In general CNG buses are thought to be less harmful to the environment than conventional diesel engines, producing a lower mass of particulate emissions and less carbon dioxide and oxides of nitrogen (which are a trigger for smog, a lung irritant and sometimes a greenhouse gas). Diesels pump out a number of toxic substances including polycyclic aromatic hydrocarbons and formaldehydes, both of which are known carcinogens. By contrast the main pollutant from CNG vehicles is methane, which is a greenhouse gas but not a health hazard to humans.

The decision to replace diesel vehicles with CNG has seemed like a no-brainer, but now there are concerns that CNG vehicles may not be as benign as they first appeared.

Zoran Ristovski, from the Queensland University of Technology in Brisbane, Australia, and colleagues compared the emissions produced by 13 different CNG buses with nine modern ultra-low-sulphur diesel buses. Each of the buses was taken to a specially set-up test centre and run under four different conditions: idle engine and three steady engine loads of 25%, 50% and 100% of maximum power. In all cases the buses had a fixed speed of 60  kilometres per hour. Exhaust gases were funnelled into a stainless steel pipe and then diverted towards detectors measuring particle concentration, carbon dioxide levels, oxides of nitrogen levels and methane levels.

The scientists were not surprised to find that carbon dioxide emissions were 20% to 30% greater for diesel buses, while oxides of nitrogen were similar for both sorts of bus. However, when it came to particle emissions the findings were not so clear cut.

Although the diesel buses emitted a greater mass of particles, both types of bus pumped out a similar number of particles. This discrepancy was explained when the scientists analysed the particle sizes. "When it comes to nanoparticles and ultrafine particles CNG buses emit as many particles as diesel buses," Ristovski told environmentalresearchweb. Diesel buses tend to pump out large sooty particles, while CNG buses emit an exceedingly fine dust. The results were published in Science of the Total Environment.

As yet no one knows exactly what the CNG exhaust fume particles are made from, or how toxic to human health they are. Ristovski and his colleagues discovered that the CNG particles were more volatile than diesel particles. "This means that they are most likely composed of some more volatile organics," said Ristovski.

One major concern is that these exceedingly fine particles (less than 50 nm diameter) may be more toxic than larger particles because they are able to penetrate deeper into the human lung.

It is no longer clear that diesel buses should be sidelined in favour of CNG. Particle filters on diesel buses could help to significantly reduce the particle load, while diesel oxidation catalysts could help reduce gaseous emissions like oxides of nitrogen.

In the meantime, Ristovski suggests that people reduce emissions by thinking about the way they drive. "Any kind of acceleration will increase the emissions significantly. It is important to drive carefully and avoid unnecessary braking and acceleration," he said.