Starting your car A/C / benzene chain letter has popped up in several places again recently. That letter is FAKE. No need to worry.

DETAILS:
It’s a rewrite of a 2009 chain letter, which is false, and based on cherry picked information out of a 2001 report on automobiles in Korea, and a 2007 article on new cars in Germany.

The article claims benzene exposure is related to the A/C, and that is false. Benzene levels are related to combustion engines, because gasoline contains benzene. A properly maintained car will emit very little benzene. A poorly maintained car will emit more while the engine is running. It does not build up or accumulate when the car is off, unless you have a fuel leak.

REF: http://www.sciencedirect.com/science/article/pii/S0048969701011019?via%3Dihub

The article claims the benzene comes from plastic, which is false. All plastics DO offgass, and usually that is formaldehyde. This is more of an issue for new cars, and has nothing to do with the A/C itself. If the car stinks like new car smell, then air it out. The only observed risk was for people with allergies, it increased those temporarily.

REF: http://pubs.acs.org/doi/abs/10.1021/es0617901

A more detailed write-up about this chain letter is here:
http://www.snopes.com/medical/toxins/benzene.asp