I recently read an article where a blogger described their decision to start masking on the subway:

I found that the subway and stations had the worst air quality of my whole day by far, over 1k ug/m3, ... I've now been masking for a week, and am planning to keep it up.

While subway air quality isn't great, it's also nowhere near as bad as reported: they are misreading their own graph. Here's where the claim of "1k ug/m3" (also, units of "1k ug"? Why not "1B pg"!) is coming from:

They've used the right axis, for CO2 levels, to interpret the left-axis-denominated pm2.5 line. I could potentially excuse the error (dual axis plots are often misread, better to avoid) except it was their own decision to use a dual axis plot in the first place! Hat tip to Evan for pointing this out in the substack comments.

The actual peak was only 75 ug/m3 and the subway time averaged below 50 ug/m3. While this isn't great, I don't think this is worth masking over.

Since I'm making a big deal about this error, though, I wanted to make sure I had my facts straight: I decided to replicate their work. I used the same meter they used (a Temtop M2000) and took the same subway journey (round trip from Davis station to Kendall station on the MBTA's Red Line). To be generous I took my time in the morning, intentionally missing a train at Davis and then lingering in Kendall after my train departed until the following train had also departed. Here's what I found:

Decent replication! And really not that concerning. Lower particle levels would of course be better, but I'd wear a mask when cooking before I'd wear it on the subway. It's possible that other subways or stations are worse, but I haven't seen evidence of that.