An FR jacket is often the most visible part of a worker's protective clothing system, but it is rarely the only component that determines safety. What workers wear underneath an FR jacket can significantly influence comfort, mobility, thermal regulation, and in some cases, overall protection against heat and flame exposure. Yet many workers spend hundreds of dollars on certified FR outerwear while giving little thought to their base and mid-layers. A non-FR T-shirt, an overly bulky hoodie, or moisture-retaining clothing can undermine comfort and create additional risks in hazardous environments.