Frequently Asked Questions on Firefighting Clothing

What is Total Heat Loss (THL)? 

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.

What is Thermal Protective Performance (TPP)?

Thermal Protective Performance (TPP) is a test that indicates the amount of protection a material or material system provides against both convective and radiant heat. To determine the actual time to burn, the TPP score is divided in half and the resulting number is the time, in seconds, that human tissue reaches second-degree burn in a flash-over situation. The NFPA 1971 (2013 edition) standard requires a TPP of 35 Cal/cm2, which is the equivalent of 17.5 seconds to second-degree burn.

Can outer shell materials be damaged by UV light?

Yes. Any outer shell material using Nomex or Kevlar is susceptible to color changes or loss of strength when exposed to UV light. All protective garments should be shielded (as much as possible) from direct and indirect sunlight and fluorescent lighting.