A mask with a filter on both sides is more preferable to one with a single filter connection. This not only offers more flexibility but also allows the user to positively attach a replacement filter on the opposite port before removing an expired one.
Field of view is a common frustration with many masks on the market. It’s important to have an unobstructed view during times that warrant the use of a gas mask. Many older surplus masks have small goggle-type eye holes, which virtually eliminate all peripheral vision.
How about prescription glasses? It’s important to make sure the mask in consideration accommodates spectacles. There are various upgraded designs available in the market that you can buy today. You can also buy a complete gas mask kit via https://www.gasmaskpro.com/gas-masks/.
The filter does not only have a shelf life, but it also doesn't last long while in use as you might imagine. Most need to be replaced after only a few hours of use depending on environmental and gas concentration. Even if breathing in “uninfected air,” they last less than 24 hours.
What does this mean? First, it’s not hard to imagine the need for several filters per person depending on exposure times. Second, it’s important to keep track of the expiration date for any filters on hand. At $40-$50 a pop, the cost of replacement filters can add up quickly. I would suggest purchasing a mask that accepts 40mm NATO threaded filter canisters. These tend to be the most readily available and popular.