Total Heat Loss (THL) measures the heat stress reduction capability — or breathability — of firefighter turnout gear. The more heat that gets trapped inside a firefighter’s turnout gear, the more likely an individual will experience dangerously elevated skin and core temperatures, as well as an increased heart rate. Material systems that provide a higher THL number will benefit the firefighter in the form of more breathable turnout gear. The NFPA 1971 (2013 edition) standard requires a THL of 205 W/m2.