Hazmat Shirt are an item of Clothing was added to the game in Alpha 13. They are currently not available in the game except through the creative menu. they can not be looted or crafted at the moment. They were put into the game for crossing irradiated zones that are going to be implemented soon. They will provide protection from the radiation and other environmental issues.
The level of protection is displayed in game by hovering over the item to show its tool tip, the values shown on each item are not the actual level of protection offered by the item of Clothing but rather a total of the protection offered when worn as a set of five items.
A set includes Chest, Feet, Hands, Head, and Legs. Currently Eyes and Face are not part of a set.