a pebble in the ocean. What is required is a ‘war’ that every section of the society is a part of.” When Arvind Kumar of New Delhi’s Sir Ganga Ram Hospital started as a chest surgeon in 1988, 90 percent of his lung cancer patients were middle-aged male smokers. Now, he says, 60 percent of his cases are non-smokers, while half are women.

Fourteen Indian cities are among the world’s 20 most polluted, according to the revised World Health Organization (WHO) data. The study shows the northern city of Kanpur tops the list of world cities with the highest PM2.5 levels. Kanpur’s average PM2.5 level 173 is 17 times higher than the WHO’s safe limit. The study adds that nine out of 10 people in the world breathe polluted air. This list contains the top 499 cities by PM2.5 annual mean concentration measurement whose measurement is greater than or equal to the air quality guideline (AQG) of 10 as documented by the World Health Organization covering the period from 2008 to 2016, with the majority of values being more recent than 2013. The 2018 version of the WHO database contains results of ambient (outdoor) air pollution monitoring from almost 2700 towns and cities in 91 countries. Air quality in the database is represented by the annual mean concentration of particulate matter (PM10 and PM2.5, i.e. particles smaller than 10 or 2.5 micrometres, respectively). Maria Neira, Director of the Department of Public Health, Social and Environmental Determinants of Health at WHO, says India should follow the example of China, which had made big improvements in air quality. “There is a big step at the government level (in China) declaring war on air pollution," she says. “We would be very happy if we would see a similar movement now in India which is one of the countries for which we are particularly concerned.” Though the government promises to initiate steps to curtail hazardous air while also launching many ambitious development plans, any gains would have to be enough to override other facets of rampant growth, from the dust left by thousands of new construction sites to exhaust from millions of new cars, especially now during the winter season.

If strict policies to battle smog were successfully implemented, we would be much richer. By the World Bank’s calculations, health-care fees and productivity losses from pollution cost India as much as 8.5 percent of GDP. At its current size of $2.6 trillion that works out to about $221 billion every year. While India is currently the world’s fastest growing major economy, China's $12.2 trillion economy is five times larger. The Centre is still trying desperately to promote basic manufacturing, which could cause pollution to worsen, said Raghbendra Jha, an Australian National University economics professor. “It’s too simplistic to assume a smooth transition” to clean economic growth in India, he said. Meanwhile, after two decades of expansion that reshaped the global economy, China is orchestrating a shift to less-polluting services and consumption. So while its cities still see smoggy days, they’ve also seen improvements. The number of “very unhealthy” days or worse - when levels of dangerous particles called PM2.5 crossed 200 - rose to 84 in New Delhi in 2017 from 66 in 2015, according to analysis of air quality data from AirVisual, which tracks air quality. In Beijing they dropped to 20 days from around 43 over the same period. “The major challenge is that people are not consistently demanding improvements in air pollution, as happened in China,” Michael Greenstone, director of the Energy Policy Institute at the University of Chicago, said via e-mail. “This is because the extent to which air pollution is causing people in India to lead shorter and sicker lives is not yet fully recognized.”

Though you can find unhealthy air in pretty much any country, the worst pollution these days is accumulating in developing countries with growing populations. The highest-ranking city from an OECD country in the PM2.5 list is Hobart, Australia, coming in at 206. “Air pollution threatens us all, but the poorest and most marginalized people bear the brunt of the burden,” said Tedros Adhanom Ghebreyesus, the WHO’s director general, in a statement. Cookstoves, heating fuel, and kerosene lighting are all common sources of pollution in big cities in developing countries. Poor governance is also a major factor in pollution since lax enforcement of standards for car exhausts, crop burning, or dust from construction sites leads to more particulates in the air. “Many of the world’s megacities exceed WHO’s guideline levels for air quality by more than five times, representing a major risk to people’s health,” says Dr Maria Neira. “We are seeing an acceleration of political interest in this global public health challenge. The increase in cities recording air pollution data reflects a commitment to air quality assessment and monitoring. Most of this increase has occurred in high-income countries, but we hope to see a similar scale-up of monitoring efforts worldwide.”

