Seven of the largest U.S. airports still allow smoking indoors, exposing millions of U.S. airline passengers to secondhand smoke, U.S. health officials say.

A report by the Centers for Disease Control and Prevention in Atlanta says numerous studies have shown exposure to secondhand smoke can cause heart attacks, lung cancer, asthma attacks and other diseases.

The CDC study says more airports prohibit smoking than when the study began in 2002, but smoking is still allowed inside seven of the nation’s largest airports, including three of the five busiest airports - Hartsfield-Jackson Atlanta International Airport, Dallas/Fort Worth International Airport and Denver International Airport.

Las Vegas McCarran International Airport, Charlotte Douglas International Airport, Washington Dulles International Airport and Salt Lake City International Airport also allow smoking.

Smoking was banned on domestic airline flights in 1990, but there is no national policy on airport indoor smoking.

The CDC study says of the 29 airports studied, 76 percent currently are smoke-free indoors, compared to 42 percent of 31 airports in 2002.

“Every year, millions of people who travel through and work at these airports are unnecessarily exposed to secondhand smoke,” Dr. Thomas R. Frieden, director of the CDC says in a statement. “Even ventilated smoking rooms do not eliminate secondhand smoke exposure. Eliminating smoking at airports is the only way to fully eliminate exposure for people who pass into and through airports. This is a no-cost, high-impact strategy that will protect millions of people from secondhand smoke while traveling.”

The study was published Thursday in the CDC’s Morbidity and Mortality Weekly Report.

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ATLANTA, Nov. 18 (UPI)