Secondhand Smoke in Motor Vehicles Study

We have published a new article on secondhand smoke levels occurring in automobiles for a variety of speeds and ventilation conditions.

Please see the Stanford News Article and visit the Download Page.

How severe are the pollutant levels in a car with a smoker?

The pollutant levels in cars are affected by the size of the passenger compartment and the rate at which interior air flows out of the car – the car’s “air change rate.”

The air change rate has not been studied systematically in prior research. Hence, we made more than 100 measurements on four motor vehicles of the ventilation characteristics of the cars under a great variety of conditions. To measure a car’s air change rate, we filled the car with a controlled amount of tracer gas and measured the decay of the gas with time using monitoring instruments located inside the vehicle.

In some experiments, volunteer smokers smoked inside the vehicles under controlled conditions with different window positions, vent settings, air conditioner settings, and vehicle speeds while we measured the interior pollutants such as carbon monoxide, fine particulate matter, and the carcinogen benzene.

With the windows closed and the vent set on “recirculation,” the air change rate was less than 7 air changes per hour for all speeds and all cars tested. Opening a window by just 3” increased the vehicle’s air change rate by 8 to 12 times. With passive ventilation – air vents open and no fan or air conditioning – the air change rate was directly proportional to the vehicle’s speed from 14 to 72 mph. Particles decrease more rapidly than predicted by the air change rate because they stick to interior surfaces.

The air change rate determines the persistence of pollutants inside a vehicle, and it also is important for understanding the rate at which pollutants on the roadway enter the vehicle, or the "sheltering effect” of a car. With the windows closed and recirculation on (or the air conditioner set to maximum), we found that a single cigarette caused extremely high interior concentrations of fine particulate matter (PM2.5, or particles less than 2.5 micrometers in diameter), with levels over 2,000 micrograms per cubic meter.

PM2.5 can enter deep into the lungs and has a health-based outdoor air quality standard of 35 micrograms averaged over 24 hours set by the USEPA. If averaged over 24 hours, a single cigarette smoked in a car would cause a personal exposure of 21 micrograms, and smoking two cigarettes would bring the average to 42 micrograms, exceeding the federal air quality standard for PM2.5. The high fine particle concentrations inside a motor vehicle from smoking suggests there is a serious health risk to both adults and children traveling in a car with a smoker.

Legislation banning smoking in cars with young children present was adopted in Arkansas in 2006, and more recently in 2007 by California. Similar smoking bans with children present have been introduced, but not yet adopted, in Georgia, Michigan, New Jersey, New York, Pennsylvania, and Vermont. We hope that the findings of the present study will contribute to the scientific dialog about smoking policies in motor vehicles.