An Open Letter to the Supreme Court of India Against Outdoor Air Purifiers

"For some reason, the misconception that outdoor air-purifiers can clean the air never seems to die away."

Members and advisers of Care for Air, a nonprofit organisation that works on issues of air pollution and its awareness, recently penned a letter addressed to the Supreme Court of India requesting that it not promote “band-aid fixes” like smog towers and other outdoor air-purifiers to deal with north India’s debilitating winter smog. The letter is reproduced below, edited for brevity and style.

Dear Supreme Court:

We, the people, would like to thank you for your concern about the health harm being caused by the toxic levels of air pollution in the entire country, especially in the country’s colder north, now reeling under smog. In this time of a public health emergency, the honourable courts seem to be the only ones to care for our health. For that, we are truly grateful.

However, India’s leading air-pollution scientists and researchers unanimously say outdoor air purifiers are inefficient to the point of being useless. We earnestly request you: please don’t direct any governments, local, state or central, to spend public money to purchase expensive smog towers. These are completely useless in bringing down PM2.5 levels by any significant amount, and could even add to pollution when their filters are disposed in our already overflowing landfills, and eventually burnt.

Resources for environmental protection are scarce. Why spend it on ineffective band-aids? Air pollution, specifically the more lethal PM2.5 particulate matter can only be controlled by eliminating emissions at all known sources. Any other way is bound to be inefficient, ineffective and unscientific, and can only buy time – but which we are paying with our breaths every day. The air we care about is the air we breathe.

The air flowing through a city is generally of the order of a million to a billion times the volume of air we inhale in the same area, compared to 1,000-10,000 times in a closed room. This is the reason why indoor air purifiers work and outdoor air purifiers don’t.

As Joshua Apte, one of Care for Air’s international advisers, said, “Many professors assign this as homework or exam problems to explore why this approach simply cannot work at scale.”

Sarath Guttikunda, a member of Urban Emissions (as well as the Care for Air group), described a back-of-the-envelope calculation for outdoor air-cleaning:

Delhi’s airshed is approximately 80 km x 80 km. If we assume Delhi is a box (like a room), atmospheric height is at least 1 km (and up to 3 km in the summer). Let’s assume an average wind speed of 2 m/s (direction is irrelevant). So in any given hour, the amount of air passing through the city is 5.76 billion m3 per hr. The capacity of the smog tower in Xi’an, China, is a fraction of that, at 460,000 m3 per hr. Even assuming 100% efficiency of the tower at all times, it can purify only 0.00007233796% of the air every hour. In sharp contrast, capturing all emissions at one outlet, say, a cement-making unit or coal-fired power plant will mean zero emission at source – i.e., 100% efficiency.

Please, honourable judges, help save our public money for more useful ways to clean our air. Like the odd-even road rationing scheme did nothing to reduce air pollution, smog towers also won’t have any impact. But, again like the odd-even scheme, the best one can expect from smog towers is better public awareness of air pollution – and political mileage. Worse, in the case of smog towers, the only people who will profit will be their manufacturers and sellers.

Outdoor air purifiers are inadvisable. Such installations give citizens a false sense of complacency and assurance, making them feel they are getting healthier and have access to air of better quality. Such populist decisions make the people feel that the government is doing something positive to solve this giant problem. It becomes very difficult to oppose them in spite of the science and evidence pointing clearly to their ineffectiveness.

As Dr Randeep Guleria, director of the All India Institute of Medical Sciences, New Delhi, recently said about masks, smog towers provide more reassurance than actual protection. At this point of a health emergency, we need much more than reassurance. We need action – and not just any action. We need strong action that yields the desired result: a real and significant reduction in PM2.5 levels.

As an organisation, we have also checked with an independent scientist and consultant closely involved with government efforts in China, and who reconfirmed that smog towers there are used more for awareness than for actual results.

“There is a consensus among air quality scientists that any outdoor air-cleaning approach is unlikely to be effective, regardless of the specific technology used,” according to Apte.

Some cities have already made this colossal mistake and wasted not just public money but also CSR money, as we have read in news reports. The Bruhat Bengaluru Mahanagara Palike’s well-meaning but foolish decision to install air purifiers at busy traffic intersections is a complete waste of both public and CSR resources. Mumbai and Delhi also placed purifiers on roads with no results.

For some reason, the misconception that outdoor air-purifiers can clean the air never seems to die away. We feel it is because people are beginning to understand the health harm caused by toxic air and are keen to find quick solutions, or at least a shortcut, to improve air quality.

The only solution to air pollution for the honourable court to act on is to control emissions at sources. Please be assured that China and other countries haven’t brought their pollution down by using smog towers. Instead, they have strengthened and enforced industrial emissions standards, phased out small and/or outdated outdated factories, promoted clean fuels, strengthened vehicle emission standards and encouraged the use of public transportation.

Honourable judges, please allow one of our experts come and present to you more specific reasons for why smog towers don’t work efficiently, and why this is not the way to go. Our experts can also respectfully offer some concrete suggestions on how the honourable court can issue urgent directions to reduce pollution by controlling emissions from all sectors that contribute to it.

Today, we request you to prevent public money from being wasted in such populist and ineffective strategies that aren’t backed by science, data and evidence.

Care For Air
Mission: Clean Air for All

Care for Air’s governing board consists of Abhishek Bhartia (Sitaram Bhartia Institute of Science and Research), Amrita Bahl, Barun Aggarwal, Bhargav Krishna (senior air pollution scientist at the Harvard T.H. Chan School of Public Health), Chetan Bhattacharji (Managing Editor, NDTV), Gopal Sankaranarayanan, (Senior Advocate, Supreme Court), Jyoti Pande Lavakare (President, Care for Air), and Manjali Khosla. Our scientific advisers are Sarath Guttikunda, co-director of Urban Emissions (India), Joshua Apte at the University of Texas (USA) and Pawan Gupta, senior scientist at NASA. Apte and Gupta advise us in their personal capacities.