My partner and I have been feeling really off lately in our apartment. It's super stuffy, and we both have constant headaches and feel tired all the time. We don't smoke, and we clean regularly, so we're not sure what's causing it. We have an old HVAC system, and we keep the windows closed most of the time because we live in a noisy area. Has anyone else dealt with something similar? Any tips to fix this?

I have another suggestion. You should consider getting an air quality monitor to check for high CO2 levels and other pollutants. This way, you'll know exactly what's going on.