Air conditioning primarily cools and dehumidifies recirculated indoor air, but it does not inherently bring in fresh outdoor air, meaning a tightly sealed, continuously cooled room can gradually accumulate indoor pollutants without some form of separate ventilation. Carbon dioxide levels from human breathing, volatile compounds from cleaning products or furnishings, and cooking odors can all build up in a well-sealed air-conditioned space over extended periods, particularly in smaller rooms with limited natural air exchange.
Some newer air conditioning systems include a fresh air intake option that draws in and conditions a small amount of outside air alongside recirculating indoor air, offering a partial solution without requiring a fully separate ventilation system. In spaces without this feature, briefly opening windows periodically, even for short intervals when outdoor conditions allow, helps refresh indoor air quality without significantly compromising overall cooling efficiency if done during cooler parts of the day.
Exhaust fans in kitchens and bathrooms play a complementary role by actively removing pollutants and moisture at their source rather than relying on general room air exchange, and keeping these fans in good working order supports overall indoor air quality alongside whatever cooling system is in use.