Water vapour from your breath comes from your lungs and is in general 10 to 100 microns in droplet size and will, as far as I'm aware, be where the virus is carried (as would most respiratory viruses). The water in your breath is not (generally) from saliva and this is not what the mask is required to defeat. Saliva is easy to stop, water vapour is somewhat more difficult.
You in fact don't want that "gas" to penetrate the mask because it is carrying the virus when it is this "gas" escaping. N95 masks are effective in this regards. Other masks, especially when worn by the public - not so much.
I've seen another commenter on here note that masks reduced spread by 15% and only with a host of caveats. They didn't get down-voted to oblivion so I presume that either nobody witnessed that post or it is generally accepted to be ball-park accurate. I'd want a lot better than a 15% reduction to consider something effective. That, to me at least, puts them in the minimally effective if not largely ineffective bracket.