When you look at the database’s ranking of particulate pollution in cities, 11 of the 14 cities with the highest levels are located in India. Kanpur, India, population 3 million, tops the list with a yearly average of 319 micrograms per cubic meter of PM2.5, the most hazardous particle commonly measured. (Bamenda, Cameroon, is the one city outside of India in the top 12.)

Cities with the highest small particulate measurements in the world

Country                  City                 PM2.5 (μg/m3)

 India                      Kanpur               173

India                       Faridabad           172

India                       Varanasi             151

India                       Gaya                   149

India                       Patna                  144

India                      Delhi                   143

India                      Lucknow             138

Cameroon                 Bamenda        132

India                       Agra                  131

India                       Muzaffarpur       120

India                       Srinagar             113

India                       Gurgaon            113

World Health Organization

Many different air pollutants can impact health — nitrogen oxide, carbon monoxide, and ozone, among them. But the database classifies air pollution in two ways: by PM 2.5, particles smaller than 2.5 microns in diameter, and by PM 10, particles that are 10 microns in diameter. The smaller PM2.5 particles from sources like open flames, factory chimneys and diesel exhaust can linger in the air longer and penetrate deeper into the lungs than larger particles, which is why they’re the bigger concern for health officials and a high-priority target for reduction. Technically, the WHO doesn’t treat its data as a ranking but rather a measurement of where risks are. But it’s clear from the report that India is one of the riskiest countries in the world to breathe, up there with Bangladesh and Georgia. When it comes to comparing PM10 measurements of the world’s largest cities, India’s capital Delhi comes in with an annual average of 292, ahead of Cairo (284), Dhaka (147), Mumbai (104), and Beijing (92). Frequent unhealthy levels of pollution from sources ranging from vehicles to the burning of coal and wood for cooking, dust storms, or forest fires affect most of the country. India’s hills and mountains also act as basins that trap toxic air over vast swaths of the country, sometimes making the air too dangerous to breathe.

The government acknowledges that more needs to be done. Its officials have also tried to ban farmers from burning crops. In a statement, the ministry said as most of the polluted cities lie around Delhi and along the Indo-Gangetic plain, it is critical that the governments of Uttar Pradesh, Bihar and West Bengal are sensitised to take up urgent action on cleaning air. The WHO report indicates that Delhi is placed at number six with an annual average PM2.5 concentration as 143 micrograms per cubic metre in 2016. However, the government has made serious efforts to deal with air pollution. “Data for the year 2017 for PM2.5 shows an improvement over 2016 and so far in 2018, it shows a further improvement, as compared to 2017. The government has also taken several bold initiatives, including leap-frogging from BS-IV to BS-VI,” it said. The ministry said out of the 94 non-attainment cities, action plans for 61 cities have been received and the states concerned have been urged to steer implementation of action plans in these cities. It asserted that action has been taken in Delhi and NCR, including the formulation of Graded Response Action Plan by CPCB, and referred to other measures, including a comprehensive action plan by the ministry, ban on petcoke in Delhi and NCR, and strict surveillance over coal-fired plants. It also referred to the launch of a mobile app - SAMEER - for grievance redressal. But environmentalists are still waiting for more concrete targets from a national clean air plan that has yet to be officially launched. “Any plan at this stage requires reduction targets and there has to be a clear compliance strategy," says Anumita Roychowdhury, who’s advocated on air pollution for more than 20 years at New Delhi's Centre for Science and Environment. There’s an additional challenge. In a chaotic democracy, where poverty and unemployment are often seen as bigger concerns, different branches of government run by competing political parties sometimes have little incentive to collaborate on pollution. China, by contrast, had Premier Li Keqiang declare a national war on pollution. The central government also told local officials they wouldn't get promoted without meeting air quality targets, and the country has sought to move away from polluting industries. Government policies forced millions of homes and businesses to switch from coal to cleaner-burning natural gas.

