Issue link: https://hi.iaq.net/i/191637
Climate Change, the Indoor Environment, and Health INFECTIOUS AGENTS AND PESTS 157 tory viruses. Fomite spread is affected by ambient humidity, which can in turn be affected by indoor air. Influenza Viruses Influenza viruses continue to account for substantial annual morbidity and mortality interspersed with periods of increased activity. The 2009– 2010 H1N1 influenza epidemic is estimated to have involved around 61 million infections, 274,000 hospitalizations, and more than 12,000 deaths in the United States (CDC, 2010b). Although there has been prolonged controversy over the environmental correlates of influenza epidemic spread, it appears that absolute humidity— the amount of water vapor in a given volume of air—is a critical determinant (Shaman and Kohn, 2009; Shaman et al., 2010a,b). In contrast, relative humidity—the amount of water vapor in a given volume of air at a given temperature expressed as the percentage of the maximum possible for that temperature—is well regulated in the indoor environment and appears not to be as important a determinant of influenza transmission and spread. However, studies by Myatt et al. (2010) show that increased absolute humidity and relative humidity, achieved by the use of indoor air humidification, can lead to substantial reductions in viable influenza virus.2 Overall, the effects of humidity on influenza virus outbreaks and peak epidemic periods are greater in temperate than in tropical environments. In some tropical and subtropical settings, relative humidity has been more closely associated with influenza epidemics (Tang et al., 2010a,b). Because periods of high relative humidity corresponded to periods of increased indoor time and air conditioning, the population-based correlations are confounded. However, because indoor air conditioning affects indoor temperature and humidity, these require more investigation to determine whether the critical aspects of influenza spread are determined by the indoor or outdoor environmental conditions. The different results in temperate and tropical zones may reflect differences in viral and human biology in those regions. However, comparative studies for tropical and subtropical regions for respiratory transmission have not been completed in the United States. Respiratory Syncytial Virus RSV is the greatest cause of bronchiolitis and pneumonia in infants worldwide and causes up to about 125,000 hospitalizations in US in2 As discussed later in this chapter, though, increased humidity may create a more hospitable environment for mold growth and accelerate the degradation and subsequent off-gassing of building materials and furnishings. Copyright © National Academy of Sciences. All rights reserved.