Low Air Humidity Reduces the
Body’s Barrier Function

1

Mucous membrane barrier
‍
The epithelial cells of the airways have cilia which are covered by a mucus layer. The majority of inhaled viruses, bacteria and toxic substances in the air adhere to this mucus.

The cilia convey the mucus together with the microorganisms and toxic substances towards the larynx, where they can be coughed out or swallowed.

2

Innate immunity
(early phase of infection control)
‍
‍Microorganisms that have been able to cross the first line of defense are recognized and destroyed by white blood cells, the “police officers” of innate immunity.

The scavenger cells release messenger substances (interferons), which trigger the production of proteins with which they jointly combat the invading microorganisms.

3

Acquired immunity
(late phase of infection control)
‍
In the late phase of the infection, when the first two barriers have been crossed, pathogen-specific antibodies are formed.

This acquired immune response emanates from B and T lymphocytes, triggered by vaccinations or previous infections and stored in the immunological memory.

‍Low Air Humidity Reduces the
Body’s Barrier Function 
‍
Influenza-Aviruses (IAV) cause infections all over the world which lead to half a million fatalities each year. These influenza outbreaks occur in temperate regions during the winter months. They peak between November and March in the northern hemisphere and between May and September in the southern hemisphere. These epidemics involve the following factors: temperature fluctuations, low airhumidity, too many people indoors, lack of sunlight and associated vitamin D deficiency. 

The detailed study by Akiko Iwasaki et al. shows once again* that excessively low humidity significantly increases susceptibility to influenza infections. (*Lipsitch et al. 2010, 30-year study) Anexcessively low humidity of 10–20% RH leads to a reduction of the self-cleaning mechanism of the respiratory tract, to a reduction of resistance against viruses and to a reduction of the functionality of the memory cells of the immune system. At a higher relative humidity (50% RH), the reduction effects do not occur. 

Three immune barriers to protect against infections
‍
Protection against a viral infection should be enabled by 3 barriers of the immune system.The first barrier consists of the mucous layer, the liquid layer on the mucouslayer and the cilia on the surface. They enable the immune system to fight off pathogens and particles entering via the respiratory tract. When the virus breaks through this first immune barrier, interferon is released to activate genes that fight and block the viruses.

If the virus succeeds in penetrating this congenital second layer of defense, as the third level, the adaptive immune system is activated in order to trigger virus-specific immune responses of the memory cells using B cells and T cells. These three barriers become ineffective if the humidity is too low (10–20% RH). The influenza virus breaks out. The severity of the illness worsens at 10–20% relative humidity regardless of the viral burden. In addition, the ability ofcellular tissue to repair itself is inhibited. Influenza disease shows a weaker course at 50% RH with lower fever and shorter duration.

Original title:
‍Low ambient humidity impairs barrier function and
innate resistance against influenza infection

Authors:
‍Eriko Kudo, Eric Song, Laura J. Yockey, Patrick W. Wong,
Robert J. Homer und Akiko Iwasaki

Published: 2019