Watch these Marines take part in a training exercise in a gas chamber. The process looks pretty painful as they can be seen in the room for several minutes continually taking off and putting their mask back on during the exercise.
This is a common exercise for United States Marines since it teaches them how to survive in an environment that contains a deadly or hazardous substance. The exercise usually lasts between 3 and 5 minutes depending on how well the Marines deal with the situation.
You can see some of the Marines struggling to make it through this exercise as their eyes and face start burning.