> Upgrading face masks to filtering face piece (FFP3) respirators for healthcare workers on covid-19 wards produced a dramatic reduction in hospital acquired SARS-CoV-2 infections, according to research carried out at Addenbrooke’s Hospital in Cambridge.
> Once FFP3 respirators were introduced [as opposed to simple surgical masks], the number of cases attributed to exposure on covid-19 wards dropped dramatically—in fact, our model suggests that FFP3 respirators may have cut ward based infection to zero.
https://www.bmj.com/content/373/bmj.n1663
FFP3 is a bit better than N95 (it's equivalent to N99).
N95 masks came from industry, but there were no RCT studies there. People just noted that they obviously worked, through arguably one can easily feel that a saw dust mask works as opposed to one against viruses.