Environmental tobacco smoke (ETS) contains over 3000 chemicals of which at least 50 are known or suspected to be human carcinogens, while over 200 are regarded as poisonous.^1,2 Although passive smoking often occurs in private and public workplaces, with extensive research already conducted in hospitality venues, casinos, airplanes, airports and homes, to our knowledge there are insufficient data (if any) published on motor vehicle secondhand smoke (SHS) levels and no comparisons have been made with other areas where people are involuntarily exposed to ETS.

Measurements of airborne respiratory particles under 2.5 μm (RSP_2.5) in diameter were made using the TSI SidePak AM510 Personal Air Monitor during March 2005 in Crete, Greece. Respiratory particles of this size are not specific to tobacco smoke, since background levels arising from cooking or vehicle fumes are also of this size, but elevated levels of such particles can be attributed almost solely to ETS and are used to measure it.^3–5 To reduce background environmental pollution, all measurements took place in rural areas of Crete with low baseline RSP_2.5 measurements (baseline measurements 0.013 mg/m³). During the measurements, the inlet of the machine was placed at the base of the passenger front seat headrest, approximately 0.7 m above the seat and the cigarette in the car’s ashtray.

Real-time measurements were taken every 10 seconds representing the average measurements of the 10-second intervals. In each test a cigarette was left to burn inside the vehicle until it extinguished itself, usually between 10–20 minutes, depending on air circulation. In all cases all windows other than the driver’s were closed, outdoor wind was not noticeable and the heating/air-conditioning fan was turned off. Weather conditions were similar during all tests.

SHS exposure was found to vary according to the vehicle’s interior passenger cabin volume (PCV) and air circulation (table 1).

In all three study vehicles, SHS exposure varied according to the driver’s window status. When windows were closed so as to simulate driving in a cold and/or rainy day, RSP_2.5 levels were higher than when the windows were left half or fully open, allowing air to circulate (p < 0.001). Lower RSP_2.5 values found in the city car (with window open), compared with those of the smaller PCV utility car, could be attributed to the larger windows in the city car and therefore to larger air exchange rates. Under all other conditions and for all cars tested, RSP_2.5 levels varied according to PCV size.

Motor vehicle SHS levels of exposure are much higher than the levels found, for example, in hospitality venues such as casinos, bars and restaurants in New York and Delaware in the United States, and in pubs and bars in the United Kingdom—up to 112 times higher than the mean levels found in the smoking areas of UK pubs and bars, and 55 times higher than the levels found in venues in Delaware, before smoking was banned.^3,4,6

The main limitation of our study is the fact that all measurements were taken with the car stationary, thus resembling driving with the windows closed or in very heavy city traffic. It is most likely that under normal driving conditions the measurements would differ according to air circulation, which we could not measure. Secondly, we were unable to measure mainstream smoke emitted from cigarettes, and PCV was estimated without taking into account interior seating or plastics. Even though further studies are needed to evaluate SHS exposure taking into consideration such parameters, our findings demonstrate that under the above conditions motor vehicle SHS poses a significant threat to public health.