What the scientists say
Most of the published studies wherein benzene levels were measured inside passenger vehicles have been done under driving conditions, in traffic. So, while such studies have indeed found that in-vehicle benzene levels can significantly exceed those outside the vehicle, and could pose a human health hazard, this is mainly attributed to the presence of exhaust fumes.
Also, the amounts of benzene actually detected by researchers, albeit significant, were much, much smaller than the amounts stated in the email. A 2006 study summarizing all the data collected to date reported in-vehicle benzene levels from exhaust fumes ranging from .013 mg to .56 mg per cubic meter — a far cry from the 400 mg to 4,000 mg per square foot (do they mean cubic foot?) reported in the email.
Benzene levels in parked cars
In the one study I was able to find that measured benzene levels inside parked cars with their engines turned off, the results were more benign. Toxicologists took samples of the air inside both a new and a used vehicle under simulated hot-sunlight conditions, measuring the levels of volatile organic compounds (VOCs) including C3- and C4-alkylbenzenes, and exposing human and animal cells to the samples to determine their toxicity. Despite the detectable presence of VOCs (a total of 10.9 mg per cubic meter in the new car and 1.2 mg per cubic meter in the old car), no toxic effects were observed. Apart from noting the slight possibility that allergy-prone individuals might find their condition exacerbated by exposure to such compounds, the study concluded there is "no apparent health hazard of parked motor vehicle indoor air."
If in doubt, ventilate
Despite this finding, some drivers may still be concerned about the presence of any benzene vapors inside their car, especially given the World Health Organization's stated position that there is "no safe level of exposure" to the carcinogen. They may also worry, per the email warning above, that turning on the vehicle's air conditioner might exacerbate their exposure to trapped toxins by recirculating contaminated air. If that's the case, there's no harm done — and much peace of mind to be gained — by simply opening the windows and ventilating the car before turning it on.
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Sources and further reading:
Benzene Facts
Centers for Disease Control and Prevention, 22 February 2006
Toxicity of Parked Motor Vehicle Indoor Air
Environmental Science & Technology, 2 March 2007
That New Car Smell? Not Toxic, Study Finds
LiveScience.com, 6 April 2007
Benzene and Its Methyl Derivatives: Derivation of Maximum Exposure Levels in Automobiles
Toxicology Letters, 5 January 2006
Concentrations of Volatile Organic Compounds in the Passenger Side and the Back Seat of Automobiles
Journal of Exposure Analysis and Environmental Epidemiology, May-June 1999
Benzene
World Health Organization
Last updated: 06/06/11
