Identifying and reducing air pollution

Creating a museum environment that welcomes visitors while protecting collections from airborne pollution is one of the more difficult balancing acts in collections care. Pollutants enter museum spaces as particles, gases, or liquids. They can damage objects almost as severely as poor temperature and humidity control.

Museums must allow fresh air into the building, as the law requires good indoor air quality. At the same time, you need to protect vulnerable items from harmful organic and inorganic compounds.

This challenge is greatest in towns and built-up areas, where outdoor air carries pollutants such as car fumes and construction dust. Visitors also bring pollutants in with them, from clothing fibres to loose skin particles. Multiple surfaces inside a museum create more opportunities for chemical reactions than you would find outside.

Start by understanding the sources and types of pollutants most likely to damage your collections. Then limit them through robust preventive conservation.

This guide covers pollutants coming from outdoors. For internal pollution, read our guide on storage and display materials.

Power stations, factories, vehicles, and heating systems emit harmful chemicals. These enter museums through open doors and windows, air exchange units, and on people.

Temperature and humidity can speed up the reactions these chemicals cause. The concentration of chemicals and the compounds they interact with indoors also affects how they behave.

Sulphur dioxide

Burning fuel, oil, coal, and diesel emits sulphur. Combined with oxygen, this forms sulphur dioxide (SO2). This compound is already naturally present in the air due to volcanic activity and has become the most significant pollutant in Britain. It reacts with water molecules to form sulphuric acid (H2SO4), which wears away even the hardiest materials.

Sulphur also combines with other gases to create compounds such as hydrogen sulphide, carbonyl sulphide, and carbon disulphide. Many fuels now have reduced sulphur content to help lower sulphur dioxide pollution.

Nitrogen dioxide

Cars, other vehicles, and power stations emit nitrogen dioxide (NO2) as a by-product of burning petrol and diesel. Combined with water, it creates nitric acid (HNO3), another form of acid rain.

Nitrogen dioxide is a highly reactive gas. It reacts in sunlight to form ozone (O3) and PAN (peroxyacyl nitrates), two pollutants known collectively as photochemical smog. Many vehicle exhaust systems now include a catalytic converter that splits nitrogen dioxide back into harmless nitrogen and oxygen.

Ozone also occurs naturally at 20-30km above the earth, where the ozone layer protects life from harmful short-wave ultraviolet radiation. Rising ozone levels at ground level now threaten both people and objects.

Gaseous pollutants fall into two main groups: those that are acidic and those that have an oxidising effect.

Sulphuric and nitric acids react with many different materials and cause permanent changes:

• Limestone, marble, and other calcareous materials weaken, discolour, or dissolve
• Iron and other metals corrode
• Leather suffers from red rot, losing strength and flexibility as the leather fibres break down, and eventually becoming a powder
• Cotton, linen, and viscose materials discolour and become weak and brittle
• Wool and silk weaken, although as they are naturally acidic they are also more resistant
• Paper objects become yellowed and brittle
• Silver in photographic images yellows and fades
• Gelatin and the film base polymers of negatives break down
• All kinds of dyes and pigments may fade

An oxidising substance is a chemical that causes other substances to lose electrons or combine with oxygen in a chemical reaction. These lead to the formation of free radicals and acids in many materials, especially organic ones. Reactions reduce the chain length in polymers, break double bonds in long-chain carbon molecules, and create new cross-links in the molecular structure of materials. Materials become significantly weaker, more brittle, and discoloured as a result.

Ozone and PAN are powerful oxidants. They react with almost any material they encounter and can trigger further unpredictable reactions. Many organic materials contain antioxidant compounds that fight oxidation. Deterioration can speed up rapidly once those antioxidants run out.

Oxidants have the following effects on museum objects:

• Dyes and pigments fade or change colour
• Rubbers and plastics crack
• Textiles become brittle
• Paint binder resins break down
• Shell, marble, and some geological specimens suffer surface changes
• Metals such as silver, copper, and iron tarnish more quickly

Particulate pollution ranges from large, abrasive particles to particles small enough to enter display cases through the tiniest cracks. Large particles scratch surfaces during cleaning. Small particles only settle when trapped or held down by electrostatic attraction. High humidity causes particles to absorb moisture and clump together. It can also allow secondary particles to form that harbour allergens.

How particles damage collections

Particles absorb sulphur dioxide, making them acidic and water-absorbent. This causes corrosion and fungal growth. Traces of metals in particles can also speed up these reactions and accelerate the breakdown of organic materials.

