By John Teresko Researchers are reevaluating a naturally occurring refrigerant that was shelved in the 1930s by man-made materials such as chlorofluorocarbons (CFCs). The drawback of those early carbon dioxide systems was their need to be operated at high pressures -- up to five times what's required by today's commonly used refrigerants. That posed engineering challenges and necessitated the use of heavy steel tubing, explain researchers at Purdue University, West Lafayette, Ind. Armed with new manufacturing technology, the researchers see renewed merit in carbon dioxide. For example, they note that its high operating pressures mean that systems can be small and light -- definite advantages for automotive and portable units. In addition, the use of carbon dioxide would obviate the environmental requirement that CFCs not be leaked to the air during repair. Expensive refrigerant recovery apparatus would no longer be needed. Researchers estimate that carbon dioxide-based systems will take five to 10 years to perfect.