During warm calm summer weather, well built homes do not get enough fresh air to purge indoor pollutants and renew oxygen. I have measured many homes that are getting and an air change in 12-24 hours with high levels of common indoor pollutants like formaldehyde and nitrates. Also with several occupants, these homes have very high CO2/low Oxygen levels.
You should have a fresh air change in 4-5 hours when occupied. This amounts to 75-100 cfm of fresh air at a minimum when occupied.
Try a small fan in a window, see if it makes a difference. Easy to setup a small duct fan or ventilating dehumidifier to fresh air and maintaining <50%RH for low/no cooling conditions in green grass climates.