Occupants of buildings spent most of their time indoors and posed themselves to the indoor biological contaminants (i.e. bacteria and fungi) in the built environment. However, the factors that influenced the prevalence of suspended bacteria and fungi in the air of the built environment are not well recognised. Authorities must take the growth of biological contaminants seriously to prevent serious health effects among occupants of buildings. This review summarises the factors (i.e. outdoor air, dust, ventilation problems, humidity/moisture surface problems and human occupancy) of indoor biological contaminants and their species in buildings that could worsen the indoor air quality. Bacteria species could exist in various indoor buildings (i.e. schools, cafeterias, mosques, faculties, child day-care centres, hospitals, dwellings, educational buildings, homes, elderly care centres, libraries, restaurants, and factories).  The dominant bacteria were Bacillus spp. (24.1 %), Staphylococcus spp. (20.7 %), and Micrococcus spp. (20.7 %) whereas for fungi species were Penicillium spp. (25 %), Cladosporium spp. (21.9 %), and Aspergillus spp. (21.9 %).