Biological particles are a diverse group of particulate matter present almost everywhere, in the atmospheric environment, soils and waters, and in indoor environments. Common to all of these particles is the biological origin: microbial, plant, or animal sources. Examples of microbial particles are moulds and yeasts, bacteria, and viruses; particles of plant origin are pollen and algae, and various particles also originate from animals (including humans). Biological particles may be viable or non-living, dead material. The health effects and biological characteristics of these particles are also diverse; pollen and microbial spores being essential parts of the organism’s multiplication system, some viruses and bacteria having dramatic potential as pathogenic organisms, and some components representing other types of health-related biological activity, such as plant and fungal allergens and bacterial endotoxin.