The smallest particles that can be seen with the
naked eye are around 40–50 µm in size. (1 µm is
one thousandth of a millimetre.)
If a human being was the size of a PM1 particle,
PM2.5 would be big as an elephant and PM10 the
equivalent of a sperm whale (about 20 metres
long).
All of these PM sizes might be very small in the
microscopic world of particulate matter, but
the difference between them is enough for the
smallest particles to potentially cause us a lot
more harm.