https://thepreppingguide.com/gas-masks-you-need-in-a-nuclear-attack/
There are some important points to note when considering a gas mask and its use in a nuclear fallout. Having used gas masks in toxic gas situations before, I know that these will ensure that you are able to use your gas mask to provide clean, safe, oxygen in a nuclear, biological or chemically contaminated area. They are:
Your gas mask needs to be NBC or CBRN approved
You need to use NBC or CBRN gas mask filters that fit your mask
Facial hair will not allow you to create an airtight seal on your gas mask
Gas masks are not to be lived in, they are a temporary measure
A gas mask should be used in conjunction with other protective equipment