The function of aerosols in transmitting and infecting of respiratory infectious diseases and its risk prediction

Abstract

The significance of respiratory infectious diseases has been iterated by outbreaks of severe acute respiratory syndromes (SARS), influenza A (H1N1), Middle East Respiratory Syndrome (MERS), etc. in recent decades. Those outbreaks of infectious diseases mostly occur in indoor environment. The exhaled droplets and droplet nuclei, the residues of droplets, from the infector are considered as the main transmitted vehicles of respiratory infectious diseases. When the infectors are breathing, speaking, coughing, sneezing, etc., they will exhale droplets with different numbers, sizes and speeds. Large droplets will deposit on floor quickly due to gravity, while small droplets will evaporate in seconds. The droplets spreading distance varies with temperature, humidity and air distribution systems. The distance is usually less than 2 m. Some studies also find that there exists a threshold distance of droplet nuclei spreading, where droplet nuclei concentration decreases quickly, indicating that short distance airborne infection risk is much higher than long distance airborne infectious diseases. The viability of microorganism in droplets is related to temperature, relative humidity, sensation of oxygen, exposure in ultraviolet, etc. The death rate of microorganism in droplets is much higher at beginning seconds due to evaporation, iterating the higher risk of short-range infection. Some studies also show that the pathogens survived well in low RH or high RH, which indicates the significance of controlling RH in indoor environment not only for thermal comfort but also for risk control. Wells-Riley equation is introduced here, which is developed by Riley and formulated on following assumptions: the infectious diseases are transmitted via droplet nuclei only; the distribution of droplet nuclei in the room is uniform; the death rate of pathogens is ignored. Wells-Riley equation and its revised models have been used to predict cross-infection risk and have clarified the important role of ventilation rate on reducing risk of airborne infection. However, the models for estimating risk of respiratory infectious diseases almost based on statistical methods. It needs to be improved to evaluate the role of close range, droplet-borne and short-range airborne infection risk. Some methods to cut off short-range airborne routes are also introduced. With the improved understanding of the spread, movement and evaporation of human exhaled droplets in different indoor environment, the understandings of transmission routes of respiratory infectious diseases deepen. However, these understandings have not been well integrated with the risk prediction model to predict and evaluate the infection risk of respiratory infectious diseases. There is a need to combine the model for predicting dispersion of droplets with the viability and pathogenicity of microbes, which can help human evaluate risk and take proper public means to reduce the cross-infection. According to the current understanding of transmission route of respiratory infectious diseases, in addition to reducing the contact probability by surface disinfection, the ventilation methods can be applied to reduce cross-infection. It is suggested to cut off the spread of droplet borne and short-range airborne, of which infection risk is much higher than that of long range airborne. Improving the effective ventilation rate can reduce the risk of long-range airborne. Airflow direction between rooms can be controlled by pressurization to prevent cross-infection between rooms. To control respiratory infectious diseases efficiently, it needs cooperation of engineering, medical, fluid mechanics, public health, environment and other disciplines. After the outbreak of SARS in 2003, much progress has been made. However, due to incubation, instantaneous, biological diversity of respiratory infectious diseases, it is extremely difficult to do fine quantitative study. A variety of infection control measures and methods are mainly based on experience. Many scientific problems, such as the mechanism of infection and the method of prediction, need further studies in multi disciplines.