I am not a expert in medicine, so you should consult other people, but I have some translation experience in this field. There are several very closely related usages, all with the terms "impairment + percent". Some also use "impairment rating + percent" :

"...a 10 percent permanent impairment of each lung..."
http://www.dol.gov/ecab/cases/2001/Aug/01-0077.htm

"...claimant had a 70 percent lung impairment from pulmonary asbestosis alone..."
www.dol.gov/brb/cases/lngshore/unpublished/Janjun93/91-1800...

There are some 190 similar usage examples on the dol.gov site

Also:

"...if Winn had only the work-related asbestosis, his impairment rating would be at least ten percent less..."
http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?court=4th&n...

"...Dr. Reid's assessment that claimant's 25 percent impairment due to asbestosis..."
www.labor.gov/brb/cases/lngshore/published/00-0370.htm

"...claimant was diagnosed as suffering from a twenty-five percent impairment due to asbestosis..."
www.law.emory.edu/1circuit/nov97/96-2162.01a.html