The U.S. Environmental Protection Agency (EPA) identifies indoor air pollution as particulate matter or gas.
Particulate matter consists of solids and liquid droplets that vary in size. Some particles are small enough to be inhaled and can cause sinus problems, burning eyes, bronchitis, and respiratory illnesses. Dust, fumes and smoke, viruses, pollen, dust mites, pet dander, bacteria, and mold are some examples.
The EPA notes that hundreds of gaseous pollutants also have been found indoors. Sources include tobacco smoke, gas stoves, exhaust fumes, laser printers, paints, building materials, furniture, adhesives, caulks, cleaning products, craft materials, pesticides, and other chemicals.
Other contributors could be inadequate ventilation, poorly maintained heating, ventilation, and air conditioning (HVAC) systems, and excessive moisture creating mold and bacteria.