What is the weird "u" symbol in chemistry

It looks just like a lower case u only the left side hangs down more. It is used in the following way in a chemistry problem:
According to air quality standards for carbon monoxide in the u.s., the maximum concentration is 40,000 u/m^3.

Thanks for any help! :)

It's the Greek letter mu. It stands for "micro," which means 10^-6, or one part in a million.