Does holding your breath prevent you from inhaling certain materials present in your surroundings?

Answer 1

When my daughter was in basic training in the army, they were taught to hold their breath while putting on their gas masks in case of a chemical attack. Sio holding your breath will work for inhaled substances. Others such as 'mustard gas' are absorbed through the skin, and holding your breath is not effective.

Answer 2

Not fully. You cannot hold your breadth for long and the "materials in the surroundings" travel very fast and spread widely. But you can reduce their intake considerably.