Fighting the flu with humidifiers?

Coming to university is a big step in life: it’s the first time for most of us to be away from home, meeting new people and figuring out how to do laundry without Mum. It is also the time to meet new microbes and partake in what is known as “Freshers Flu”. 

Freshers flu is a blanket term, used to describe all the common colds, and flu experienced by students around the beginning of the academic year, which also happens to be the beginning of flu season in the UK. Although for most students Freshers Flu might not actually be caused by the influenza (flu) virus, it is still a serious threat in many places, causing between 290,000- 650,000 deaths every year, according to the World Health Organisation. 

There are 3 types of Influenza- imaginatively named A, B and C- of which A is the most serious and is responsible for causing epidemics. Influenza can also lead to more serious complications such as ear infections, meningitis, and even sepsis (a system wide infection of the whole body). Treatments encompass plenty of rest, fluids and painkillers, although the flu vaccine now is becoming a more common approach too, especially for people who appear to be more at risk e.g. healthcare workers, the elderly population, etc. 

It has been known that cold weather promotes influenza, thought it was not well understood. A recent study by Kudo et al. (2019) from Yale shed some light on how humidity affected influenza infection in mice models. They found that mice who were in low humidity conditions were more susceptible to Influenza A infections. 

Kudo et al. used special environment chambers to house their mice, in either low Humidity (20%) or high Humidity (50%) . The temperature was kept at 20-22 degrees throughout. 

Dry air condition was found to reduce how effectively the cells lining the respiratory tract can clear mucus. It also hampered the innate immune system and their defence against these pathogens. 

The study found that low ambient humidity seemed to be mediating this effect via the inflammasome-caspase route. Inflammasomes are basically a defence mechanism in response to pathogens invading the cell. When the mice had their inflammasomes genetically removed (Knocked out), the ambient humidity did not aggravate the influenza infection. 

Kudos to Kudo et al. Figure shows that mice in low humidity conditions (blue) had a lower survival rate to mice kept in high humidity conditions (green).

Additionally, usually after a viral infection, the tissues of the airways get damaged, The study also showed that mice in lower humidity also had a reduced ability to repair this tissue damage, and the cells lining the airways in the lungs (the epithelial cells) were dividing less after infection compared to the cells from mice kept in higher ambient humidity.

So why is this the case? The authors speculate that it may be due to the body’s immune system not being able to cope with the dry and dehydrated conditions of respiratory cells, allowing build up on particles from air, and also the cells not repairing the tissue. Whether the dry air also contributes to reducing the body’s anti-viral mechanisms is also something that requires further investigation. It is also interesting to note that influenza is still rampant in tropical countries, where it is warmer and more humid, so further work from the lab would focus on why this might be the case. 

Though for this study, the mice were kept at set temperatures between 20-22 degrees Celsius, it would be really interesting to see what effects humidity has on colder temperatures, as you would expect outside during the winter season. 

Overall the findings of this study has great implications, especially for our increasing elderly population, and also patients who are immuno-supressed, to maintain humid conditions in hospitals and care settings, amongst other places, to prevent outbreaks of flu. 

Whilst we might not have all the answers yet, it appears that high ambient humidity is beneficial for our immune system in the case of influenza, and might help to manage viral infections. So whether or not you consider experimenting on yourself this flu season by getting a humidifier, make sure to keep the ibuprofen, bottle of water and Vicks on stand-by…

Read the original research paper from https://www.pnas.org/content/116/22/10905 

Additional notes from other sources: 

  • Martínez A, Soldevila N, Romero-Tamarit A, Torner N, Godoy P, Rius C, et al. (2019) Risk factors associated with severe outcomes in adult hospitalized patients according to influenza type and subtype. PLoS ONE 14(1): e0210353. https://doi.org/10.1371/journal.pone.0210353
  • Oxford Vaccines webpage http://vk.ovg.ox.ac.uk/vk/influenza-flu 
  • Zamboni DS, Lima-Junior DS. Inflammasomes in host response to protozoan parasites. Immunol Rev (2015) 265(1):156–71.10.1111/imr.12291