Humidity levels and their connection to health

“Exposure to dry air impairs host defence against influenza infection, reduces tissue repair, and inflicts caspase-dependent disease pathology”

 We know that air quality plays a big role in how we function in our day to day environments, but a recent article from the National Academy of Sciences of the United States of America (PNAS) shared by CAREL, delves deeper and sheds light on the reasons why a minimum level of humidity may be a viable strategy to reduce disease symptoms and to promote more rapid recovery in influenza-infected individuals.

 Raul Simonetti | HVAC/R Corporate Business Manager

‘The previous studies by Dr. Taylor and Dr. Reiman demonstrated a correlation between healthcare acquired infections (HAIs, by Taylor) and pre-school absenteeism (by Reiman) when the humidity was lower than approx. 40 %rh, but they did not explain the reasons. The attached article, instead, explains the reasons why “Low ambient humidity impairs barrier function and innate resistance against influenza infection”:

1. The surface of the trachea is composed of a mucus layer, the surface liquid layer, and the cilia
   Note: cilia are minute hair-like filaments present inside the trachea that move to expel foreign bodies (e.g., pathogens and particles)
2. “During influenza infection, the respiratory mucosal barrier provides the first line of defense [from pathogens and particles]. The mucus layer, the surface liquid layer, and the cilia of the surface of the bronchus epithelia promote mucociliary clearance (MCC) of invading pathogens and particles”
   Note: MCC (= mucociliary clearance) is the movement of cilia
3. “We show that mice kept at low relative humidity (10–20% RH) experience more severe symptoms than those kept at higher relative humidity (50% RH). Lower RH impaired mucociliary clearance and tissue repair and blocked the induction of ISGs known to restrict IAV, resulting in higher viral burden“
   Notes:
   IAV = influenza A virus
   ISG = IFN-stimulated genes
   IFN = type I interferon
   “If the virus breaches these layers [mucus layer, and the cilia of the surface of the bronchus epithelia], the innate immune defense mechanisms … will induce secretion of type I interferons (IFNs) to turn on hundreds of IFN- stimulated genes (ISGs) to block virus spread”
4. “… These mechanisms, such as impaired MCC and ISG induction may in part underlie the epidemiological correlation of a drop in absolute humidity preceding death from seasonal influenza infection in temperate regions“
5. “It is worth noting that humidity does not seem to affect host defenses against influenza viruses in all situations, as tropical and subtropical climate regions, which are wet and warm, allow for influenza virus to thrive”
6. “Our study suggests that increasing ambient humidity may be a viable strategy to reduce disease symptoms and to promote more rapid recovery in influenza-infected individuals” ‘

 Click HERE to view the full article