For more info, you can visit:
https://altcourt.org/blog/35167/17940/the-best-gas-mask
https://www.login.ps/blogs/27601/CIVIL-GAS-MASK
https://community.wongcw.com/blogs/390214/Sizes-of-gas-masks-the-main-differences-and-the-right
https://social.studentb.eu/read-blog/76783
https://www.flokii.com/blogs/view/55788
https://rollbol.com/blogs/1529397/Which-gas-mask-to-choose-filtering-or-insulating
http://www.barberlife.com/read-blog/142725
https://tokemonkey.com/read-blog/132310