CNG is not without environmental drawbacks says a new Central Pollution Control Board study. The study says burning CNG has the highest rates of potentially hazardous carbonyl emissions. These conclusions have attracted criticism from many, including environmentalists.
The study also made a case for regulating CNG and other fuels for methane emissions. Methane, a greenhouse gas, is a key contributor to climate change. Among the study's finds were that retrofitted CNG car engines emit 30% more methane than original CNG engines. Almost all CNG car engines in India are retrofitted.
CNG or compressed natural gas, has been touted as the greenest of the fossil fuels. There is little doubt that in terms of carbon emissions and well-known pollutants like sulphur dioxide, CNG is more environmentally friendly than oil or coal.
But the CPCB study argues that when it comes to carbonyl emissions, CNG is worse than a "dirty" fuel like Bharat Stage II diesel.
Carbonyls are a class of chemicals that range from harmless acetic acid (vinegar) to suffocating formaldehyde. Some of them have been shown to be linked to cancer after prolonged exposure.
Critics have questioned the study's conclusions on carbonyl emissions.
"There are a lot of scientific discrepancies in the study," said Anumita Roy Chowdhury, associate director, Centre for Science and Environment, an NGO instrumental in getting the government to introduce CNG public transport in Delhi in late 1990s.
Chowdhury, in a letter to the CPCB, asked why certain highly toxic carbonyl compounds, like butadiene, were ignored by the study.
The CPCB said the study represented the first attempt to generate data on carbonyl compounds.
"A comprehensive study on carbonyl compounds can be done," PCB chairperson SP Gautam said.
Another criticism of the study, was that only non-efficient retrofitted CNG vehicles, not done by auto manufacturers, were tested despite company-fitted CNG vehicles being available.
In India, except one company, most car makers sell CNG retrofitted vehicles. A CNG engine is about 20-30% more efficient than a retrofitted CNG kit.
"I can't understand why only one greenhouse gas methane was checked and others like carbon dioxide, whose concentration in emissions is higher, was left out," Roy Chowdhury said.