Common indoor air pollutants from natural sources include:
airborne mold spores
pet dander
pollen from plants
bacteria
Exposure to biological contaminants can cause a variety of symptoms, the severity of which can depend on the pollutant type, concentration, and how long the exposure lasts. Symptoms can range from short-term discomfort—sneezing, watery eyes, etc—to more severe health issues, including coughing, shortness of breath, allergic reactions, and asthma.
Maintaining a clean space and improving airflow can help improve indoor air quality from some biological contaminants—especially mold infestations. Ventilate damp environments, and in places without windows—like closets—get a fan or a dehumidifier. If mildew or mustiness has already set in, or if mold is growing on surfaces, here are the EPA’s recommendations for dealing with mold infestations.