Air quality worse inside your car?

Consumer guide to the chemicals found in your car:

http://www.healthycar.org/home.php

It is an interesting article, but it only addresses new cars. I'd like to know how much these chemicals dissipate over time, for instance if I buy a used car, is there less chemical presence or is it worse? common sence and a couple of science classes in school tell me it's probably less of a concern in a used car.