NEW DELHI: Delhi's air pollution measures like introducing compressed natural gas, building the Metro and moving polluting industries outside city limits have been praised universally. But evidence has now emerged that, since these interventions, areas bordering the city have been experiencing very poor air quality.

As polluting industries, built-up area and population over there rises, satellite towns of Delhi may be silently bearing the brunt of the resulting increase in air pollution.

A study of satellite-based aerosol optical depth data by scientists at division of environmental health in department of public health sciences at University of Miami and Nasa has found that Gurgaon, Noida and Faridabad borders saw a significant deterioration in air quality after 2002.

AOD—the degree to which aerosols (airborne solid and liquid particles) prevent transmission of light in an area—is also considered an indirect proxy for air quality. Naresh Kumar, associate professor at University of Miami, who has authored the study, has correlated land use change in Delhi and surrounding areas between 2000 and 2004 with rate of increase in AOD.

Overall, AOD in the study area was 6-7 times higher than many less polluted areas of United States. Areas outside Delhi experienced 1.7% higher increase in AOD between 2000 and 2004 as compared to parts of central Delhi. "After the interventions, rate of increase of AOD increased rapidly with increase in distance from the city centre," the study found. Rise in AOD was about 2.5% within 10km from the city centre while areas more than 50km away from the city recorded a 6.5% increase in AOD, suggesting a jump in aerosol loading from anthropogenic sources.

Unlike Delhi, there has been no intervention in the bordering areas to deal with air pollution. Most of these places don't even have air quality monitoring stations.

The air pollution interventions in Delhi led to registration of 1,00,000 CNG vehicles. More than 25,000 industries that were previously in Delhi—in three industrial areas—have relocated to peripheral areas..

There was also a massive increase in population here. Districts bordering Delhi experienced 1.5-3 times higher population growth than Delhi between 2001 and 2011. Population growth in Gurgaon was 73% as compared to 20% in Delhi. This had a trickledown effect in peripheral areas.

Add to this the effects of increase in built-up area and deforestation.

"Our study suggests an intensifying burden of air pollution with increase in distance from the city centre—especially in eastern, southern and southwest parts of Delhi," says the study. "This calls for policy makers' attention to checking the unabated increase in air pollution in areas outside Delhi. An important lesson we learn from this research is that the lack of uniform policy interventions is likely to result in disproportionate distribution of emission sources and hence air pollution."

Experts have raised these concerns earlier. Environmentalists in Delhi have been demanding all NCR be treated as a single air-shed so that there is uniform impact of the government's air pollution interventions.

Kumar feels "there is an urgent need to quantify the burden of morbidity and mortality associated with air pollution, and develop effective air quality management strategies to combat air pollution in the world's most polluted city. PM2.5 concentration in and around Delhi is 6 to 12 times higher than WHO standards, depending on location".