Now a days more and more people are becoming concerned with the stuff they eat, the products they use and most importantly the stuff they bring into their homes. So you are not alone in asking this question.
This question in particular has brought up quite a bit of talk in the carpet cleaning industry. Although, some chemicals are made a bit stronger for some of the heavier soiled carpeting; most of, and usually all of it is extracted out of the carpet and your home. Thus leaving your carpets clean and not full of chemicals. This is also considering the company you have hired has technicians that are trained and certified (an uninformed and uneducated tech can ruin your carpet ).
What can be even more harmful to you and your family is what is in the carpet before it is cleaned. A build up of soil can be a breading ground for bacteria, fungus and other micro-organisms. Tracked in decaying matter, pest control chemicals, spilled food that spoils (and many more). I believe that the idea of using chemicals to clean a carpet is far superior to the alternative, not having your carpets cleaned. These things may cause more harm than the limited exposure you would have using us and our products.
I need to give credit to Scott Warrington of http://www.interlinksupply.com/ for some of this information.