In a country largely under construction – by some rough projections, around 70% of the buildings that will exist in India’s cities by 2030 are yet to be built – controlling the dust produced by roads and worksites is an important, but largely neglected. “Between now and 2030, we need to build something like 700 to 900 million square meters of urban space every year, which means one Chicago every year,” says a spokesman for the housing and urban affairs ministry. “Can this be done? I don’t think that’s an appropriate question - you have no option but to do it. And you have to do it in a green and resilient manner.” But, it is also a fact that inequality is growing. Many are above the official urban poverty line of Rs 47 a day but are not quite middle class and are vulnerable to shocks, according to Sumila Gulyani, programme leader for infrastructure and sustainable development in India at the World Bank. What's more, non-monetary measures of poverty, like access to healthcare and education, are sometimes worse for the poorest urban inhabitants compared to their rural counterparts. “Housing hasn’t kept pace and environmental issues have lagged,” she says. “The inability to manage this massive influx, to keep pace with needs for infrastructure, services and housing, and sustain the environment, has reduced the kind of economic returns that you would expect to see from this massive urban population growth.” Urban planner and architect Ranjit Sabikhi says that type of geographical segregation of the poor can create serious social stresses in cities. “It’s a very unhealthy concentration, and large numbers of our population are living in conditions with no schools, no health centers, no open spaces for children to play. Instead, you could have a whole series of mixed housing, for high, middle as well as for low-income groups.”

“It’s totally haphazard, mad growth of population with no regard” for the effects on the environment and health,” asserts Dr Arvind Kumar. “There’s just been massive growth, shift and migration that is adding to the problem.” In India, much of the migration to cities is ‘hidden’, with the urbanization rate probably closer to 55 percent, rather than the official estimate in the 2011 census of around 32 percent, according to Gulyani. And the bigger the city, the more attractive it becomes for migrants. By 2030, the world is projected to have 43 megacities with more than 10 million inhabitants, most of them in developing regions. “A lot will depend on how cities are managed," says Gulyani.

Key Findings

Air pollution levels remain dangerously high in many parts of the world. New data from WHO shows that 9 out of 10 people breathe air containing high levels of pollutants. Updated estimations reveal an alarming death toll of 7 million people every year caused by ambient (outdoor) and household air pollution.

WHO estimates that around 7 million people die every year from exposure to fine particles in polluted air that penetrate deep into the lungs and cardiovascular system, causing diseases including stroke, heart disease, lung cancer, chronic obstructive pulmonary diseases and respiratory infections, including pneumonia.

Ambient air pollution alone caused some 4.2 million deaths in 2016, while household air pollution from cooking with polluting fuels and technologies caused an estimated 3.8 million deaths in the same period.

More than 90% of air pollution-related deaths occur in low and middle-income countries, mainly in Asia and Africa, followed by low and middle-income countries of the Eastern Mediterranean region, Europe and the Americas.

Around 3 billion people – more than 40% of the world’s population – still do not have access to clean cooking fuels and technologies in their homes, the main source of household air pollution. WHO has been monitoring household air pollution for more than a decade and, while the rate of access to clean fuels and technologies is increasing everywhere, improvements are not even keeping pace with population growth in many parts of the world, particularly in sub-Saharan Africa.

WHO recognizes that air pollution is a critical risk factor for non-communicable diseases (NCDs), causing an estimated one-quarter (24%) of all adult deaths from heart disease, 25% from stroke, 43% from chronic obstructive pulmonary disease and 29% from lung cancer.

