Traditionally firefighters have been issued standard cotton tshirts worth about $10 for their standard station wear and these are the tshirts that they practice in and wear under their turnouts.
Is there any benefit to other types of tshirts? This blog posts gives you some information in order to help make that decision.
There are numerous studies out there regarding this issue. You can check them out at;
NFPA 2112 Testing
Homeland Security Wildland Firefighter PPE Guide
You may want to avoid any underwear that is categorized as a thermoplastic. Check this out;
What to wear under your turnouts
Here are some excerpts from ArcFlashWear
Selection of the 'perfect' FR textile and/or fibre is a complex process. Each FR textile or fibre has particular properties which either provides benefits or shortcomings. Textile manufacturers blend fibres in an attempt to balance these individual properties to maximise a textile's performance. It is imperative that assessing Corporations be aware of each fibre's properties to ensure that garments selected meet the specific requirements of a given application. To read more about the individual properties of each fabric and/or fibre used in the construction of our FR Range, click here.
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