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?

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    $\begingroup$ The Reference Guide To Odor Thresholds For Hazardous Air Pollutants Listed In The Clean Air Act Amendments Of 1990 is published by the EPA (US) and is 89 pages long. You might find some of your answers there. And yes, some people have a better sense of smell than others. $\endgroup$ Feb 16, 2022 at 3:34
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    $\begingroup$ Also see: en.wikipedia.org/wiki/Odor_detection_threshold $\endgroup$ Feb 16, 2022 at 3:52
  • $\begingroup$ Smell is subjective attribute with consensual conclusion. $\endgroup$
    – Poutnik
    Feb 16, 2022 at 5:59
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    $\begingroup$ It's possible that many of the gases Wikipedia lists as odorless really are completely odorless, period—i.e., that they would not be detected by any human even at 100% concentration. So I think the question is: Do all "odorless" gases fall into that category, or are there gases that can be detected, but only by a small percentage of individuals, and only at 100% concentration, that are also designated as "odorless"? [Also, there's a typo in your question—that should be Ar (argon), not Ag (silver).] $\endgroup$
    – theorist
    Feb 16, 2022 at 7:17

1 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.

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    $\begingroup$ There is an advantage to smelling carbon dioxide, if you're a bedbug. They identify where their prey is sleeping, or has been doing so, by olfactories that detect this slightly acidic gas. $\endgroup$ Feb 16, 2022 at 11:49
  • $\begingroup$ @OscarLanzi which illustrates the point. Bedbugs don't create much CO2 but benefit from finding creatures like us who do. we exhale quite a lot so detecting it by smell would be problematic. $\endgroup$
    – matt_black
    Feb 16, 2022 at 13:02

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