variety and amount of synthetic household materials Oxygen deficient atmosphere (21% O2 is normal and 19.5% O2 is considered oxygen deficient) Elevated temperatures Toxic atmospheres To deal with such hazards, firefighters carry a self-contained breathing apparatus (SCBA; an open-circuit positive pressure system) to prevent smoke inhalation. These are not oxygen tanks (oxygen as a powerful fire accelerant would represent a grave risk when combined with virtually anything combustible in the presence of fire) but use compressed air in a similar manner to SCUBA diving gear. A firefighter's SCBA usually hold 30 to 45 minutes of air, depending on the size of the tank and the rate of consumption during strenuous activities. While this gear helps to eliminate the risks, firefighters are still exposed to smoke, toxic dust, fumes and radiation that have contributed to firefighters being 14% more likely to develop cancer. Obvious risks associated with the immense heat generated by a fire, even without direct contact with the flames (direct flame impingement), such as conductive heat and radiant heat, can cause serious burns even from great distances. There are a number of comparably serious heat-related risks, such as burns from hot gases (e.g., air), steam, and hot and/or toxic smoke. Prolonged, intense exertion in hot environments also increases firefighters' risk for health-related illnesses, such as rhabdomyolysis. Accordingly, firefighters are equipped with personal protective equipment (PPE) that includes fire-resistant clothing such as Nomex or polybenzimidazole fiber (PBI) and helmets that limit the transmission of heat towards the body. No PPE, however, can completely protect the user from the effects of all possible fire conditions. Heat can cause flammable liquid contained in tanks to explode violently, producing what is called a BLEVE (boiling liquid expanding vapor explosion). Some chemical products such as ammonium nitrate fertilizers can also explode, potentially causing physical trauma from blast or shrapnel injuries. Sufficient heat causes human flesh to burn as fuel, or the water within to boil, leading to potentially severe medical problems. Furthers risks include the occurrences of backdrafts. Backdrafts occur when there is a large amount of oxygen introduced to an oxygen-depleted fire. If a fire is compartmentalized and most or all of the oxygen has been burned up, there is a high risk of backdraft if something such as a window or door is opened. Introducing oxygen to a low burning fire can be devastating as it will ignite all of the oxygen