Mucociliary clearance is humidity dependent–contrary to common belief

Efficient mucociliary clearance (MCC) is a precondition for the moistening capacity of human noses. A review of the literature reveals that only young and healthy individuals can maintain efficient nasal MCC in low ambient humidity. Aging, concomitant diseases and stress factors, all diminish MCC and therefore the ability of maintaining homeostasis of the airways. In an aging population this finding calls for action. We cannot change the outdoor climate nor the physical laws that cause indoor dryness. But we must take responsibility for the anthropogenic indoor climate and its undesired consequences.publishedVersio

Human airways in context with indoor and outdoor climate Technical Specification of human nose

The bulk of literature on humans as occupants of buildings, is focused on comfort issues, especially thermal comfort. This review focuses on climate impacts on human airways and related health aspects. The paper describes design specifications, capacities, and limitations of conducting airways. The focus will be on the crucial role of the nose for the conditioning of breathing air to the physiological requirements of gas exchange in the alveoli. Throughout several million years of humankind’s evolution, the nose adapted to extreme climate situations in terms of moisture and temperature conditions. Low absolute humidity (AH) was the climate factor with the greatest impact on the shape of human noses. Up to the present day, low AH is the crucial challenge for the vital air conditioning. Nose and bronchi of a growing aging population are over stressed with indoor winter conditions in temperate climate. The paper explains the potential consequences for infection defense and seasonal infections.publishedVersio

Expiratory Aerosol pH: The Overlooked Driver of Airborne Virus Inactivation

Respiratory viruses, including influenza virus and SARS-CoV-2, are transmitted by the airborne route. Air filtration and ventilation mechanically reduce the concentration of airborne viruses and are necessary tools for disease mitigation. However, they ignore the potential impact of the chemical environment surrounding aerosolized viruses, which determines the aerosol pH. Atmospheric aerosol gravitates toward acidic pH, and enveloped viruses are prone to inactivation at strong acidity levels. Yet, the acidity of expiratory aerosol particles and its effect on airborne virus persistence have not been examined. Here, we combine pH-dependent inactivation rates of influenza A virus (IAV) and SARS-CoV-2 with microphysical properties of respiratory fluids using a biophysical aerosol model. We find that particles exhaled into indoor air (with relative humidity ≥ 50%) become mildly acidic (pH ∼ 4), rapidly inactivating IAV within minutes, whereas SARS-CoV-2 requires days. If indoor air is enriched with nonhazardous levels of nitric acid, aerosol pH drops by up to 2 units, decreasing 99%-inactivation times for both viruses in small aerosol particles to below 30 s. Conversely, unintentional removal of volatile acids from indoor air may elevate pH and prolong airborne virus persistence. The overlooked role of aerosol acidity has profound implications for virus transmission and mitigation strategies

Inactivation mechanisms of influenza A virus under pH conditions encountered in aerosol particles as revealed by whole-virus HDX-MS

Multiple respiratory viruses, including influenza A virus (IAV), can be transmitted via expiratory aerosol particles, and aerosol pH was recently identified as a major factor influencing airborne virus infectivity. Indoors, small exhaled aerosols undergo rapid acidification to pH ~4. IAV is known to be sensitive to mildly acidic conditions encountered within host endosomes; however, it is unknown whether the same mechanisms could mediate viral inactivation within the more acidic aerosol micro-environment. Here, we identified that transient exposure to pH 4 caused IAV inactivation by a two-stage process, with an initial sharp decline in infectious titers mainly attributed to premature attainment of the post-fusion conformation of viral protein haemagglutinin (HA). Protein changes were observed by hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS) as early as 10 s post-exposure to acidic conditions. Our HDX-MS data are in agreement with other more labor-intensive structural analysis techniques, such as X-ray crystallography, highlighting the ease and usefulness of whole-virus HDX-MS for multiplexed protein analyses, even within enveloped viruses such as IAV. Additionally, virion integrity was partially but irreversibly affected by acidic conditions, with a progressive unfolding of the internal matrix protein 1 (M1) that aligned with a more gradual decline in viral infectivity with time. In contrast, no acid-mediated changes to the genome or lipid envelope were detected. Improved understanding of respiratory virus fate within exhaled aerosols constitutes a global public health priority, and information gained here could aid the development of novel strategies to control the airborne persistence of seasonal and/or pandemic influenza in the future. IMPORTANCE: It is well established that COVID-19, influenza, and many other respiratory diseases can be transmitted by the inhalation of aerosolized viruses. Many studies have shown that the survival time of these airborne viruses is limited, but it remains an open question as to what drives their infectivity loss. Here, we address this question for influenza A virus by investigating structural protein changes incurred by the virus under conditions relevant to respiratory aerosol particles. From prior work, we know that expelled aerosols can become highly acidic due to equilibration with indoor room air, and our results indicate that two viral proteins are affected by these acidic conditions at multiple sites, leading to virus inactivation. Our findings suggest that the development of air treatments to quicken the speed of aerosol acidification would be a major strategy to control infectious bioburdens in the air

Mucociliary clearance is humidity dependent–contrary to common belief

Efficient mucociliary clearance (MCC) is a precondition for the moistening capacity of human noses. A review of the literature reveals that only young and healthy individuals can maintain efficient nasal MCC in low ambient humidity. Aging, concomitant diseases and stress factors, all diminish MCC and therefore the ability of maintaining homeostasis of the airways. In an aging population this finding calls for action. We cannot change the outdoor climate nor the physical laws that cause indoor dryness. But we must take responsibility for the anthropogenic indoor climate and its undesired consequences

Human airways in context with indoor and outdoor climate Technical Specification of human nose

The bulk of literature on humans as occupants of buildings, is focused on comfort issues, especially thermal comfort. This review focuses on climate impacts on human airways and related health aspects. The paper describes design specifications, capacities, and limitations of conducting airways. The focus will be on the crucial role of the nose for the conditioning of breathing air to the physiological requirements of gas exchange in the alveoli. Throughout several million years of humankind’s evolution, the nose adapted to extreme climate situations in terms of moisture and temperature conditions. Low absolute humidity (AH) was the climate factor with the greatest impact on the shape of human noses. Up to the present day, low AH is the crucial challenge for the vital air conditioning. Nose and bronchi of a growing aging population are over stressed with indoor winter conditions in temperate climate. The paper explains the potential consequences for infection defense and seasonal infections

Seasonality of Respiratory Viral Infections

The seasonal cycle of respiratory viral diseases has been widely recognized for thousands of years, as annual epidemics of the common cold and influenza disease hit the human population like clockwork in the winter season in temperate regions. Moreover, epidemics caused by viruses such as severe acute respiratory syndrome coronavirus (SARS-CoV) and the newly emerging SARS-CoV-2 occur during the winter months. The mechanisms underlying the seasonal nature of respiratory viral infections have been examined and debated for many years. The two major contributing factors are the changes in environmental parameters and human behavior. Studies have revealed the effect of temperature and humidity on respiratory virus stability and transmission rates. More recent research highlights the importance of the environmental factors, especially temperature and humidity, in modulating host intrinsic, innate, and adaptive immune responses to viral infections in the respiratory tract. Here we review evidence of how outdoor and indoor climates are linked to the seasonality of viral respiratory infections. We further discuss determinants of host response in the seasonality of respiratory viruses by highlighting recent studies in the field. Expected final online publication date for the Annual Review of Virology, Volume 7 is September 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates