The simple answer is that memory foam is made of urethane foam (a petroleum product) which emits volatile organic compounds (voc) from the time it is made to the time it is tossed in the landfill. This process is usually called mattress off gassing or sometimes out gassing. If this sounds strange to you just think about how a new car smells. That smell is produced by all the new plastic parts and all the foam seat cushioning etc.