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 [...]

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First published:
July 9th, 2025

Source:
https://www.lesswrong.com/posts/rj75cGJhPCHnPMDvF/subway-particle-levels-aren-t-that-high

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Narrated by TYPE III AUDIO.

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Images from the article:

Graph showing PM2.5 and CO2 measurements during a workday with timestamps.

Graph showing MBTA Red Line PM2.5 levels during morning and evening commutes.

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“Subway Particle Levels Aren’t That High” by jefftk

LessWrong (30+ Karma)

“Subway Particle Levels Aren’t That High” by jefftk