How odorless is "odorless"?

Question

Wikipedia has entries for many odorless gasses such as Ar, CO, etc. How rigorous is odor testing? Is it possible someone with a better sense of smell could smell Ar, CO, etc?

Answer 1

Odourless probably does mean odourless for good physiological reasons

The reason why common gases, such as those mentioned, are odourless is probably a consequence of they physiology of smell and its evolutionary history.

The reason why we or any other animal can smell is due to some fairly complex interactions between particular molecules in nerve cells in the nose. But those cells evolved for particular reasons. There is a big advantage, for example, to being able to detect putrefying food as it is a bad thing to eat food that isn't fresh. Putrefaction often produces a variety of gases containing nitrogen and sulfur (hydrogen sulfide and ammonia are very simple examples to which our noses are exquisitely sensitive). The same may be true for some pleasant smells for the opposite reason.

But there is absolutely no advantage in being able to smell oxygen, nitrogen, argon or carbon dioxide as they are ubiquitous in the air. Even carbon monoxide is common in the air once fire is used (though at low levels and, even though there would be an advantage in smelling it, we have not had fire and poorly ventilated rooms long enough to evolve the ability to smell it).

Even if the nerves in our noses could detect those common gases, this would be futile as the brain would edit out the constant signal–as it does with almost any repeated stimulus–and we would not notice it.

In short, smell is an adaptation to detect uncommon things that affect our survival. We can't smell ubiquitous things as there is no point at all of detecting them. For those things, odourless really does mean odourless.