"The levels were above the threshold for what's considered unhealthy for sensitive groups -- people like children and the elderly -- as determined by the Environmental Protection Agency," said lead study author Vaughan Rees, Ph.D., a research associate at the Harvard School of Public Health.
During 45 driving trials, the researchers strapped a pollution monitor into a child-safety seat, and then asked a smoker-volunteer to light up at different times along the near hour-long route. The road tests were conducted under two different ventilation conditions: all car windows rolled down, then with just the driver's side window cracked about two inches.
"Common sense tells you if you smoke in a pretty confined space, such as a car, without ventilation, there's going to be a lot of secondhand smoke which is potentially dangerous," said Rees.
He added, "Before this study we had no idea what sorts of levels of secondhand smoke were generated. And we had no way of comparing that with other studies that have looked at secondhand smoke levels in other indoor environments like bars and restaurants."
The study appears in the November issue of the American Journal of Preventive Medicine.
PM 2.5 is one often-used gauge of air quality, which reports the amount of "particulate matter" or particle pollution in the air that is 2.5 micrometers in diameter or smaller. The smaller the particles, the easier it is for pollution to pass through the nose and throat and penetrate into the lungs.
According to the U.S. Environmental Agency's Air Quality Index, 24-hour exposure to PM 2.5 greater than 40 micrograms per cubic meter is unhealthy for sensitive people -- which can include children, older people and people with certain medical conditions. PM 2.5 levels above 250 micrograms are hazardous for everyone.
The Harvard study found an average secondhand smoke level of 272 micrograms, when the driver's side window was opened slightly. When the car windows were wide open, the average secondhand smoke level was 51 micrograms.
"At 40 miles an hour, on an open road, there's quite a lot of air movement inside the vehicle but that wasn't sufficient to completely remove the secondhand smoke," Rees said. "In other words, the smoke really hangs around."
The Harvard team observed much lower levels of tobacco smoke pollution when the car windows were fully opened, but Rees cautions that this difference should not be interpreted as an effective way to clear secondhand smoke to harmless levels.
In June, a U.S. Surgeon General's report concluded that there is no safe level of exposure to secondhand smoke and reiterated that tobacco pollution is linked to sudden infant death syndrome, ear infections and asthma attacks in children.
"There is an argument that even exposure for very short periods of time, perhaps even 10 seconds can precipitate asthmatic episodes in children," Rees said. He added that ventilation won't likely overcome secondhand smoke pollution that sticks to surfaces like child-safety seats.
Andrew Hyland, a tobacco control researcher at Roswell Park Cancer Institute in Buffalo, N.Y., predicts that the Harvard study will provide new ammunition to smoking-ban proponents. "No one has ever measured just how polluted it can get inside a vehicle," Hyland said.
This year two states -- Arkansas and Louisiana -- adopted private-vehicle smoking bans to protect children from secondhand smoke.
Hyland said: "One of the potential policy remedies is to say you shouldn't smoke in the car when little Johnny is in back in a car seat. But I think there's an equal communication opportunity to tell people whether it's kids, it's your spouse or your best friend – the exposures are there at hazardous levels, and they are dangerous."
Lisa Esposito | EurekAlert!
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