Biological contaminants include bacteria, viruses, animal dander, mites, cockroaches, and pollen from many sources in homes. Controlling relative humidity between 30-50% and avoiding standing water can minimize growth of some biological sources. Biological contaminants thrive in damp, warm areas and where dust collects. They can trigger allergic reactions and infectious illnesses with symptoms like sneezing, coughing, and shortness of breath.