How effective are anti-smog guns in Delhi? Here’s what experts have to say

The Public Works Department is also rolling out more vehicle-mounted anti-smog guns this year than it did last year. There are 120 anti-smog guns on hire that are running on roads between 8 am and 8 pm, and 30 more will be added by the end of the week, said a PWD official. The vehicles that are being used to mount the anti-smog guns are either CNG or BS-VI engines, the official added.

Additionally, they are being placed atop 10 buildings of around 7-10 storeys. “Last year, we had fewer numbers, only 10 or 12 (vehicle-mounted ones), and those on buildings were smaller last year,” the official said.

But how effective are these devices in mitigating air pollution?

Experts say that there has been no thorough scientific analysis so far to determine what impact they have, and there could be some brief effect at best.

In winter, conditions are dry and with repeated action on the top soil, it gets lifted, explained Dipankar Saha, former head of the CPCB’s air laboratory. Due to low humidity, this dust is unable to settle.

“If the anti-smog gun is used along the roadside, the particulate matter may settle. It may reduce particulate pollution, but it is not a permanent solution. In emergency situations, where there is a fire or construction and demolition dust, this can be applied so that the situation is not aggravated in nearby areas. For the entire city, it is not possible. Water itself is an important resource,” Saha said.

There has been no study so far, but the effectiveness needs to be monitored scientifically — to see if it is effective or if it’s an eyewash, Saha said.

Sachchida Nand Tripathi, professor at IIT Kanpur, said he was not aware of any study or data to support or refute the effects of anti-smog guns. “We know that in China, they tried artificial rain to control haze. But that works at scale. Rain droplets remove pollutants, water has that ability which is called wet scavenging. That is the idea that is being pursued. Whatever comes beneath the droplets, the droplets will carry the particles and gases. But how many guns can you employ? You’re firing at a small place, and the overall volume is small, so it’s almost inconsequential whether you look at removal or other effects,” he said.

“The best is to act at the source (of pollution),” Tripathi added.

In November 2019, the Supreme Court asked the CPCB to ascertain the efficacy of anti-smog guns and submit a report in 10 days. A Supreme Court order from January 2020 then mandates the use of anti-smog guns at large construction sites, road construction stretches and dust-prone traffic corridors.

According to the CPCB’s guidelines on anti-smog guns, water throw distance ranges from 30 metres to 100 metres depending on the device. Anti-smog guns can also be rotated. Treated waste water is not to be used in the anti-smog gun. The guns that are “customised for urban areas” use 40 to 250 litres of water per minute depending on the device, according to the guidelines.

Anumita Roychowdhury, executive director of research and advocacy at the Centre for Science and Environment, also said she was unaware of any evaluation that has been done to examine how effective the anti-smog guns are. “We can understand if it’s being used at construction sites to douse the dust, but spraying in ambient conditions (the truck-mounted ones), we haven’t really looked at it,” she said.

On the other effects that water sprinkling may have, Professor Tripathi said, “Humidification may make the particles bigger… they will grow, that is a possibility. That can lead to larger drops, but I don’t know that since we don’t have enough information.”