When I was writing my first post on coronavirus back in March, I advised against wearing a mask. This was then consistent with the WHO position and seemed to have sense.
The WHO advice has changed quite recently and it seems that masks are now recommended in situations when social distancing is not possible. There is also evidence that they actually affect the transmission, largely by blocking the droplets on which virus can travel.
This means that there is no need to be very strict with the quality of the mask, so it does not necessarily need to block the virus from passing through. Hence, any face covering that blocks the droplets will do the job.