Flight Crew Commercial; Flight Crew Flight Instructor
Distraction; Physiological - Other; Time Pressure; Training / Qualification; Workload
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Illness / Injury
Flight Crew Landed As Precaution
Aircraft
On our training flight we went out north to a practice area X. We were at about 5;000 feet MSL heading North when a CO level warning came on. My student quickly canceled the message but I was able to catch a quick glimpse of the message. At this point I decided to turn back to ZZZ and conclude the lesson. I had opened the storm window and shut off the heat/defrost.  I took flight controls and started a descent down to 2;500. The descent was steep but nothing out of the usual (taking into consideration the rate of descent during a simulated emergency descent). However; on this descent my student bent over and grabbed his eye and started complaining of severe pain behind his eye. I contacted approach an informed them about the high CO level warning and that my student was in considerable pain. I requested priority for landing and a straight in approach for the RNAV XXR. We leveled off at 2;800 and proceeded towards the Initial Approach fix. During this time period I was in constant communication with my student. He remained responsive but continued to experience sever pain. I told him I would keep talking for the remainder of the approach discussing items related to the approach. I told him to listen to me to make sure I was making sense when I was talking just in case I started succumbing to the 'CO' without knowing I was. During the flight inbound we both noticed a chemical smell in the cockpit. We landed the aircraft and were escorted by the firetruck and ambulance. I asked to taxi on XYL to taxiway X onto ramp. Once we were there I shutdown and had my student go into the ambulance to get checked over. They said he should go to the doctor the following day.I believe there was a leak from the exhaust into the cabin. I did have the heat/defrost on which utilizes the heat from the exhaust. I assume there might have been a crack in the exhaust system that resulted in CO coming into the cabin.Our aircraft are usually kept to a very high maintenance standard so I would not recommend anything  on their part. However; I would  recommend a possible redesign of the heating system/exhaust systems from the manufacturers that would make it harder for CO to get into the cabin.
Flight Instructor reported a CO level warning came on during cruise and the instructor initiated an immediate return to departure airport. Instructor stated the student had physiological reactions requiring medical attention.