science
Cheap air-quality monitors have spread to homes and streets worldwide, and many of them report a number labelled PM2.5.
PM2.5 means fine particulate matter under 2.5 micrometers wide, tiny enough that many together would fit across a human hair's width. This size matters for health because particles this fine can bypass the nose and throat's filtering and reach deep into the lungs and bloodstream, making PM2.5 a closely tracked pollution measure.
Oxygen percentage is a different measurement of breathable gas, not particle pollution. Air temperature is a thermal reading unrelated to particulate content, so neither describes what PM2.5 detects.
Cheap sensors typically estimate PM2.5 with a small optical sensor shining light through air and measuring scatter off particles, less precise than lab equipment but affordable enough to spread into ordinary homes.
science
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