Just recently, there's a particular smell that's been found to emanate from carpets, and it isn't that pleasant "new carpet smell" at all. In fact, the presence of this smell has caused many people to complain about what is called "sick building syndrome," or a workspace where some undefined thing is making people feel queasy all the time.
In response, William Nazaroff and Glenn Morrison at the University of California at Berkeley started doing experiments on carpet samples to see what the culprit was. They found a bunch of chemicals called aldehydes that were responsible for the nasty smell.
After further testing, Nazaroff and Morrison found that these chemicals came as a result of the oxidation of oils that are used in carpets-- oils that originally came from vegetables. The rotten vegetables came about as a result of chemicals found in smog.
This is mainly an urban problem, where air pollution is serious enough that chemicals in smog can react with the chemicals in carpets, creating an odor the researchers described as being "an unpleasant cucumber smell." Aside from the bad odor, the real bad news is that smog might be getting into the building and causing bad air quality inside.