CHI87MA101

After arriving at the gate in Chicago, fumes & smoke were noted in zone c of the cabin, then the cabin lgtg went out & the emerg lgtg came on. Maint psnl fnd the #2 APU had an auto-shutdown due to a malfunction. The APU was placarded as inop & the acft was rtrnd to svc. As it was clbg, shortly after takeoff, smoke bcm noticeable & the crew elected to rtrnto the arpt & lnd. After lndg, the smoke had dissipated & fire dept psnl rprtd no indcn of smoke or fire. Suspecting only an air conditioning problem, the crew elected to cont to the gate. As they bgn taxiing, a flt attendant rprtd the smoke was increasing, so the capt stopped the acft & directed an evacuation. A x-wind prevented proper deployment of theslides on the rgt side of the acft. Drg the evacuation, an elderly pax, using slide L-4 on the left side, was seriously injured by others who came down the slide after him. Also, 5 pax received minor injuries drg evacuation. An inv revealedthe smoke was caused by oil from a failed APU bearing which leaked into the air conditioning sys.