When materials burn, the smoke that is released creates chemicals, and the gases and this can cause inhalation. More than 50% of deaths are fire-related or due to smoke inhalation. By breathing in the smoke around a fire is smoke inhalation. The smoke releases gases that are very harmful and it is this smoke that can kill you.
Fires love oxygen and humans need oxygen to breath. It Hugh fires most of the oxygen is used up, thus making it difficult to breath. Smoke also produces carbon dioxide and this limit oxygen. So if you find a close area with lots of smoke, you need to go to a open area.