(See: Northcross AL, Trinh M, Kim J, et al. Particulate mass and polycyclic aromatic hydrocarbons exposure from secondhand smoke in the back seat of a vehicle. Tobacco Control 2012; doi: 10.1136/tobaccocontrol-2012-050531.)
The methods of the study were as followed: "SHS exposures in stationary vehicles with two different window configurations were monitored. A volunteer smoked three cigarettes in a one-hour period for twenty-two experiments. PM2.5, CO, nicotine and PAH where measured in the backseat of the vehicle."
The results were as follows: "We estimate that a child spending only ten minutes in the car with a smoker at the mean PM2.5 concentration measured in the first window configuration −1697 mg/m3 – will cause a 30% increase to the daily mean PM2.5 personal average of a child."
The study concludes: "Reducing SHS exposures in a vehicle would substantially reduce personal exposure for both children and adults. Particularly in the case of children, measures to reduce or eliminate SHS exposure in vehicles could have a major impact in reducing the risks of SHS-related illness.
The Rest of the Story
There is a significant flaw in the article which renders its conclusion questionable.
The article makes an assumption that is not explicitly stated. The assumption is that smokers who smoke in cars in the presence of their children do not smoke in the home. This is not only a questionable assumption, but it is most likely incorrect.
A parent who smokes in the enclosed environment of a car with his or her child present is most likely not going to refrain from smoking in his or her own home. Chances are that the child is exposed to secondhand smoke not only in the car, but in the home as well.
Since the study assumed that no smoking takes place in the home (it used measurements taken from smoke-free homes in estimating the increased exposure attributable to tobacco smoke exposure in the car), it has grossly overestimated the true fraction of tobacco smoke exposure due to the car compared to the home.
In fact, if one uses the same levels of particulate matter and nicotine as were found in cars in this study and estimates the proportion of overall tobacco smoke exposure for children who live in a home with a smoker, one realizes that the percentage of exposure from the car is only about 2% to 10% of the child's total secondhand smoke exposure.
Although the concentration of secondhand smoke in cars is very high, the duration of exposure in the car is much lower than in the home. Dose is equal to concentration multiplied by duration. Because the duration of exposure in the home is so much greater than in the car, the dose received in the home dwarfs that from the car.
The rest of the story is that for a child exposed to tobacco smoke in a car, that exposure probably accounts for only 2% to 10% of his or her overall tobacco smoke exposure. Between 90% and 98% of that child's exposure to tobacco smoke occurs in the home. For this reason, banning smoking in cars is going to slightly reduce the child's secondhand smoke exposure, but it is unlikely to cause a major reduction in health effects because the child is still heavily exposed to tobacco smoke in the home. A reduction of 2% to 10% is unlikely to substantially reduce the risk of illness due to secondhand smoke.
If anti-smoking advocates want to protect children from the health effects of secondhand smoke, and they feel that banning smoking in private cars is justified, then why are they not also calling on laws to ban smoking in homes with smokers?