4. Your heating and/or cooling system
One of the more surprising sources of disease-causing bacteria and fungi is your A/C unit. If they are not kept well-maintained and cleaned, they can become a hotbed for germs. Germ-ridden A/Cs can cause breathing problems, headaches, fatigue, and a general sense of feeling ill. The culprits behind these maladies include mold, dust, and animal dander. Mold arises due to moisture build-up from condensation that forms when cold air is brought in, and the dust and dander come from within a building and are recirculated with old air.
The coils in an A/C unit are the most susceptible to microbe growth due to their often wet, cold conditions. Researchers have found a correlation between reducing these organisms with UV light and people in the associated building feeling better and are now working to make people more aware of the importance of A/C maintenance. Using ultraviolet devices can bring relief, but simpler measures like changing the filter when necessary and keeping your carpets clean can also have a big impact.