Before anything else, let’s understand what THL and TPP would be in a simple way:
THL (Total Heat Loss).
Total heat loss is, in other words, how breathable the garment is.
TPP (Thermal Protective Performance)
This refers to how much heat the fabrics together retain.
It’s important to say that both THL and TPP tests are conducted on all layers of the structural firefighting garments. We cannot simply take the value of one layer in isolation and add in the values of another.
There must be a balance between the values to achieve acceptable performance of a garment. The ideal is to have good thermal protection allied to a set with good breathability. It’s no good having a light, breathable garment, but with a high rate of heat transfer from flames or something similar. The same goes with garments that have a high thermal protection rate but aren’t breathable, causing fatigue to the firefighter and, consequently, sudden health problems.
Today one of the biggest challenges for clothing manufacturers together with their fabric suppliers is exactly to manage ways to deal with the heat stress of firefighting through textile engineering.
One of the biggest factors in firefighter accidents is heat stress, which is listed on the NIOSH (National Institute of Occupational Safety and Health) website, USA.
Source: http://www.cdc.gov/niosh/topics/heatstress/