Burning fuel in vehicle engines, power stations, and heating systems produces black soot and tar particles. These are released into the air and cause soiling in urban museums, visible as grime around window frames.

New concrete and plaster give off alkaline particles. These darken oil paint films and discolour many dyes and pigments. Wool, silk, gelatine, and other protein-based materials lose strength when exposed to alkalis.

Museums near the coast face the additional risk of salt crystals in the air. Salt absorbs airborne moisture and creates salty droplets that corrode most unprotected metals. High moisture levels around salt-containing dust particles also support the growth of fungi and micro-organisms, even when the surrounding environment appears quite dry.

Particles produced outside, such as soil grains, pollen, and fungal spores, can combine with particles produced inside, such as textile fibres and skin fragments. Together they form an attractive food source for insects and fungi, which may then affect museum objects.

Museums can take practical steps to control and remove many of the dangers posed by air pollution. Contact the Environmental Health Department of your local council to find out pollution levels in your area. Then devise a plan to protect your collections.

Monitor air quality

The Scottish and UK governments publish information about pollutants on publicly accessible websites. Find current data on the Scottish Government air quality pages. If you suspect a specific problem in your museum, invite an environmental scientist with experience in monitoring indoor air pollution (IAP) to assess your museum’s air.

Museum staff can also play a role more information is available in our guide on monitoring museum environments.

Prevent pollutants from entering the building

Stop external pollutants entering the building by controlling airflow in and out. Make sure windows and doors close properly and open them as little as possible. Add seals to the bottom of doors and the edges of windows to create a more controlled environment.

Keeping doors open can make your museum feel welcoming, but it increases the amount of pollutants entering the building. Create a porch or lobby with a separate set of external glass doors that can stay closed where possible.

Design considerations for new and refurbished buildings

• Keep the building free of leaks and draughts. Carry out a leak test to identify problems and control any ventilation the building needs.
• Place stores deep within the building structure to keep exposure to outdoor air as low as possible.
• Use internal lobbies as safe spaces where active pollutants can settle. Providing lockers for coats, bags, and umbrellas reduces pollutant levels.
• Use internal doors to control airflow between exhibition spaces.
• Fit HEPA high-performance particle filters on ventilation systems.
• Use a water spray system or activated carbon filters on any ventilation drawing in exterior air to filter out gaseous pollutants.
• Keep airspeed in ventilation systems slow to trap more pollutants.

Ventilation systems with good filters can be expensive to install and maintain and often need contractors. You can reduce costs by recycling filtered air and taking regular performance reports from the systems.

Preventive conservators can offer further advice for your museum. Talking to the planning authority can sometimes lead to relaxed ventilation requirements for public spaces in new or refurbished buildings.

Your museum is unlikely to be completely free of airborne pollutants, especially if your budget or building consents limit what you can do. The key is stopping pollutants from reaching objects.

Dust is the most common way pollutants settle on and react with collections. Exhibitions can also bring in chemicals over short or longer periods.

Protect your collections by:

• Placing objects in an enclosure such as a cupboard, storage box, or display case
• Choosing enclosure materials carefully and using conservation-grade materials that do not give off pollutants themselves
• Using cleaning materials that are chemically inert and do not create static
• Avoiding open display and storage
• Using dust covers for items that cannot be stored in enclosures, and cleaning them regularly using environmentally friendly detergents
• Allowing any exhibition materials to off-gas (release chemical vapours) before they come into contact with objects
• Avoiding high-VOC paints near objects
• Using pollutant absorbers such as buffered, acid-free paper and board. Do not use buffered material near items affected by buffering agents, such as photographic materials and textiles
• Using activated carbon products such as carbon cloths in showcases and storage boxes. These absorb pollutants well but can become saturated over time
• Choosing conservation storage materials designed to protect collections. More information is available in our advice guide on choosing display cases
• Maintaining a regular housekeeping programme to prevent the build-up of dust, dirt, and fibres
• Using air cleaners, which attach filters to a fan that draws in room air. Some models remove both particles and gaseous pollution. Air cleaners are particularly useful in listed buildings where physical changes are limited
• Monitoring and controlling temperature and relative humidity to slow reactions. Aim for cooler, less humid conditions
• Blocking or reducing air ventilation in storage rooms, which do not need the same standards as public areas, especially where external vents lead onto a busy street

For more information on collections care, see our other advice guides.