The study, commissioned by Navistar International, states that while natural gas does burn cleaner than diesel, more research needs to be done on the health effects of emissions and suggests that natural gas emissions may also cause cancer.
"Little is known about whether particulates from natural gas engines have the same effects as diesel particulates in terms of their possible carcinogenicity," the study says.
The Harvard researchers say the health effects from inhaling particulates may depend on the size as well as the mass of the particles. Smaller particles can be more toxic because they penetrate deeper into the respiratory tract.
"Some studies suggest that while natural gas engines emit a smaller mass of particulates, the number of 'ultra-fine' particulates may be higher than for diesel engines," the researchers reported.
The study also revealed that using natural gas instead of diesel may lead to a 5% to 10% increase in greenhouse gas emissions due to the potency of methane as a greenhouse gas. When natural gas engines are fueled, methane escapes into the atmosphere. Since it is about 20 times more potent as a greenhouse gas than carbon dioxide, even small quantities will have an impact.
The study said policy makers and vehicle manufacturers in Europe have leaned more toward diesel to comply with the Kyoto agreements to lower greenhouse gas emissions. "In designing strict emissions standards for NOx and particulates that could discourage the use of diesels, regulators in California do not appear to have taken into account the diesel's advantage in control of greenhouse gas emissions," the researchers stated.
Dr. William Bunn, Navistar's vice president of health, safety and productivity, said the study exemplified the need to do a complete risk assessment before favoring a particular fuel. "California is only focusing on one part of the environmental puzzle," he said. "We have to look at the big picture and do a complete risk assessment before issuing regulations. The study is the beginning of an effort to look at all sides of the debate."