Thick white haze that lasts for days in Delhi (File)
Every winter for at least a decade now, Delhi has been bracing for extreme air pollution that harms the health of its population, particularly children and the elderly.
When temperatures drop and wind speed declines, toxic particles from car tailpipes, industrial chimneys, and construction sites remain trapped over Delhi and its satellite cities. This is also the season when farmers in neighbouring states harvest summer crops and burn the residue, leaving Delhi in a thick white haze that lasts for days.
This year, once again, the city's authorities are rolling out emergency measures such as reducing road traffic, banning farm fires, and activating the city's smog towers to improve air quality. Along with that is the revival of a debate about the effectiveness of the towers.
What are smog towers?
Delhi residents often keep air purifiers - small fans connected to a high-efficiency particulate air filter, or HEPA - at home. The fan pushes the air through a filter that traps dust, particulate matter, pollen, and other pollutants. A smog tower follows similar principles and is intended to be a giant air purifier installed in the open.
The tower, equipped with thousands of filters and can be as high as twenty meters (66 feet), is designed to capture pollution particles and improve air quality in densely populated areas. Although the technology they rely on is simple, smog towers need a lot of maintenance. They are also costly to set up - the one serving the city's iconic business center, Connaught Place, set up in 2021, cost about $2.5 million.
How did India come up with the idea?
Inspired by a smog tower set up in the city of Xi An in China, the Supreme Court asked the federal government several years ago to install similar equipment in Delhi to control air pollution, with funds from private companies and technical support from researchers at the Indian Institute of Technology Bombay.
There are at least two main units in Delhi, which are switched on when air quality worsens. When a small smog tower was tested in 2020 in a busy market to filter 600,000 cubic meters of air per day, the Delhi-based think tank Council on Energy, Environment and Water worked out that the city would need to deploy 2.5 million such filters to clean its air. The operation would cost the equivalent of over $20 billion, about 2.5 times the Delhi state budget for the year.
Do smog towers help in improving air quality?
As they operate outdoors, smog towers cannot clean enough air or fast enough to make a difference. As soon as filtered air is released, it mixes again with the surrounding pollution, which offsets any improvements.
A team of researchers at IIT Bombay recently examined the tower operating in Connaught Place and found it cleans air with 50% efficiency that drops to 30% at a distance of 50 meters from the filters, and just above 10% when 500 meters away. The researchers also discovered that the structure holding filters in place was not tightly sealed, allowing polluted air to bypass them.
While the idea still enjoys support from local authorities in Delhi and beyond, the overwhelming scientific consensus is that smog towers are costly and ineffective.