How Best To Assist With Limiting The Transmission Of Airborne Viruses

Care homes and hospitals have been of particular focus in the news recently and how best to assist with limiting the transmission of airborne viruses within these establishments.

This is part of the greater discussion around Indoor Air Quality and how we can protect people who are working, studying or being cared for in increasingly airtight buildings for the majority of their day.

Humidity control as part of the Indoor Air Quality discussion is very much part of the debate. IAQ has come into focus as we try to replicate good quality fresh air in our homes and buildings irrespective of the outside air quality, whilst being conscious of energy usage which could put greater pressure on climate change.

The benefits of humidity control are well documented – creating a more comfortable condition, adding to thermal comfort (feeling warmer for a lower air temperature) and importantly reducing the transmission of bacteria and viruses.

Scientific data does show that to maintain a humidity of 50%rh will reduce the transmission of viruses by keeping the bodies defence mechanism (mucus membrane in the nose and throat) moist and able to capture air borne bacteria and virus before it enters the body. Maintaining a relative humidity (RH) between 30-60% has been shown to influence the survivorship of viruses and reduce the transmission and infectiousness of viral diseases.

Sterling et al. (1985) (Fig. 1), synthesized the knowledge at the time into a commonly shared graph of microbial and environmental contaminants on humans at different RH.

Figure 1: Effect of pathogens, microbes, and environmental contaminants on human beings. From Sterling et al. (1985).

More recent studies have taken aim at the influence of relative humidity (RH) on specific families of viruses, like influenza and coronavirus. These studies focus on the observed survival on surfaces and in the air column, and the rates of infection under different environmental conditions. Below we outline key findings from commonly referenced or recent studies. Links to the studies are posted as well.

RH and Viral Transmission

  • Maintaining an indoor relative humidity between 40%-60% may help to limit the spread and survival of novel coronavirus. Humidification maintains hydrated and intact mucosal barriers of human occupants, resulting in an increase resistance against any microbial attack. (Dietz et al., 2020).
  • High temperature, high ultraviolet index, low wind speed, and low relative humidity are contributors to increased MERS-CoV cases (Altamimi et al., 2019)
  • Cold and dry conditions favour influenza transmission in guinea pigs (Lowen et al., 2007)
  • Maintaining RH in offices and workplaces lowers potential risk for transmission of viral diseases (Wolkoff, 2018 and references therein).

RH and Virus Survival

  • While coronaviruses are durable on surfaces relative to influenza viruses, survival rates are reduced at moderate RH of 50%. (Casanova et al., 2010)
  • The infectiousness of airborne influenza viruses was significantly reduced when RH was above 40% (Noti et al., 2013)

Figure 2. The percent infectivity of airborne influenza at a constant temperature (20°C, 68°F). Curve shows all size fractions (>4 µm, 1–4 µm, and <1 µm). From Noti et al., 2013. 

RH and Recovery

  • Humidification in homes can reduce survival of influenza and promote recovery, by improving restfulness, in sick individuals (Myatt et al., 2010)
  • Maintaining 40-45% RH in hospitals reduced perceived air dryness and airway symptoms of patients and hospital staff (Nordström et al., 1994).

So, where a humidity control system is not already in place in a facility, is there a practical solution to retro fit into the office, care home, classroom etc? Mobile humidifiers can be used but they do require regular manual filling and take up floor space.

Commercial systems can be installed into the fresh air supply (AHU), but sometimes this is not possible, or such a system is not already installed into the building.

One excellent solution is to use the HomEvap, a cold water evaporative humidifier, which can be fitted to a heat recovery unit in a house or supplied with a fan to be neatly installed above a false ceiling in an office – out of the way, plumbed in, and providing excellent control automatically. Energy usage ranges between 20 -90 watts dependant on if you require a fan or not. A single humidifier can evaporate 5 litres of water per hour which will provide humidity control to a space with a volume of approximately 1200m3. The result is a comfortable, healthier environment for net zero energy usage (temperature can be reduced by 2⁰C when the humidity is raised from 30 – 50%rh for the same thermal comfort).

In a care home the solution is often not to put the humidifier in the day room or bedrooms but to control the humidity in corridors serving these rooms and to allow the humidity to migrate to the areas of concern. This way there is no noise issue (although very low in a bedroom a small fan can still be a nuisance) and maintenance when required does not disturb the occupant of the room.

Humidity Solutions Ltd has a wealth of experience, expertise and application knowledge in the field of humidity control please do contact us for free advice.

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