More countries taking action: More than 4300 cities in 108 countries are now included in WHO’s ambient air quality database, making this the world’s most comprehensive database on ambient air pollution. Since 2016, more than 1000 additional cities have been added to WHO’s database which shows that more countries are measuring and taking action to reduce air pollution than ever before. The database collects annual mean concentrations of fine particulate matter (PM10 and PM2.5). PM2.5 includes pollutants, such as sulfate, nitrates and black carbon, which pose the greatest risks to human health. WHO air quality recommendations call for countries to reduce their air pollution to annual mean values of 20 μg/m3 (for PM10) and 10 μg/m3 (for PM25).

While the latest data show ambient air pollution levels are still dangerously high in most parts of the world, they also show some positive progress. Countries are taking measures to tackle and reduce air pollution from particulate matter. For example, in just two years, India’s Pradhan Mantri Ujjwala Yojana Scheme has provided some 37 million women living below the poverty line with free LPG connections to support them to switch to clean household energy use. Mexico City has committed to cleaner vehicle standards, including a move to soot-free buses and a ban on private diesel cars by 2025.

Major sources of air pollution from particulate matter include the inefficient use of energy by households, industry, the agriculture and transport sectors, and coal-fired power plants. In some regions, sand and desert dust, waste burning and deforestation are additional sources of air pollution. Air quality can also be influenced by natural elements such as geographic, meteorological and seasonal factors.

Air pollution does not recognize borders. Improving air quality demands sustained and coordinated government action at all levels. Countries need to work together on solutions for sustainable transport, more efficient and renewable energy production and use and waste management. WHO works with many sectors including transport and energy, urban planning and rural development to support countries to tackle this problem.

WHO estimates that around 90% of people worldwide breathe polluted air. Over the past 6 years, ambient air pollution levels have remained high and approximatively stable, with declining concentrations in some part of Europe and in the Americas.

The highest ambient air pollution levels are in the Eastern Mediterranean Region and in South-East Asia, with annual mean levels often exceeding more than 5 times WHO limits, followed by low and middle-income cities in Africa and the Western Pacific.

Africa and some of the Western Pacific have a serious lack of air pollution data. For Africa, the database now contains PM measurements for more than twice as many cities as previous versions, however data was identified for only 8 of 47 countries in the region.

Europe has the highest number of places reporting data.

In general, ambient air pollution levels are lowest in high-income countries, particularly in Europe, the Americas and the Western Pacific. In cities of high-income countries in Europe, air pollution has been shown to lower average life expectancy by anywhere between 2 and 24 months, depending on pollution levels.

Types of Pollutants

In order to understand the causes of Air pollution, several divisions can be made. Primarily air pollutants can be caused by primary sources or secondary sources. The pollutants that are a direct result of the process can be called primary pollutants. A classic example of a primary pollutant would be the sulfur-dioxide emitted from factories Secondary pollutants are the ones that are caused by the inter mingling and reactions of primary pollutants. Smog created by the interactions of several primary pollutants is known to be as secondary pollutant.

Causes of Air pollution

1. Emissions from Industrial Plants and Manufacturing Activities

With the current rate of industrialization and increased manufacturing activities, high levels of smoke, sulfur dioxide, and particulate matter are emitted into the air. In a typical industrial plant, for instance, it is easy to notice the long chimneys or smokestacks erected into the air, emitting large amounts of fumes and smoke from it. Industrial plants, factories, and power plants release high amounts of organic compounds, chemicals, particulate matter, and carbon monoxide into the air. Industrial plants that refine petroleum, manufacture cement, produce metals such as steel and aluminum, process plastics, or manufacture chemical products are among the industries and manufacturing activities that release lots of foreign harmful materials into the air. Petroleum refineries, for example, emit high levels of hydrocarbons into the air. Most manufacturing plants release the pollutants in small amounts, but continuously over extended time periods that eventually leads to cumulative negative effects. In some cases, manufacturing plants have accidentally emitted high scores of air pollution in a very short time span that has lead to massive harm to human and animal health and destroyed plant life.

