Bioaerosols are airborne particles derived from plants, animals or are living organisms, including viruses, bacteria, fungi, and mammal and bird antigens. Bioaerosols can range in size from roughly 0.01 micrometer (virus)to 100 micrometer (pollen). These particles can be inhaled and can cause many types of health problems, including allergic reactions (specific activation of the immune system), infectious disease (pathogens that invade human tissues), and toxic effects (due to biologically produced chemical toxins). The most common outdoor bioaerosols are pollens from grasses, trees, weeds, and crops. The most common indoor biological pollutants are animal dander (minute scales from hair, feathers, or skin), dust mite and cockroach parts, fungi (molds), infectious agents (bacteria and viruses), and pollen.