While using the cabin heater during flight on a cold day, you detect a faint exhaust odor followed shortly by a dull headache and drowsiness. The MOST APPROPRIATE immediate action is to

During cruise flight at 4,000 feet AGL, you increase the cabin heat setting. About 10 minutes later, you develop a headache, dizziness, and mild nausea. What is the MOST APPROPRIATE immediate action?

During cruise flight at 6,500 feet MSL with the cabin heater in use, you develop a throbbing headache and notice that your passenger's complexion has a cherry-red flush. What is the FIRST action you should take?