The American Environmental Protection Agency as well as their European counterparts estimates that indoor air is 2 to 5 times as polluted as air on the other side of the window.

Following is a more detailed list of indoor air pollution sources:

1. Combustion (Oil, Kerosene, Coal, Wood),
   
2. Tobacco products
   
3. Building materials
   
4. Furnishings
   
5. Insulation
   
6. Carpets
   
7. Household cleaning products,
   
8. Pesticides
   
9. Personal care products
   
10. Hobbies
    
11. Paint and paint strippers
    
12. Central heating/cooling systems
    
13. Humidifying devices
    
14. Outdoor sources
    
15. Biological contaminants
    

1. Airborne Microorganisms: Mold spores, mildew, fungus, viruses, bacteria
2. Animal dander
3. Animal urine
4. Mites
5. Pollen

1. Central air handling systems Humidifiers, AC and Large building ventilation systems
2. Home heating and cooling systems, evaporation trays in humidifiers, dehumidifiers, refrigerators, AC
3. Water damaged carpets and building materials