2. Combustion From Fossil Fuels

In the modern world, fossil fuel combustion is the biggest contributor to air pollution. The leading culprit today is traffic, but factories and power plants also continue to contribute to some extent. Conventional power plants that combust fossil fuels to produce energy emits hazardous gases such as oxides of nitrogen, Carbon Monoxide, particulates, and hydrocarbons into the air. The number of cars on our roads are overwhelming and increasing, with an estimated number of more than half a billion cars on the road. Heavy duty trucks, shipping vessels, trains, and airplanes also combust lots of fossil fuels to function. All these transportation utilities are power-driven by diesel and gasoline engines that combust petroleum to produce energy. The petroleum contains hydrocarbons. Thus, in the process of combustion, it releases Nitrogen Oxides (NOx), Carbon Monoxide (CO), Volatile Organic Compounds (VOCs), lead, and particulate matter into the air. Collectively, these are the major contributors to air pollution and have persisted as one of the most challenging to manage because humans heavily rely on these forms of transportation.

3. Farming Chemicals and Household Products

Use of household and farming chemicals produce considerable amounts of harmful foreign substances into the atmosphere and have the potential to cause air pollution. Dusting of crops, painting supplies, home fumigation, household cleaning products, fertilizer powder, insect/pets sprays, hair sprays, and deodorant sprays release harmful chemicals into the air, causing pollution. High concentrations of these chemicals within a confined area can be hazardous and can cause serious health and breathing problems. Since they are regularly used products, they also qualify as major causes of air pollution as they release toxic particles and chemical gases into the atmosphere.

4. Natural Causes of Air Pollution

The majority of people only perceive air pollution as a consequence of human actions. In some cases, natural events can as well cause air pollution. However, they are rarely witnessed, and some of them are very disastrous and difficult to prevent from happening. Examples of natural events that lead to air pollution include volcanic eruptions, whirlwinds, forest fires, and gaseous releases from decaying plants and animals or radioactive decay of rocks. Forest fires often begin naturally and can release huge amounts of smoke and dust particles that float in the air. The smoke and dust can be carried for miles within a short time leading to widespread air pollution. Some of the biggest wildfires have emitted smoke that drifted several miles across bordering cities and countries. A massive volcanic eruption has the potential of discharging huge swathe of dust into the atmosphere to an extent of obstructing sunlight from reaching the earth surface. High amounts of gases released from plant and animal decaying matter or radioactive rock decay can also result in the pollution of air. Collectively, these examples provide a clear picture of some of the natural causes of air pollution which are beyond human control.

5. Other Causes

The majority of developing countries burn charcoal, wood, and crop waste to produce fuel used for cooking and heating. As such, the traditional practice of wood and charcoal burning is usually a chief contributor to indoor air pollution in developing nations. Burning charcoal, wood, and crop waste release Carbon Monoxide (CO), Carbon Dioxide (CO2), and particulate matter into the atmosphere that potentially causes air pollution. In some regions, waste is incinerated instead of being land-filled or recycled and as such, it substantially contributes to air pollution. Some agricultural practices like burning agricultural land after harvest or burning as a mechanism for clearing land cover is also a significant cause of air pollution.

Effects of Air pollution

1. Respiratory and heart problems: The effects of Air pollution are alarming. They are known to create several respiratory and heart conditions along with Cancer, among other threats to the body. Several millions are known to have died due to direct or indirect effects of Air pollution. Children in areas exposed to air pollutants are said to commonly suffer from pneumonia and asthma.

2. Global warming: Another direct effect is the immediate alterations that the world is witnessing due to Global warming. With increased temperatures world wide, increase in sea levels and melting of ice from colder regions and icebergs, displacement and loss of habitat have already signaled an impending disaster if actions for preservation and normalization aren’t undertaken soon.

3. Acid Rain: Harmful gases like nitrogen oxides and sulfur oxides are released into the atmosphere during the burning of fossil fuels. When it rains, the water droplets combines with these air pollutants, becomes acidic and then falls on the ground in the form of acid rain. Acid rain can cause great damage to human, animals and crops.

4. Eutrophication: Eutrophication is a condition where high amount of nitrogen present in some pollutants gets developed on sea’s surface and turns itself into algae and adversely affect fish, plants and animal species. The green colored algae that is present on lakes and ponds is due to presence of this chemical only.

5. Effect on Wildlife: Just like humans, animals also face some devastating affects of air pollution. Toxic chemicals present in the air can force wildlife species to move to new place and change their habitat. The toxic pollutants deposit over the surface of the water and can also affect sea animals.

6. Depletion of Ozone layer: Ozone exists in earth’s stratosphere and is responsible for protecting humans from harmful ultraviolet (UV) rays. Earth’s ozone layer is depleting due to the presence of chlorofluorocarbons, hydro chlorofluorocarbons in the atmosphere. As ozone layer will go thin, it will emit harmful rays back on earth and can cause skin and eye related problems. UV rays also have the capability to affect crops.

How viable are these short-term solutions?

Face masks: Despite looking deeply dystopian, surgeons’ masks are an increasingly common sight in cities around the world, but are largely pointless, claim scientists “Surgical masks are pretty useless because air just leaks in around the side,” according to Prof Ally Lewis, director of the UK’s National Centre for Atmospheric Science. As for more sophisticated anti-pollution masks? “Others are designed to be far more airtight and do remove particles, but don’t remove gases. Nitrogen dioxide can pass right through.” What’s more, if the seal is good enough to keep small particles from leaking in, it may also require uncomfortable amounts of energy to suck air through the mask.

The humble extractor fan: Cooking can cause massive spikes in indoor air pollution, so “extractor fans are a very good idea,” says Rob MacKenzie, professor of atmospheric science at University of Birmingham, “as long as they’re venting outside – and especially if you have a gas hob, because flames produce nitrogen dioxide.”

Personal air purifiers: These come in all sizes – from take-out coffee cup to big industrial-looking drums. “If you run a big blower with a fine particle filter on it,” says Lewis, “as long as your house isn’t too leaky, it will make a meaningful difference to the particle numbers. The question regarding these products is: is the volume of air it’s filtering significant relative to the volume of the house?” A home might cover hundreds of cubic metres, and in most houses the air is completely renewed every hour. “If your unit is the size of a drinks can,” asks Lewis, “does it seem reasonable that it is going to make its way through tonnes of air?” A frequent complaint about the plug-in machines – which have become common domestic appliances in China – is their size. “It’s like having a rattling old air-conditioner,” says Lewis “it takes a lot of energy.” Mark Jacobson, director of civil and environmental engineering at Stanford University, has an even more fundamental problem with the growing adoption of personal purifiers. “They are a short-term way for people to save their lungs but they do not solve air pollution problems – which also harm animals, agriculture and structures. People should not have to breathe through an air filter their entire life.”

In-car filter systems: Consumers can’t know how effectively their car filters the air they breathe. “Cars are small, sealed boxes,” says Lewis, “but they are driven in the most polluted place there is: the middle of the road. Their filters have a tough job.” As well as most particulates, filters in modern cars should catch noxious gases such as nitrogen dioxide with charcoal. But performance will vary and filters become less effective with use – so they need replacing around every six months. Filters should also work more efficiently, says MacKenzie, “if you limit the air exchange between inside and outside, by switching to recirculation rather than continual fresh air. It’s somewhat morally bankrupt, though, to be sitting in your luxury 4X4 with pristine air, churning out god knows what from your exhaust.”

