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CHICAGO — Passengers who fly aboard planes that re-circulate cabin air are no more likely to catch colds than travelers on aircraft that pump in fresh air, a study suggests. 

Still, all of the air travelers in the study got a lot of colds — significantly more than would be expected in non-fliers. 

Health experts have long suspected that re-circulated air carries more germs and causes more colds. 

Researchers called the latest findings encouraging, because planes that pump in fresh air are being phased out in favor of less costly, more fuel-efficient models with ventilation systems that re-circulate air. 

“Re-circulation of cabin air did not emerge as a risk factor for the development of upper respiratory-tract infection symptoms in our study,” Dr. Jessica Nutik Zitter of the University of California, San Francisco and colleagues wrote. 

The study, published in Wednesday’s Journal of the American Medical Association, involved questionnaires given to 1,100 passengers leaving the San Francisco area and traveling to Denver between January and April 1999. 

A week after their flights, 21 percent of the fresh-air passengers and 19 percent of the re-circulated-air passengers reported having a cold. The researchers said the incidence of colds in non-travelers is about 3 percent. 

Researchers said the high number of colds among passengers in both groups could result from factors unrelated to cabin air, such as stress, sleep loss and poor eating habits sometimes associated with travel. 

Judith Murawski, an industrial hygienist for the Association of Flight Attendants, said the rate of colds in air travelers seemed high and could suggest that cabin air in general is poor. 

Many airlines use ventilation systems with filters designed to remove viruses and bacteria from re-circulated air, but not all do, she said. 

She also said there is no minimum standard for how much outside air is brought into airplane cabins. Her union, which represents 50,000 flight attendants at 26 airlines, opposes a move in the industry to set a standard significantly lower than that recommended for office workers.