Before attaching the toe warmers to each sock I let the air hit it and waited for them to feel warm. Since this was my first use I didn't really know how hot they could get and I still don't know. Each one was sealed in a plastic package. I just assumed that once the chemical began reacting that it would continue reacting. The instructions didn't say anything about continuous fresh air contact being necessary. After all they are designed to go inside shoes in cold weather.