Final answer:
The diameter of a human hair is about 90 µm, which is equal to 0.09 mm.
Step-by-step explanation:
The diameter of a human hair is about 90 µm.
To convert micrometers to millimeters, we need to divide the given value by 1000, since there are 1000 micrometers in a millimeter.
Therefore, 90 µm is equal to 0.09 mm.