Or more correctly called the "NBC Chamber" (for Nuclear, Biological, Chemical). This was a building that was used for familiarizing basic combat trainees in the army with chemical weapons. The trainees were suited up in their M17 protective masks, then herded into the Gas Chamber. Once in the chamber, they were instructed to take their masks off to experience the effects of CS gas. The purpose of this was to give the trainees confidence in their NBC gear. The idea was to get them in there with their masks on, take the mask off briefly, then put it back on and clear it (get the gas out of the inside of the mask) but it didn't always work this way. Some trainees would panic and try to run out the door without having attempted to clear and reinstall the mask. The CS gas used in the Gas Chamber was a formulation of what civilians call "tear gas". It causes you to tear up, and your lungs and sinuses want to fill up from the chemical irritation. It isn't pleasant, but every basic trainee goes through this.
The building in the picture is now delapidated, not having been used for many years. It's the real deal, however, and I passed through this building in 1969 like many other trainees.