An interesting take on performance enhancement from andrew sullivan

[Baby Lee]

At a most simplistic level much of the opposition to steroid use is based on the ingestion of artificial 'substances' that boost performance without there being a relationship to the amount of work or level of talent that an individual athlete has. This has the perceived effect of decoupling the characteristics of the individual from that individual’s performance. This line of arguing says that performance should be based on intrinsic rather than extrinsic factors. Well, how do you classify this one? Nike puts athletes in a house with reduced oxygen levels so that the body builds up its oxygen use/carrying capacity as a response. This allows them to 'live' in a house at a virtual high altitude and train at low altitude. In this case, the athlete’s performance is being boosted by a substance that is extrinsic to their individual characteristics (and in fact is 'ingested' in manner analogous to steroids) yet this approach is praised rather than pilloried. Is the ingestion of chemicals (yes CO2 is a chemical, albeit an abundant one) to boost performance desirable or not? You can't have it both ways so to be consistent the anti-steroid crowd should be calling for the expulsion of most of the US long distance running team.
So what about it? Is there a fine line where it's ridiculous to call Oxygen a performance enhancing substance?

[Rain Man]

I'm not a professional athlete, but I know that oxygen really boosts my performance. Without it, I'm good for a minute, tops.

In a serious response, changing someone's environment is a lot different than injecting them with chemicals, in my opinion. It's a bit of a stretch to say that you're giving them CO2.