system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a scientific basis for the concept of "fresh air"?

What is the difference between "fresh air" and "stale air".  Is there more oxygen?  Less CO2?  Fresh air seems to have a fairly profound physiological effect on some people, elevating mood, reducing nausea, etc.  Is this merely a placebo effect at work or is there really something about "fresh" air that rejuvenates a person?
gpt
Air is comprised of nitrogen, oxygen, argon, water vapour, carbon dioxide and minute traces of other gases.

Air entrapped within a home will tend to equalize with the outside air, but does so slowly.  Simply as a result of your presence, air composition in your vicinity changes.  Respiration produces carbon dioxide (increasing its volume fraction) and water vapour (increasing humidity).  Your skin cells slough off to create airborne dust (particulate matter).  In addition to the effects of your body, objects within your home oxidize (rust) consuming oxygen from the air in the immediate vicinity. The particulate matter load is added to by small particles breaking off from solid objects as you move them around.  Dust mites and their waste add to the airborne dust load.  Outside, emissions from vehicles and other machinery add nitrates, sulfides and particulate matter such as soot to the air.  Volatile compounds both within and outside the home add their vapours to the air.  Natural processes do remove some of these pollutants, but it takes time.  Thus the air in your home, or even surrounding your city, will have increased impurity levels compared to the worldwide average, or what is "ideal" fresh air.

As regards physiological effects, oxygen concentration is less important, as the hemoglobin in your blood is remarkably efficient at oxygen transport; however, carbon monoxide (naturally occurring, or from vehicle emissions etc.) binds to hemoglobin much more readily than does oxygen.  As a result, CO exposure can starve the body of oxygen somewhat, and it takes some time after exposure to CO for the body to eliminate it.  Carbon dioxide comprises the body's primary stimulus to breathe, and as such elevated levels of CO2 can lead to increased respiration rates, with the associated fatigue and changes in blood chemistry that that implies.  Organic compounds and toxins in the air have the same effects as those poisons delivered by other means, and although the body has mechanisms to deal with such toxins, you can be asymptomatic for poisoning but your body is still allocating energy resources to eliminate the toxins.  Airborne particulates, in addition to possible toxic effects, are eliminated from the body by various processes, consuming energy, and in some cases, cause damage to the lungs which reduces their efficiency.

In the presence of fresh air (nitrogen, oxygen, argon, water, carbon dioxide and trace compounds), the concentration of pollutants is low, making it easier to eliminate any existing pollutants in the body since there is a concentration gradient between inspired and exhaled air. 

Physiologically, this is rejuvenating, and it comes as no surprise that you would become aware of this at an objective level.