A lot of it depends on the carpet itself. When I've had carpeting work done, I've tried to buy it well in advance of having it installed, and had it stored in either the carpet warehouse or my barn so that it could do a lot of off=gassing before bringing it into the house.
A friend of mine is horribly chemically sensitive, so this is something that I've been made aware of. I myself have some dust allergies, so I went with hardwood floors and washable cotton area rugs.
http://airpurification.lifetips.com/faq.asp__Q__SiteID__E__1039__A__QuestionID__E__85769__A__type__E__0
http://www.naturalhomeproducts.com/carpetlines.html