Can humans breathe an Argon-Oxygen atmosphere, where Argon substitutes the inert Nitrogen gas which currently populates our atmosphere? I didn't quite know which forum to stick this one in; I put it here because I only want to know if such a mixture of gases can be feasibly inhaled, without the prospect of early death by some physiological component.
One may think that Argon, being an inert noble gas, causes no harm at all and can perfectly replace N2 in a mix with oxygen. After all, you've been breathing Argon your whole life, since approximately 1% of the air is Argon:
However, we are talking about a raise from 1% to 78%... and there are possible consequences here:
- Argon is way denser than N2: Argon has a density of 1.784 g/L, while N2 has a density of 1.251 g/L (O2 has a density of 1.428 g/l). So, Argon is 40% denser than N2 (and also denser than O2 and denser than air). Without taking into account ventilation and other forces, there is a risk of argon not mixing properly with the incoming "air" in your lungs.
- Argon is a narcotic: This may sound strange, but there are evidences that Argon has sedative properties. According to Nowrangi, Tang and Zhang (2014):
Argon gas is considered a small noble gas element that has been applied in a number of fields. It has been generally classified as a nonreactive or inert gas providing a view that it does not contain any biologically active characteristics. In fact, argon has demonstrated characteristics such as narcosis [...] The mechanism in which argon displays its anesthetic ability has been suggested to be from the stimulation of γ-aminobutyric acid type-A receptors. (emphases mine)
As an additional information, there is an Argon-Oxygen mix, called Argox, which is used for decompression research only, not as a scuba-diving breathing mix.
Also, this question from WorldBuilding has some interesting information (and some wrong information as well).
In conclusion: this Argon-Oxygen mix is not harmless.
Source: Nowrangi, D., Tang, J. and Zhang, J. (2014). Argon gas: a potential neuroprotectant and promising medical therapy. Medical Gas Research, 4(1), p.3.