Two people standing on a metal spiral staircase. — High contact surfaces such as staircase handrails can also be historic surfaces which are sensitive to cleaning. They can be particularly vulnerable if cleaning increases in frequency, and some cleaning materials may cause irreparable damage. © Imageplotter Travel / Alamy Stock Photo

High contact surfaces such as staircase handrails can also be historic surfaces which are sensitive to cleaning. They can be particularly vulnerable if cleaning increases in frequency, and some cleaning materials may cause irreparable damage. © Imageplotter Travel / Alamy Stock Photo

Effects of Relative Humidity on COVID-19 in Heritage Interiors

Dr. Philip Skipper

Clara Willett, IHBC

Updated 24 May 2021

Adjusting relative humidity to prevent COVID-19 spread could pose long-term damage to heritage interiors.

The COVID-19 pandemic has led to unexpected challenges for the whole country, including the heritage sector. Following news reports on the spread of the virus and lack of advice on disinfecting historic surfaces, Historic England rapidly developed guidance to provide this information.

News reports are necessarily brief and as such can raise valid concerns in the community. One such concern was the effect of relative humidity (RH) and temperature on the spread of the virus.

RH and temperature are closely related, but for the temperature range which is found in heritage interiors RH had the greater impact on virus persistence. Therefore in this article we will be addressing only RH.

Relative humidity in heritage interiors

Relative humidity is the amount of water vapour held in the air at a given temperature, relative to the maximum amount of water that it could hold at the same temperature (expressed as a percentage, with 100% being saturation).

Heritage interiors such as historic houses contain materials (building fabric, fixtures, fittings and objects ) which are sensitive to and can be damaged by RH and temperature variables.

Bodies such as English Heritage Trust and The National Trust recommend a target RH in their properties of between 40-65%. Below 50% there is the risk of shrinkage and desiccation in wood and glues. Above 65% RH there is a risk of promoting mould growth (Cassar, 2009 and National Trust, 2011) with an increased risk of mould-related diseases (Dietz et al., 2020). Anything outside of this range would result in a risk either to visitors or occupants from mould or to the property from mould, desiccation or condensation. Therefore, monitoring and managing RH and temperature at appropriate levels is essential. This is usually achieved through heating and dehumidifiers.

Over winter and into spring there have been valid concerns that the weather conditions have increased the risk of transmission of COVID-19; lower temperatures, low RH and the reduced ultraviolet light from sunlight enhance survival of the virus. With lower temperatures the use of comfort heating in occupied buildings comes into play: this can drive the RH down.

As the news reports stated that the virus was deactivated at high levels of humidity, serious concerns were raised in heritage bodies that this would lead to changes in environmental management: turning off dehumidifiers could have devastating effects on many heritage materials.

A wooden panelled corridor with wooden display cabinets on the left hand side and windows on the right hand side with sunlight shining through into the interior.

Heritage interiors contain a variety of materials which owing to their age and nature develop a patina which adds to their cultural value and significance. Inappropriate and more frequent cleaning can impact on this, sometimes causing permanent damage. © Historic England Archive

Effect of relative humidity on the persistence of the virus

The Academy of Medical Sciences report (The Academy of Medical Sciences, 2020) on preparing for winter 2020/21 states that in most indoor environments the virus is 'unlikely to persist in the air at a level that poses a risk for more than 30 minutes after an infected person leaves.'

However transmission through touching contaminated surfaces may also pose a risk, and whether heritage professionals like it or not a tactile connection to the past remains a temptation for many people.

Furthermore some historic surfaces such as handrails are frequently touched in everyday use. The majority of studies on the persistence of the virus to date include evidence of its survival on surfaces, so there was ample information for Historic England to base its guidance on.

Several sources state that the risk of surface-based transmission is greater below 40% RH (Ahlawat et al., 2020). This is demonstrated by the recent study by Biryukov et al. (2020) of the effect of RH on the breakdown of the virus: this has shown that the time the virus persists on surfaces decreases as RH increases (Figure 1). However, this is not a linear relationship. While there is a large reduction in persistence time between an RH of 20-60%, increasing the RH from 60-80% has relatively less effect.

As such RH levels of 40-60% are the most effective against the virus and maintaining them at 80% offers no significant increase in effectiveness. Viral persistence also varies substantially depending on the surface material.

The chart based on Biryukov et al. (2020) demonstrates the effect of RH on the half-life (the time required for a quantity to reduce to half of its initial value) of SARS-CoV-2 at 24°C, with a red trend line added as part of this article. The error bars for 80% RH fall within the error range for 60%. There is, therefore, no significant increase in efficacy between the two.

Show detailed description

A graph showing the half-life in hours of the SARS-CoV-2 virus on the y (vertical) axis and relative humidity on the x (horizontal) axis. Grey vertical bars (with error bars) show that the half-life of the virus decreases as the relative humidity increases: from 15 hours at 20% RH to 7 hours at 80% RH at 24 degrees. There is no significant increase in efficacy between 60 and 80%. A red line shows this decrease at the mean point of each grey bar.

Conclusion

Public health is the priority with regard to reducing transmission of the COVID-19 virus and government guidance provides information about this. Temperature and humidity affects the persistence of this virus, but fluctuations in these environmental conditions can also cause irreparable damage to historic surfaces.

Fortunately for the heritage sector, the RH levels considered appropriate for heritage materials (40-65%) lie within those which effectively reduce the persistence of the viral particles (about 60%).

While increasing the RH above 60% may reduce the persistence of active virus present on certain surfaces, the negligible deactivation of the virus is outweighed by the public health risks from mould spores generated by high levels of humidity. Additionally, heritage bodies should be mindful of the damage increased humidity may pose to the properties and their contents.

For the control of the COVID-19 virus, Historic England’s guidance on the treatment of historic surfaces recommends quarantining them, before considering cleaning or changing current RH; seeking professional advice from a conservator before doing so.

About the authors

Dr. Philip Skipper

Freelance Consultant

Dr Philip Skipper is a freelance consultant, providing advice on biological growth and biodeterioration of heritage materials. This includes research into cleaning biofouling from surfaces, monitoring mould in heritage interiors and more recently dealing with COVID-19 in heritage environments.

Clara Willett, IHBC

Senior Building Advisor at Historic England

Clara Willett, IHBC, is a Senior Building Advisor at Historic England, focusing on stone and terracotta conservation. This includes research into hard to treat stones, stone sourcing, and the effects of climate change.

Further information

Ahlawat, A., Wiedensohler, A. and Mishra, S. K. (2020) ‘An Overview on the Role of Relative Humidity in Airborne Transmission of SARS-CoV-2 in Indoor Environments’, Aerosol and Air Quality Research, 20(9), pp. 1856–1861. doi:10.4209/aaqr.2020.06.0302

Biryukov, J. et al. (2020) ‘Increasing Temperature and Relative Humidity Accelerates Inactivation of SARS-CoV-2 on Surfaces.’, mSphere, 5(4). doi: 10.1128/mSphere.00441-20

Cassar, M. (2009) Environmental Management Performance Standards: Guidelines for Historic Buildings, Swindon: English Heritage

Dietz, L., Horve, P. F., Coil, D. A., Fretz, M., Eisen, J. A. and Van Den Wymelenberg, K. (2020) ‘2019 Novel Coronavirus (COVID-19) Pandemic: Built Environment Considerations To Reduce Transmission.’, mSystems, 5(2). doi:10.1128/mSystems.00245-20.

National Trust (2011) The National Trust Manual of Housekeeping: care and conservation of collections in historic houses, Swindon, WIltshire, National Trust.

The Academy of Medical Sciences (2020) Preparing for a Challenging Winter 2020/21. London.

COVID-19: Cleaning and Disinfecting Historic Surfaces, Historic England