Best Environment For Breathing?

Question: Has there been research to determine the best possible environment for people to live in ie; humidity, temperature, air purity, and oxygen concentration. If there is where could the results be found.

From Mike
Probably different opinions depending on nutrition, lifestyle, stress. My choices would be moderate to low humidity, cool to comfortably warm climate, absolute clean air, 21.8 % oxygen, sea level to 1,700 feet*. Lots of negative ions. *Studies show that that is as high as you want to go before the altitude stresses instead of conditions.
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