Serum chromium levels are measured in mcg/mL. What level of measurement is this variable?

A researcher in a Veterans Affairs hospital wants to determine whether patients who had metal-on-metal hardware versus ceramic-on-metal hardware used for their hip replacements have different levels of chromium ions (measured in mcg/mL or parts per billion [ppb] from 0 and up) in their blood 2 years later. A sample of all patients who had full hip replacements with these two types of hardware at the hospital in the last 2 years is collected for a total of 145 subjects in the study. The researcher sets her alpha at 0.05 and her power at 0.80.