Micron, aka micrometre, often written as μm = one thousandth of a millimetre, or 0.001 millimetres. In metres it is one millionth of a metre.
Nanometre = one thousandth of a micron, or one millionth of a millimetre (one billionth of a metre).
So, for example, 0.3 microns = 300 nanometres.
A Covid particle is around 60-140 nanometres, or 0.06-0.14 microns (μm).
The unaided human eye can generally see (in optimum viewing conditions, up close) particles larger than 40 microns, or 40,000 nanometres. This is 0.04 millimetres, or one-twenty-fifth of a millimetre.
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