The primary purpose of a filtering mask [is to protect the wearer][0]. They coincidentally protect the environment, too, [unless of course they have a built-in

The primary purpose of a filtering mask [is to protect the wearer][0]. They coincidentally protect the environment, too, [unless of course they have a built-in check valve][1] that passes unfiltered air out of the mask. [0]:https://ohsonline.com/articles/2014/05/01/comparison-respiratory.aspx https://ohsonline.com/articles/2014/05/01/comparison-respira... [1]:https://www.amazon.com/gp/product/B008MCV1HY/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B008MCV1HY&linkCode=as2&tag=ohs19-20&linkId=c80249fb63f2d7ef9790a4467e33e163 https://www.amazon.com/gp/product/B008MCV1HY/ref=as_li_tl?ie...