Or is it just unpleasant?
When “stuffy” starts to equate to “insufficient oxygen”, then yes.
Or too much CO2.
For example, in the UK, the government has agreed to a set of standards for acceptable CO2 levels in indoor spaces:
800 ppm or below indicates that an indoor space is likely to be well ventilated and has acceptable indoor air quality.
Consistent levels of 1500 ppm and above indicate that the room is badly ventilated. In these cases, measures should be taken to improve ventilation.
Guidelines can differ according to nation. The United States Department of Labor’s Occupational Safety and Health Administration (OSHA), for example, has set a permissible exposure limit of 5000 ppm of CO2 as a time-weighted average over 8 hours. And, according to ASHRAE, the American Society of Heating, Refrigerating and Air Conditioning Engineers, levels of indoor carbon dioxide should be below 700 ppm.
Many nations, including Japan, Korea, Portugal, France and Norway, have set 1000 ppm of CO2 as the standard for specific indoor environments, including school and office buildings.
You have to define stuffy though. In winter when indoors tends to be dry as a bone (pushing 10% RH), walking into a properly humidified space (40% ish) may seem stuffy at first, but it actually is better for you than the dry air.
I was just thinking of being somewhere and thinking “It’s stuffy in here.”
Could you clarify?
I mean, there’s insufficient oxygen on Mars, at least for humans. I wouldn’t say “It’s stuffy here” while being outside without some breathing apparatus on Mars. I wouldn’t say anything of course, but neither would those words pass through my mind.
Same for high altitudes on earth. While the oxygen level is the same, low pressure makes it difficult to get enough oxygen, as tourists hiking to Machu Picchu and mountaineers in Nepal have discovered. Again, ‘stuffy’ isn’t a word I’d use.
Well, “stuffy” usually refers to an enclosed space with poor air circulation, along with factors like mustiness. I tend to envision a Victorian parlor with antimacassars on the chairs and all the windows shut up tight.
You might covet a nice breeze in such a situation, but it doesn’t sound actively unhealthy.
You have to rebreathe air for a long time to deplete the O2 concentration to a problematic level. As the astronauts of Apollo 13 showed, CO2 buildup causes problems much sooner. @commasense 's link shows that CO2 buildup is a known issue for indoor environments, and lately it’s also become an issue in cars: