Cleaning and Disinfecting Historic Surfaces

Cleaning and disinfecting

Cleaning is essential to remove dust and dirt from historic surfaces to prevent damage, and it is often part of a housekeeping routine. It is primarily a ‘dry’ process, involving dusting and vacuuming; the ‘wet’ cleaning of historic surfaces is only carried out when absolutely necessary.

Care should be taken when emptying vacuum cleaners to prevent the dispersal of dust and appropriate PPE, in other words masks and gloves, should be worn. When deep cleaning after a known case of COVID-19 the NHS infection prevention control guidance recommends the use of a high-efficiency particulate air (HEPA) filtered vacuum cleaner.

Disinfecting is the process of eliminating or reducing harmful microorganisms (such as viruses, bacteria and so on) from objects and surfaces. Before the outbreak of COVID-19, there was no need to disinfect historic surfaces, but it is now known that a person who touches a contaminated surface then touches their mouth, nose or eyes can become infected.

The amount of time that the SARS-CoV-2 virus which causes COVID-19 (we will refer to it as the COVID-19 virus throughout this guidance) remains active on a particular material varies. Research on several materials used in historic surfaces gives a guide time of 72 hours in most cases. For some materials, the persistence time is less than 72 hours (see information below). Current guidance (November 2020) from PHE suggests that in non-healthcare settings the risk of residual infectious virus is likely to be significantly reduced after 48 hours​.

Persistence time on surfaces is also affected by temperature and relative humidity (RH). Current research indicates that increases in ambient temperature and humidity decrease the lifespan of the virus. Maintaining RH between 40-60% is most effective against the virus. These RH parameters also prevent the risk of mould growth which can damage historic environments and lead to respiratory illness, but RH of below 50% can cause shrinkage in wood. However, you should not alter temperature and RH to accommodate these parameters without first getting professional advice. ​

To prevent the spread of COVID-19 and other viruses, disinfecting surfaces may be included in routine cleaning in certain circumstances. However, as this is a new virus further research is needed to fully understand how effective different types of disinfectants are and how they will interact with various historic surfaces. For example, standard (commercial, industrial, household) liquid disinfectants and hand sanitisers containing chemicals that eliminate microorganisms can cause irreparable damage to historic surfaces, especially if used repeatedly.

It is always best to avoid or mitigate any virus contamination where possible. Using chemical disinfectants to treat historic surfaces is a last resort. If this is the only option, it is important to manage the risk and follow appropriate health and safety procedures.

Avoid contamination

If a surface cannot be touched, it does not need to be disinfected.

Consider the following:

• Can access to a room or area with historic surfaces be restricted?
• Can barriers be put up to keep people away from historic features?
• Can a ‘no touching’ policy be introduced (using signs to explain the reasons)?
• Can the historic surfaces be covered?
  • If it is impossible to prevent the historic surfaces being touched, it may be helpful to cover them with something that can be removed and then disinfected or replaced as required. For example, furniture can be covered with a tightly woven or synthetic fabric (such as Tyvek ™) and flat surfaces with polyester (such as Melinex ™), acrylic (such as Perspex ™) or glass sheets. Foam protectors can be fitted where people have to touch historic handrails to use stairs safely. Door handles can be covered with acid-free tissue and then cling film. Floors can be covered with mats. Care should be taken not to leave impermeable coverings on historic surfaces for too long since this could cause a build-up of moisture.
• Can the historic surfaces be protected?
  • Regularly reapplying wax (for example microcrystalline or bees) on existing waxed surfaces should give additional protection against increased cleaning and disinfecting.
  • Wearing disposable gloves may protect surfaces from becoming contaminated as well as protecting the user. All gloves will need to be disposed of appropriately.
  • Where possible, wash hands rather than use hand sanitiser. Most hand sanitisers contain alcohol and when wet may damage historic surfaces.

If there is good reason to suspect that an infected person may have touched a historic surface, or that an area may be contaminated, it will need to be protected, quarantined or disinfected.

Mitigate the risk

Are there any alternative approaches to using standard (commercial, industrial, household) liquid chemical disinfectants?

Consider the following:

• Can the rooms, areas or surfaces be quarantined? Using time to let the virus naturally deactivate on historic surfaces is the safest, cheapest and most environmentally friendly method of disinfection.
• Can you close the affected areas or rooms for at least 48 hours before attempting to clean them? The COVID-19 virus persists for longer on some non-porous materials so it may be necessary to extend the quarantine; please see the information on individual historic surfaces below.

After surfaces have been quarantined for the appropriate period, they do not need to be chemically disinfected and normal cleaning can be carried out.

Manage the risk

How can the risk be managed to minimise potential damage to the historic fabric?

If avoiding contamination or imposing a period of quarantine are not possible and using a chemical disinfectant is the only option, a risk assessment and management approach will be required to achieve a proportionate response. You need to balance the need to disinfect with the potential damage to the historic surface.

Before disinfecting any historic surface, it is important to consider the following:

• Understand the historic surface you are wanting to disinfect:
  • What is the substrate material?
  • Has it got an applied surface finish? For example, paint, varnish, shellac, gilding or wax.
  • What is its condition? If it is fragile – flaking, cracking or in poor condition, for example – a liquid chemical disinfectant may cause further damage and should not be used.
• Understand the chemical composition and concentration of the disinfectant, as well as the potential effect of the disinfectant chemicals or proprietary products (for example, wipes, sprays) before using them. Material Safety Data Sheets (MSDSs) for proprietary products should be available, and manufacturer’s instructions for storage and use should be followed.
• Do NOT use any detergents or disinfectants which are strongly acid or alkaline.
• Do NOT use any detergents or disinfectants containing chlorine, such as household bleach (containing sodium hypochlorite) and water purification treatments (containing chlorine dioxide). These are effective COVID-19 virus cleaning agents, but may also cause permanent damage to historic surfaces. Such products contain chloride anions, which can react to produce damaging salts.
• Do NOT use any detergents or disinfectants containing QACs (quaternary ammonium compounds) on any historic surface because they may cause permanent damage. Many commercial biocidal products currently being suggested for disinfection contain QACs. These chemicals can sometimes be difficult to identify in product labelling or MSDSs, so it is best to check on the Internet. QACs can be strongly acidic or alkaline, and either would damage historic surfaces. They may also leave damaging salt residues if not rinsed thoroughly.
• Do NOT use heat or steam devices. Heat (including steam) destroys microorganisms by breaking down the proteins. Its effectiveness is primarily determined by the temperature and exposure time, as well as the type of material being treated. Latest research indicates that the COVID-19 virus can survive long exposure to high temperatures (60°C for 1 hour). The high temperature and sustained time period required for deactivating this virus (92°C for 1 hour) is not safe for historic materials. Heat treatments used to kill insect pests, and household or garment steamers are unlikely to be effective.
• Do NOT use UV or UVC (ultraviolet and ultraviolet C) light. Although UVC light (254 nm wavelength) is effective in deactivating the COVID-19 virus (research shows that five minutes’ exposure kills the virus), this method is not recommended for historic surfaces. UV and UVC light is not only potentially damaging to historic textiles, paper, wood and pigments, but it is also a health hazard to skin and eyes. Research into far-UVC light (222 nm) may have application for some heritage settings in the future.
• Ozone treatment has been suggested, but it has not yet been proved to be effective against the COVID-19 virus and it could be harmful to human health and historic surfaces.

Disinfectant chemicals/methods

The following chemicals have been found to be effective in deactivating the COVID-19 virus on a variety of materials. These materials correlate to historic surfaces (see information below). This information is based on our understanding of currently available research and links to this are given.

Alcohols

Examples: ethanol, industrial denatured alcohol (IDA), such as industrial methylated spirit (IMS), Isopropanol (also known as 2-propanol, isopropyl alcohol, IPA or surgical spirit)

• Alcohols need to be least 70% (70 parts alcohol, 30 parts purified water) to be effective. Both components play a crucial role in deactivating the virus by breaking down the lipid shell. Consult the manufacturer’s instructions to make sure the ethyl alcohol component is not diluted below 70%.
• Industrial alcohols can be difficult to buy and transport. You will need to register with HMRC.
• Methylated spirits bought from DIY stores are not generally suitable for treating historic surfaces because they usually include harmful additives and dyes that could stain historic surfaces.
• Isopropanol is readily available at 70% and can also be purchased as impregnated wipes. Always read the product labelling or MSDS to check that isopropanol and deionised water are the only ingredients.

Concentration and minimum effective contact time

Ethanol and Isopropanol content

• 70%: 2 minutes (reference document)
• 80%: 30 seconds (reference document)

IMS with ethyl alcohol content

• 70%: 2 minutes (reference document)
• 80%: 30 seconds (reference document)

Peroxygen compounds

Example: hydrogen peroxide

• Peroxygen compounds such as hydrogen peroxide can break down the virus’ essential components, but are powerful oxidising agents and can also damage historic surfaces.
• Peroxygen compound-based proprietary disinfectants are available. Always read the product labelling or MSDS to check that the peroxygen compound percentage doesn’t exceed 0.7% (7,000 ppm).

Concentration and minimum effective contact time

• 0.7% (7,000 ppm): 2 minutes (reference document)

Surfactant/detergent and water

• Detergents and surfactants are able to deactivate the COVID-19 virus by breaking down the lipid shell around the virus and/or protein components within it. Read more about how detergents deactivate COVID-19. This process reduces the levels of contamination and may be helpful when used in combination with quarantining, but further research is needed to establish its full effectiveness.
• Detergent/surfactant and water solutions should only be used on surfaces that are able to withstand being damp. Excessive moisture from these mixtures can cause damage, such as lifting coatings or paint layers, and staining porous materials.

Concentration and minimum effective contact time

• As per manufacturer’s instructions: 1 minute (reference document)

The following list gives examples of historic surfaces, the persistence of the COVID-19 virus (where known) and the suitability of the selected disinfectant chemicals.

When disinfecting historic surfaces, the following guidance should be observed:

• Wear appropriate Personal Protective Equipment (PPE), as advised by PHE and according to health and safety precautions in relation to any products, chemicals or methods being used.
• Always carry out a trial in a discreet area and check that no damage has been caused. If damage is noted, do not use the disinfectant chemical or proprietary disinfectant product.
• Apply disinfecting chemicals in a controlled way, applying the chemicals to a clean cloth or paper towel (not the historic surface), and paying close attention to the effective contact time (see information above). This can be achieved by careful and repeated wiping. The wiping action also helps break down the virus cells. If the surface is dirty or greasy (which could reduce the efficiency of the surfactant at breaking down the virus), it is advisable to repeat the treatment. Remove any residue from the surface using a clean cloth or paper towel. Detergent/surfactant and peroxygen compound residues should be removed with a clean cloth, dampened in deionised water. Minimise excessive moisture as this can easily cause damage. The surface should then be dried immediately with another paper towel or soft cotton cloth.
• Apply standard disinfectants to non-historic surfaces carefully to avoid accidental contact with historic surfaces. Fogging, misting or spraying (including electrostatic spraying) should not be carried out in spaces that contain historic surfaces, fixtures and fittings, objects or collections (even if they are in display cases).
• When disinfecting other surfaces, take care to avoid accidental contact with historic surfaces. For example, the wooden elements of a door should be protected when cleaning a metal door handle with a disinfectant chemical.
• Take care not to damage other adjacent historic surfaces. For example, the wooden elements of a door should be protected when cleaning a metal door handle with a disinfectant chemical.
• Dispose of all cleaning and disinfecting materials as advised by PHE.
• If in doubt, seek advice from a conservator who specialises in the historic surface you are trying to treat.