What are the four major indoor air pollutants?

The four major categories of indoor air pollutants that pose the most significant health risks to human occupants include particulate matter, volatile organic compounds, biological contaminants, and combustion gases. Particulate matter consists of microscopic dust, soot, and smoke particles that penetrate deep into human lungs. Volatile organic compounds are harmful chemical gases emitted from paints, pressed-wood furniture, cleaning supplies, and building materials. Biological contaminants encompass mold spores, bacteria, pet dander, and dust mites that thrive in humid, unventilated spaces. Combustion gases include toxic agents like carbon monoxide, nitrogen dioxide, and sulfur dioxide released by unvented or faulty fuel-burning heaters, stoves, and fireplaces, all of which threaten respiratory health.