Save your $$. The chances of you having it when TSHTF are slim unless you carry it with you 24-7.. The canisters you need for the chem(s). you want to protect yourself from don't last for ever on the shelf (shelf life), plus the canisters are good for a onetime shot and can be expensive to buy. As for variables there are a few such as organic or in-organic?, concentration of Chem(s), wind, temp, humidity and others like mask fit. You can have the most expensive mask there is and still have leakage around the edges if you are walking, talking, running, sweating, laughing etc. I have to get a mask fit test every year to be sure there is no leakage. As for the masks that NATO & our military issue, keep in mind that they are there for use by soilders that are in a war & are expendable, they do offer limited protection to a point. Compared to what I use, I consider them false a sense of security.