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Volatile Organic Chemicals (VOCs) are carbon-based chemical compounds that become gases at fairly low temperatures, such as room temperature. It includes hundred of compounds found in everyday items from air fresheners to most anything that sprays. It's also found in furniture, wallboard, upholstery, adhesives, paints, etc. Foam mattresses have traditionally had a strong chemical smell when first bought. Some people are sensitive to it and others aren't.
I found one company that makes very low VOC memory foam mattresses. http://tempflow.com/biogreen_memory_foam.htm If you check at the bottom of the page, there is a link to the dealers where it is available or you can email them for additional dealers.
GreenGuard® is one certification body and that is what you should look for when buying a memory foam mattress. GOTS (Global Organic Textile Standard), and CertiPUR-US are others. GreenGuard certified mattresses for adult-size beds start about here: http://www.greenguard.org/en/SearchResults.aspx?CategoryID=28&SubCategoryID=124&pageNumber=4
I was unable to find anything to compare VOCs from one company with another or even one model to another among popular brands and models. In fact models can be very confusing, since the exact same mattress may be sold by different dealers under different names.
Please let me know if you have questions. I'm always ready to try to answer follow-up questions or opt out for another expert.