Everyone's an expert

The patient in room 6 was a middle-aged male, a heavy smoker, who was complaining of chest pain and shortness of breath. We tend to take chest pain pretty seriously in the emergency department, as there is a zero tolerance policy for missing MIs (myocardial infarctions or heart attacks). Still, about 3% of people with an MI will be sent home from the ED. His EKG was ok, but his story was convincing. I had him on oxygen at 2L/min, but his oxygen saturation was still low. I turned it up to 4L/min. He looked at the regulator and told me that he was receiving too much oxygen. I explained that 4L was not an unusual amount of oxygen and that he needed it. He guffawed (really) and laid his hand on my arm, "No offense, ma'am, but I deliver medical equipment and I think I know how much oxygen is too much." How could I argue with those credentials?