Driving in a car with a smoker can expose you to secondhand-smoke levels as high or higher as those in a smoke-filled bar, and even rolling down the window or cranking up the AC doesn’t provide full protection, according to researchers.

Reuters reported Aug. 25 that researchers who measured nicotine levels with sensors installed on the front passenger-seat headrest and the rear seat found that all passengers are exposed to high-levels of secondhand smoke when they ride with a smoker.

Nicotine levels in nonsmokers’ cars were undetectable, but in smokers’ cars they averaged 9.6 micrograms per cubic meter, higher than the levels typically detected in public or private spaces where smoking is permitted. “This is because the car is a very small place,” said researcher Ana Navas-Acien of the Johns Hopkins Bloomberg School of Public Health.

Nicotine concentrations in cars doubled for every cigarette smoked, the researchers found.

The study was published online in the journal Tobacco Control.