Invisible Agents of COVID-19 Transmission? Common Sources, Characteristics, and Implications of Indoor Aerosols

The positive association between outdoor atmospheric particulate matter and the morbidity and mortality of Coronavirus Disease 2019 (COVID-19) infections was concluded by recent studies. Since people spend most of their time indoor, aerosol particles generated by indoor activities have inherently closer links to human occupants and more direct implications on the airborne transmission of COVID-19. In light of this risk, we reviewed the characteristics of aerosol particles emitted from common indoor sources and how exposure to those particles affects human respiratory infections as well as the transport of airborne pathogens. Activities such as tobacco smoking, cooking, vacuum cleaning, laser printing, and burning candles, mosquito coils or incenses can generate large quantities of particles, mostly in the ultrafine range (< 100 nm). These tiny particles – each may have unique properties and aerodynamics depending on the sources and environmental conditions upon their release – could stay airborne and deposit in the deeper regions of human airways while being difficult to clear by the respiratory system. Adverse effects can be induced by inhaled aerosol particles via oxidative stress and inflammation. Early epidemiology evidence and animal studies showed the confounding effects of particle exposure in respiratory infections. Specifically, inhaled particles can impair human respiratory systems and immune functions and induce the upregulation of angiotensin-converting enzyme 2, causing vulnerability to COVID-19 infection. Moreover, co-production of inflammation mediators by COVID-19 infection and particle exposure magnifies the cytokine storm and aggravates symptoms in patients. We also discuss the possibility of indoor aerosol particles being virus carriers in the airborne transmission of COVID-19. Although many hypotheses were proposed in recent discussions, no rigorous evidence was provided to elucidate interactions between aerosol articles and virus-laden droplets or droplet nuclei, and studies are needed on how these interactions affect the persistence, transport, and lung deposition of COVID-19 and other airborne pathogens.