Human beings are facing threats from bio-aerosols like pollens, mold, bacteria and viruses in indoor environments. Improving indoor air quality by applying air cleaners in air handling units can reduce high-energy consumption of ventilation and extra expenses related to absences from work, reduced productivity and medical treatments. Currently, there are several main types of air cleaning technologies (ACT) which are already well applied in air conditioning systems. Filters are the most commonly used ACT, which can offer relatively stable filtration efficiency. Ultraviolet germicidal irradiation (UVGI) systems use electromagnetic energy in the UVC spectrum to damage and prevent replication of microbial DNA and RNA. However, most standards for filters are based on data obtained from accelerated tests in labs, which may be quite different from actual situations. There is not enough evidence to verify the direct relationship between in-duct UVGI and the concentration of bio-aerosols in indoor environments. More studies should be performed to provide a more reliable connection between concentration of bio-aerosols and sick building syndrome or sick leave under the effect of different ACTs.