Check your mask prior to every use
Each time you reuse the mask, something may have happened to the mask so it no longer fits properly. A valve may have cracked, the filter cartridge may no longer be properly attached, or the straps may no longer be properly adjusted. Before you know it, you will be inhaling contaminated air, but you can prevent that by conducting a leak test prior to each use. It will cost you a minute, but spare your health.