Iono, from page 6:
"However, taking into account the available studies evaluating pre- and asymptomatic transmission, a growing compendium of observational evidence on the use of masks by the general public in several countries, individual values and preferences, as well as the difficulty of physical distancing in many contexts, WHO has updated its guidance to advise that to prevent COVID-19 transmission effectively in areas of community transmission, governments should encourage the general public to wear masks in specific situations and settings as part of a comprehensive approach to suppress SARS-CoV-2 transmission (Table 2)."
The later stuff is on filtration by cloth masks. I don't think that's why cloth masks are effective.
They're terrible filters. Don't think of cloth masks as filters. N95 does that, but cloth masks are useless for that.
What they do is alter the trajectory of respiratory droplets coming out of your mouth.
When you talk, your vocal cords vibrate, and that aerosolizes tiny liquid droplets from your respiratory tract. These droplets may contain viral particles.
When you wear a cloth mask, you're not filtering out those particles, you're altering their trajectory.
With the cloth barrier, the droplets catch on the mask, or are deflected at an angle that is not straight-ahead at the level of other people's mouth.
So if everyone is wearing masks, then it disrupts the vectors of virus-containing droplets being directed from one person's mouth directly into another person's airway, especially during vocalization.
The redirected particles spent more time in the air, and so have more of a chance to dry out before being inhaled.
It's one-big harm-reduction strategy, where the goal is not to completely filter out viral particles, but rather to make sure that you're not directly inhaling too many on a given exposure (sub-infectious dose).
That's how I think of it. There's no RCT or anything that proves it, but I haven't thought of any good argument for why this isn't a good model for what's going on.
Although hand-washing and physical distance are the gold-standard, part of the problem is that it doesn't seem to be feasible as an option (at least in the US), so we need to rely more heavily on the single best harm-reduction strategy, which seems to be masks at the moment. It's the difference between the best option in a lab, and the best option in a crowded grocery store or subway train where people aren't necessarily going to follow the rules.