A recent assessment by Beijing-based Greenpeace East Asia shows that between August 2014 and August 2015, Delhi's levels of PM2.5 (fine, respirable pollution particles) were far higher than those in Beijing.

Using data from Delhi Pollution Control Committee (DPCC) and Beijing Municipal Environmental Monitoring Centre stations in Beijing, Greenpeace East Asia found that the Chinese city's monthly PM2.5 averages were between 100g/m3 and 200g/m3. Delhi's monthly averages for October, November, December and January were well above 200g/m3. A microgram (g) is a thousandth part of a milligram.

Not only the Chinese government but also civil society organisations are now saying that Beijing's air quality is improving steadily due to aggressive anti-pollution policies. Average annual PM2.5 levels fell by about 25% from November 2014 to November 2015, after more modest reductions since 2012.

"This November was the worst on record in Beijing since 2010, largely due to the horrendous smog episode that the city just experienced. However, even after that, 2015 is on track to be by far the best year on record," said Lauri Myllyvirta, global campaigner, coal at Greenpeace. "This reduction is not happening only in Beijing but all across eastern and central China, and it can be verified from NASA satellite imagery along with official data."

Other Chinese cities have also shown improvement, say activists. A Greenpeace East Asia analysis of data from 360 Chinese cities released last year reveals "modest" air quality improvements in coastal regions and cities such as Beijing over 12 months. Even the industrialised Hebei province, just outside Beijing, has improved by 31%.

"Our analysis shows that the government's strict pollution control measures are working," said Zhang Kai, climate and energy campaigner at Greenpeace East Asia.

On the other hand, DPCC's average PM2.5 data for winter months in Delhi shows no improvement whatsoever. DPCC monitors air quality at RK Puram, Mandir Marg, Punjabi Bagh, Civil Lines and Anand Vihar. Some of the stations have in fact shown an increase in PM 2.5 pollution. Only IGI Airport showed considerable improvement in December 2013, January 2014, and January, February, October and November this year. This might be because construction activity-a major polluter-at Aerocity which started in 2010 finished by 2014. Spokespeople for Aerocity said they also have a traffic plan for the area.

"Delhi and Beijing started neck and neck. Both had severe pollution levels but Beijing never lost the momentum in dealing with it. They implemented restraint policies such as limiting car numbers and scaling up public transport at the same time," said Anumita Roychowdhury, head of Centre for Science and Environment's (CSE) Clean Air campaign. "Now they are considering implementing the California emission standards by December 2017 which are among the most stringent in the world. We cannot do with small steps anymore in Delhi."

Experts say Beijing may have been able to achieve rapid improvements in air quality by idling coal-burning power plants, which helps it meet climate targets along with controlling air pollution.

"Based on our analysis of weather patterns in Beijing and the fact that the fall in pollution levels is the largest since satellite records started in 2003, we are confident in saying that the fall in pollution levels is due to a real reduction in air pollutant emissions," said Myllyvirta. "China has achieved this by significantly reducing coal consumption in industry and power plants-coal-burning is responsible for an estimated 60% of PM2.5 in China."

In 2013, China had released an air pollution action plan called Air Ten, setting targets for 2017 that gave it the momentum to act quickly. Three of Beijing's four coal-fired power plants have been shut down and the last one will be closed in early-2016. A new air pollution law will also take effect next year," said a campaigner from Greenpeace East Asia.

Severe pollution has undermined public health in both cities and reduced life expectancy.

C. Arden Pope III, Mary Lou Fulton Professor of Economics at Brigham Young University, told TOI: "The estimates are that living in a highly polluted city such as New Delhi results in an estimated loss of life expectancy of about three years."

In an article about China published last year, he and Dr Doug Dockery at Harvard School of Public Health, had said life-time exposure to ambient air pollution in a city like Beijing may also result in an estimated loss of life expectancy of approximately three years.