I often need to use [number] [unit of measure] combinations such as: 32 mg kg^{-1} (i.e. 32 mg/kg ~matter of style preference). Which is the best practice to encode this?:

- $32 mg kg^{-1}$

- $32$ mg kg $^{-1}$

- 32 mg kg $^{-1}$

If the first option is preferable, should all symbols representing units of measure be done in math mode? Any justification for (or not for) using math mode to render units of measure?

Thanks to anyone who can help.