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Personalized ventilation: One possible solution for airborne infection control in highly occupied space?

Airborne diseases have always been one of the main causes of human morbidity and mortality worldwide. There are hundreds of airborne communicable pathogens resulting in millions of deaths and injuries annually. With the continuous development of epidemiology and other related disciplines, combined with the promotion of modern engineering technology, the control methods of airborne transmission have been developed markedly, but the spread of these diseases has never been stopped, resulting in endless social and economic losses. According to the WHO statistics, about 5–10% of the world’s adults and 20–30% children suffered from influenza each year, causing 300–500 million severe cases and 20–50 million deaths. Although tuberculosis can be treated and cured at present, nearly eight million people were still infected each year, and two million of these died every year. Therefore, how to effectively reduce the casualties caused by airborne infection and control the spread of these airborne diseases is one of the most urgent problems that need to be addressed in the field of epidemiology and other associated disciplines. An important reason why airborne diseases are difficult to control is that they can easily spread in crowded places. For example, schools and nursery institutions have always been prone to high incidences of influenza; tuberculosis, severe acute respiratory syndrome (SARS) and norovirus. All these have been documented to spread in aircraft cabins. Similar crowded spaces also include crowded offices, public transports conference rooms, restaurants, theatres and so on. Due to the large density of occupants and relatively long exposure time, respiratory infectious diseases are prone to outbreak in these places. Many studies have indicated that the incidence of respiratory diseases is closely related to the density of people or the degree of crowding. Although the ventilation rates (especially for fresh air) have been raised according to various standards for such highly occupied spaces, these are really aimed at maintaining good indoor air quality and thermal comfort without a proper consideration to control airborne infection. Meanwhile, the increase of air change rate of ventilation can cause a higher burden of energy consumption. The outbreak of diseases such as SARS and influenza A virus in recent years has brought profound lessons to our ventilation design. The previous simple design of airflow distribution cannot meet the needs of protecting indoor human health and reducing threats of new diseases any more. The new balanced ventilation mode with a comprehensive consideration of energy conservation, thermal comfort, air quality and health should be developed. There are mainly two ventilation strategies applicable to control the risk of cross-infection in highly occupied spaces. One way is to increase the total ventilation rate to a sufficient level to minimize airborne infection, but how much ventilation air is sufficient is still a controversial subject of debate. The other way is to improve the ventilation efficiency for individual occupants. Personalized ventilation (PV) which delivers fresh air directly to the occupant’s breathing zone can serve as a possible solution. Figure 1 depicts the relationships between ventilation modes and airborne infection risk in highly occupied spaces. Human breathing, e.g. coughing, sneezing, speaking, breathing, etc. is one of the most important sources of airborne transmitted pathogens. Effective engineering