NEW DELHI: Compressed natural gas (CNG) was pitched as a silver bullet for Delhi’s air pollution problem, but new research suggests exhaust emissions of CNG vehicles may also be harmful. They may not be as toxic as diesel fumes but, scientists at Council of Scientific and Industrial Research (CSIR) say, gases produced while burning CNG contain carbon nanoparticles that are suspected to cause cancer.

Referring to a study by CSIR-Indian Institute of Petroleum and University of Alberta (Canada), M O Garg, director general of CSIR, said nanoparticles have lower mass than particles found in diesel smoke but are far more numerous.

“This is a result of combustion which causes carbon disintegration. We have to see if the nanoparticles arise from CNG or the lubricant used. All I am trying to say is that CNG technology may have great potential but there is also a negative aspect to nanoparticles,” Garg told TOI, adding that systematic toxicity studies are needed besides trials to find out whether particulate filters can remove nanoparticles from exhaust gases.

The researchers analyzed smoke from a DTC bus, and carried out similar studies on diesel, petrol and hybrid vehicles.

But the researchers also point out that technological improvements have enabled other fuels to gradually become cleaner. “About 15 years ago, when Supreme Court ordered the implementation of CNG, diesel had 500ppm (parts per million) sulphur content. Now there are catalytic convertors and particulate filters (that make diesel engines cleaner),” said Garg, citing the introduction of unleaded petrol as another example.

The advocacy group Embarq studied emissions from buses run on different fuels in Mexico, Brazil and India, and found that CNG buses emit more micro particles. “The low-sulphur diesel particles were found to be slightly bigger than those from CNG. We also found that overloaded CNG vehicles emit even more micro particles,” said Amit Bhatt, strategy head, urban transport, Embarq. “We can say that Euro V and Euro VI diesel is as good as CNG in terms of other pollutants, and superior if you consider particulate emissions,” Bhatt added.

For now, CNG can’t be labeled a highly polluting fuel because its particulate emissions are lower than those from diesel. “For diesel, we measure the weight of particulate emissions because they are measurable in micrograms. But for CNG, their weight cannot be measured as they are ultrafine; only their numbers can be counted,” said Anumita Roychowdhury, head of Centre for Science and Environment’s (CSE) Clean Air campaign. She said European countries have started measuring ultrafine particles because they have already addressed the problem of particulate emissions from diesel. India, however, should focus on reducing PM emissions that are the fifth major cause of death now, she added.

“The challenge of ultrafine controls will come only after we clean up the particle mass emissions from diesel vehicles through improvements in emissions standards. After the mass is gone, ultrafines remain,” Anumita said.

Even some leading medical researchers TOI spoke to said more evidence is required to draw conclusions about the harmful effects of CNG emissions. “The fact that carbon nanoparticles are emitted by CNG-run vehicles does not establish their health impact. Evidence available so far shows that extremely fine particles are not responsible for any significant respiratory health issues since they are too light and are exhaled by healthy individuals,” said Dr Arup Basu, chest specialist at Sir Ganga Ram Hospital. But, he cautioned, these particles can deposit inside patients with chronic obstructive pulmonary disease (COPD).