Bus shelter pollution zappers: Air-purifying bus shelters and street furniture are being developed, some using filters and others with added oxidation, which turns gases into dust. It sounds like a good idea, but, says Lewis, “you’ve got to think about how big the atmosphere is over a city. Hundreds and hundreds of square kilometres, possibly 2km deep, so you’ve got a massive swimming pool of pollution.” Unless the bus stop is enclosed, like a mini-waiting room, he says, “the mixing of the atmosphere will completely outweigh the benefits you might get from blowing a bit of filtered air around.” Underground train networks face a similar issue: “It isn’t a sealed box that you can clean up. Every time a train goes through, it’s like a piston replenishing the polluted air. You’d need machines to move thousands, maybe hundreds of thousands of tonnes of air per hour.”

Clean buildings: Lewis sees more gains to be had by cleaning the air in offices and other workplaces than on the streets. “A modern office building is already very air-tight for energy efficiency,” says Lewis, “so you have the opportunity to filter the air because you’re not replenishing it with polluted air from outside.”

More trees: “In terms of air pollution mortality and morbidity, planting trees doesn’t help very much at all,” says Jacobson. “Planting is more useful for absorbing carbon dioxide, which affects pollution indirectly through temperatures but not directly as a chemical air pollutant.” (There are, of course, many other environmental, economic and health benefits to trees). While bunging more trees along London’s Oxford Street probably won’t touch the sides, the more leaves there are, the more fine particulates (PM10s and PM2.5s), nitrogen dioxide and carbon dioxide will be removed from the air. “You’d need two man-made filters to achieve the same effect, which would increase the energy burden,” says MacKenzie, who has a particular interest in how plants affect air. “The most useful places to put lots of vegetation are pedestrianised areas,” he explains, “because there’s nothing there to make the air dirtier. Trees offer the advantage of closing off the area from polluted air above. And tall trees are helpful near motorways because they produce turbulence that helps traffic pollution disperse.”

Green walls: For heavily trafficked streets, says MacKenzie, “green walls would appear, in theory, to be a better option than trees” – since the plants up the side of a building can do their job without risk of trapping pollution at street level. Their success, however, will depend on many factors. “Green walls could help with pollution hotspots, but not with every hotspot – you have to do careful calculations,” MacKenzie explains. “It would require a lot of green vegetation, a lot of maintenance and careful, heavy implementing. So it might be an expensive solution.”

Domestic air quality monitors: Lewis co-authored an article in the scientific journal Nature last year, warning about the proliferation of unregulated, affordable air quality sensors. “Smart” sensors now even come with apps offering breakdowns of CO2, particulates and volatile organic compounds. “There is a significant challenge in making a decent measurement of air pollution with a cheap device,” Lewis says. “The sensors may be unreliable and they are marketed at the general public, who have no way of knowing whether they’re working or not.” By contrast, he points out, air pollution monitoring equipment used by government organisations such as Defra, or academic researchers “typically costs tens of thousands of pounds. If we had a cheap way of doing it, we’d do it the cheap way.” Lewis advises against making health decisions based upon personal monitor readings. The most accurate guide to air quality in your home is to keep track of your local outdoor readings and to keep a close eye on possible internal sources of pollution. “If you’re constantly frying in a wok or have an open fire, you will be making additional sources [of air pollution]. It’s not rocket science, and you probably don’t need a sensor to tell you that.”

Responsible burning: MacKenzie says he finds it bizarre that, for fans of domestic open fires, “the smell is part of the attraction, when that’s telling you it’s a source of pollution”. If your carbon monoxide alarm goes off when you have a fire, you should assume there are other, significant pollutants in the room’s air. “However, the threshold on an alarm is set quite high,” MacKenzie warns, “because it’s about whether you’re going to fall asleep and never wake up again. If it doesn’t go off, you might still have concentrations of carbon monoxide and other particles in your house.” Good ventilation, dry fuel and high temperatures are essential for clean burning, while swept and lined chimneys provide further protection. Even with so-called smokeless fuel, you need to take care. “I’d estimate that smokeless coal produces more nitrogen oxides than wood fuel,” MacKenzie says, “and they both produce the very small particles that are the least noticeable, but the most harmful, of the smoke particles.”