1. I’m going to try to be a better twitter user. And that means bringing a little more charity to this platform than it tends to induce and avoiding snappy tweets that makes people feel like things are worse than they really are. So what to make of these two reports?
2. Here is the conclusion of the NAM report. There are no direct studies of the question at issue. We have to make inferences from the other data They suggest fabric masks might reduce transmission of larger particles but net benefits depend on make and use, so benefit uncertain. 
3. The BMJ piece says, “Although good quality evidence is
lacking, some data suggest that cloth masks may be
only marginally (15%) less effective than surgical
masks in blocking emission of particles, and fivefold more effective than not wearing masks.”
lacking, some data suggest that cloth masks may be
only marginally (15%) less effective than surgical
masks in blocking emission of particles, and fivefold more effective than not wearing masks.”
4. NAM only surveys evidence, but BMJ goes on to ask what to do. Both see cloth masks as potentially beneficial in blocking large particles. So BMJ says low risk of harm + evidence of some benefit means they are worth a shot. I don’t see a contradiction.
5. Although it’s uncomfortable, this is what decision making in the face of uncertainty looks like. Benefits of cloth masks are likely to be small and can disappear depending on construction, use, and user behavior (e.g., if they abandon social distancing).
6. But if you didn’t read the papers and only read the comments from the blue check marks, you get a very different impression. A much more dispiriting one. And right now, we have enough to be worried about without manufacturing problems.
