Over the past couple of years, anti-smog guns have become a prominent symbol of New Delhi’s efforts to combat air pollution in the winter. More Indian cities are also moving towards adopting anti-smog guns to reduce pollution.
Mounted on trucks and resembling missile launchers, anti-smog guns spray water droplets into the air to disperse suspended dust particles. But these large vehicles doing the rounds on city roads themselves contribute to pollution as they cause traffic congestion.
They are also a potent reminder, along with their cousins, smog towers, of technological fixes that dot the air pollution landscape, aiming to retroactively remedy an issue that needs to be addressed at its source. Deploying such equipment also comes with risks to the health of citizens and the environment, and raises questions of efficacy.
Anti-smog guns spray fine, nebulised – in the form of mist – water through high-pressure propellers into the air, creating a rain-like effect. This increases the mass of suspended air particles, forcing them to settle down.
Anti-smog guns with coarse nozzles are used at construction sites, while finer nozzles are better for controlling airborne particles in urban spaces. Because nozzles are tailored to the particle size, they should not be cross-applied, which means that anti-smog guns meant for construction sites must not be used in urban spaces. Yet, anti-smog guns from construction sites were used in urban spaces in the winter of 2022 in the national capital.
In 2021, the Delhi government’s environment department had issued guidelines on the use of anti-smog guns at construction and demolition projects, but there are no similar guidelines on their use in public spaces.
In September last year, the Central Pollution Control Board had issued guidelines on the deployment of anti-smog guns at construction projects with a built-up area greater than 20,000 sq mt, requiring environmental clearance in the National Capital Region.
This was in line with the directions of a Supreme Court order issued in January 2021 and the Union Ministry of Environment Forest and Climate Change. In September, the Delhi government mandated the use of anti-smog guns at construction sites with an area of over 5,000 square metres.
The Graded Response Action Plan of the Commission for Air Quality Management recommends the use of anti-smog guns at construction sites when the Air Quality Index reaches Stage 1, between 201-300, and is considered “poor”. When the Air Quality Index deteriorates to stage II – between 301-400 – and is considered “very poor”, water sprinkling along with dust suppressants in hotspots, heavy traffic corridors and “vulnerable” areas are recommended.
Over the last two years, experts have questioned how effective anti-smog guns are in reducing air pollution.
High water usage
In anti-smog guns designed for use in urban spaces, water is used at the rate of 40-250 litres per minute. At construction sites, anti-smog guns are supposed to be operated for around 30 minutes every two to three hours to suppress dust. These requirements can vary based on the site conditions and the type of construction.
According to Delhi Environment Minister Gopal Rai, Delhi has 521 water sprinklers, 233 anti-smog guns and 150 mobile anti-smog guns. Assuming water flow at 100 litres per minute, 233 anti-smog guns running thrice a day for 30 minutes will require 21 lakh litres per day.
Similarly, deploying 150 mobile anti-smog guns and 521 water sprinklers for four hours per day, would mean that the anti-smog guns require 36 lakh litres per day while the sprinklers could consume 125 lakh litres per day.
In total, these measures account for the use of 182 lakh litres of water per day, without factoring in water loss due to leakages or spills. Given Delhi’s routine water shortage problems, anti-smog guns are an unsustainable way of controlling air pollution.
Quality of water
Public health imperatives demand that the quality of water used in an anti-smog gun is free of biological contaminants. The Delhi government’s guidelines state that the water used should be free from coliforms (a kind of bacteria), viruses, bacteria and undesirable solids that may clog the nozzles.
The guidelines also advise against using treated sewage water at active work zones, like construction sites. The water should be of the quality of municipal supply water, or Class A and Class B water, and the use of treated sewage should be avoided. Class A refers to drinking water without conventional treatment but after disinfection while Class B is water fit for outdoor bathing. These are among the classifications as per surface water standards.
But news reports indicate that treated sewage water has been used for sprinkling in Delhi. Studies say that sewage treatment plants in India do not effectively treat emerging pollutants such antibiotics, and microplastics, and likely contribute to the spread of antibiotic-resistant bacteria. Spraying treated sewage water in urban spaces will add pollutants to the air.
Chemical dust suppressants
Dust suppression is usually carried out using only water, or water combined with chemical dust suppressants. The use of dust suppressants enhances the duration of effectiveness to five-six hours compared to 15-30 minutes when only water is used. This reduces the frequency of employing dust suppression, thus saving water.
But globally, studies have shown that dust suppressants have little efficacy in suppressing fine particulate matter (such as particulate matter 2.5, which measures less than 2.5 micrometres) which can enter the lungs. In a November 2019 advisory, the Delhi government recommended the use of the chemical magnesium chloride hexahydrate as a dust suppressant. But it comes with pressing concerns.
First, in cities like Delhi with a semi-arid climate, the use of water can add moisture to the air, fostering the proliferation of micro-organisms. Dust suppressants can themselves lead to further pollution. For instance, dust suppressants that adhere to soil particles can potentially be resuspended in the air by strong winds. Dust suppressants may also contain volatile organic compounds that are then dispersed into the air. This could be counter-productive for the air quality and contribute to the formation of ozone, a polluting gas that causes respiratory illnesses.
Second, municipal workers involved in applying dust suppressants are more susceptible to inhaling these chemical compounds. Dust suppressants, if used in urban areas, could also enter the skin of passers-by. Commuters, rickshaw-pullers, street vendors, pedestrians, and particularly those from socio-economically disadvantaged backgrounds, face a disproportionate risk of exposure.
Third, while both magnesium and chlorides are essential nutrients for the growth of plants, an increase in their concentration in the soil may harm plant growth. Higher levels of magnesium and chlorides make it difficult for plants to absorb water and other nutrients from the soil. When chlorides accumulate in the margins of leaves, portions of a leaf turn yellow or wilt, indicating chloride toxicity.
Finally, calcium and magnesium chlorides are highly soluble and can move through the soil, contaminating the groundwater. The consumption of groundwater could be among the major exposure pathways for the ingestion of these chemical compounds.
Given the possible health effects, high water requirements and the risk of poor water quality, anti-smog guns are not a sustainable pollution control measure. They also cannot reduce exposure to fine or ultra-fine particles unless the nozzle size is finer. The effect of dust suppressants, meanwhile, depends largely on their composition, application rate, and interaction with the air, soil and groundwater.
Before implementing these solutions, civic and government authorities should consult experts and ensure such decisions are supported by robust scientific evidence.
Abinaya Sekar (Senior Research Associate) and Bhargav Krishna (Fellow) are affiliated with the Centre for Policy Research. Views expressed are personal. The authors thank Shibani Ghosh and Anirudh Sridhar for their inputs in developing this article.