Nine students and 11 faculty members have developed rashes at a Pittsburgh-area middle school where air pollution was found to be among the worst in the nation by USA Today.

Officials say they don't know what is causing the rashes.

The newspaper reported last month that air samples near Sto-Rox Middle School were the 28th worst among 128,000 schools in the nation.

The Allegheny County Health Department found traces of the industrial solvents benzene and toluene in air samples in the school's library and faculty lounge in response to the USA Today story. But a health department spokesman says those substances aren't in high enough levels to cause an immediate health risk.

The school is near several heavy industries just west of Pittsburgh.