Killing in fiction

I cruised back to a website I visited before, one that appears to give out, shall we say, not the most accurate information pertaining to forensics mixed with some good stuff and found some more not fully accurate information and an opportunity to throw in my 2 cents worth.

The question sent in by an author:

I've got a man in his 60's who is dying (I gave him cancer, but really it could be anything convenient for the scenario I'm describing). He is being cared for at home by his wife and his niece (who is a nurse), along with help from a local hospice group. He has trouble breathing and sometimes needs a mask put over his face (oxygen, I'm guessing), to help him out. The oxygen is in a tank behind his bed.
One afternoon he needs the mask, but when they put it on him, he dies anyway. His death seems natural. It's not until autopsy that they realize he was murdered: There at the end, he wasn't breathing oxygen through the mask at all but some other substance that had been put into the tank (or that tank was switched out for another) and the substance killed him.
What deadly substance was substituted for the oxygen? Would it have a smell? How would the autopsy reveal that this is what happened?
I'd like the fatal gas to be as simple as possible, something easily attainable by any of the 5 suspects. It would also be fine if someone could've just changed the settings on the oxygen or some other gas that would be present anyway and that's what killed him. The most important part of the scene is that they have to think it was a natural death until autopsy (though a whiff of some strange odor would be fine.)

The answer posted:

There are several possibilities but three that would be very easy for your killer to employ.
1—Simply not turning on the oxygen and having him continue breathing only room air. If he were very ill with severe lung disease, and particularly if he was taking pain meds, which would suppress his respiratory drive, this alone could make him gradually slip into a coma, stop breathing, and die.
2—If he had what we call Chronic Obstructive Pulmonary Disease (COPD), which is basically emphysema and similar diseases, he would be very sensitive to high oxygen (O2) levels. The physiology here is complex but the bottom line is that people with severe COPD can slow or even stop their breathing if exposed to high levels of O2. So your killer could turn up the oxygen. Normally these tank masks combinations are set at a flow rate of 2 liters of O2 per minute. If the valve on the tank were turned up to say 10 liters/minute the victim would gradually slow his breathing and eventually stop and die from asphyxia. This could take anywhere for 15 minutes to a n hour so you have a bit of leeway here.
If the gauges were returned to normal after the victim died and before he was seen by investigators or other witnesses, then the cause of death might never be determined since there are no laboratory or autopsy findings in these situations.
3--Carbon monoxide (CO) would also work. The killer could extend a small hose—or a garden hose—from a car tail pipe in the garage or the driveway just outside the window to the mask the victim was wearing. This is a very deadly situation that comes from the unique chemistry of CO…
(the website writer goes on to actually fairly well discuss the toxicity of CO and some findings after death)

Because the questioning author listed their website I dropped her an email:

Not to be a butt-in-ski, but I ran across your question on Dr Lyle’s site and I feel there are some errors in the answer he posts.
1) Having the flow of oxygen to his mask turned off would cause him to rebreath his exhalations (and carbon dioxide) and could/would ultimately result in his death. Remember that the air hunger he would experience would get quite strong, so he would have to be pretty incapacitated to not reach for the mask.
2) Oxygen through a mask is always set at a minimum of 10 liters per minute, never at the 2 liters that a nasal cannula is set, so this one starts out wrong. High flow oxygen and the loss of hypoxic drive in severe COPD is a theoretic possibility, but is very seldom seen in real life.
3) Carbon monoxide does not normally come in tanks, but I suppose you could fill one with it. A more “natural” choice here would be nitrous oxide, used by dentists and others for anesthesia. It is lethal when used in concentrations greater than about 30% (it is normally mixed with oxygen in a “machine” for administration). Given straight it would be quite lethal and might be noticed as a sweet smell. It could be found after death when a blood gas is run found to have a very low blood oxygen content.

Maybe I should get into author consulting; I guess I have done a bit of that already. I have since corresponded with the author who sent the question above, her story has changed and I think I helped her a bit in agreeing with her new plot twist on how to kill a fictional character (kind of fun).