The number of cities where the standards of Nitrogen Oxides (NOx) exceeded norms increased from 18% to 29% in 10 years, according to the Centre for Science and Environment (CSE).
While the focus remains on particulate matter pollution, medical experts say gaseous pollutants are more dangerous.
“NOx will be a serious issue in future. Global statistics show that in the first phase of action, people are able to control particulate matter but what is very difficult to control is NOx,” AnumitaRoychowdhury, executive director of research at CSE, said.
In 2007, none of the cities were in the critical category — that is 1.5 times the permissible limit. In 2016, 12% of the cities where NOx was being monitored entered the critical list, CSE data revealed.
The main sources of NOx pollution are vehicles and thermal power plants. Some experts are also concerned that the use of cleaner fuel, which tries to curb particulate pollution, may lead to increase in NOx pollution.
NOx is a regulated pollutant in India. The CSE listed Amritsar, Aurangabad, Delhi, Faridabad, Jaipur, Kolkata, Meerut, Navi Mumbai, Pimpri-Chinchwad, Pune, Thane and Vijaywada as hotspots for NOx pollution.
“We keep focusing on particulate matter but the gaseous content is equally, if not more, harmful for all age groups, including pregnant ladies,” Sanjeev Bagai, a Delhi-based paediatrician, said.
NOx leads to the formation of secondary particulate pollution and cannot be filtered by air purifiers. “When moisture content in the atmosphere is high, the gases are oxidised. They form oxide compounds and release free radicals that kill,” Bagai said.
The environment ministry also issued norms in 2015 to regulate NOx emissions from thermal power plants. However, the deadline for meeting the norms was not met in December this year.
The industry has argued that there is no viable technology to check NOx pollution. However, a task force backed by the PMO has called on thermal power plants to implement the standards.
