Identifying and reducing air pollution

Power stations, factories, vehicles, and heating systems can emit harmful, polluting chemicals that make their way into museums and galleries. 

Sulphur dioxide 

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

Sulphur can also combine with other gases, creating common compounds such as hydrogen sulphide, carbonyl sulphide and carbon disulphide. Many fuels now have reduced sulphur content to help reduce sulphur dioxide pollution and its consequences. 

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 also reacts in the presence of sunlight to form ozone (O3) and PAN (peroxyacyl), two pollutants that are known collectively as ‘photochemical smog’. Many vehicle exhaust systems now include a catalytic converter that splits the polluting nitrogen dioxide back into harmless nitrogen and oxygen. 

Ozone is also naturally present in the air at a height of 20-30 km, where the ozone layer protects life on earth from harmful short wave ultraviolet radiation. However, the increase in ozone at ground level  has become a threat to both people and objects. 

These gaseous pollutants can be divided into two main groups: those that are acidic and those that have an oxidising effect. 

Sulphuric and nitric acids react with several different kinds of material and cause permanent changes such as: 

• Limestone, marble, and other calcareous materials weaken, discolour, or dissolve. 
• Iron and other metals corrode. 
• Leather suffers from ‘red rot’, where it loses strength and flexibility due to hydrolysis of leather fibres. Eventually it becomes a powder. 
• Cotton, linen and viscose materials discolour and become weak and brittle. 
• Wool and silk are weakened, although as they’re naturally acidic, they’re also more resistant.
• Paper objects become yellowed and brittle. 
• Silver in photographic images yellows and fades. 
• Gelatin and the filmbase polymers of negatives degrade. 
• All kinds of dyes and pigments may fade.

Oxidising substances bring about oxidation reactions, which leads to the formation of free radicals and acids in many materials, especially organic materials. These can reduce the chain length in polymers, break double bonds in long-chain carbon molecules and cause new cross-links in the molecular structure of materials. The end result is that materials get significantly weaker, more brittle, and discoloured. 

Ozone and PAN are powerful and react to form new chemical compounds with almost any material they encounter. This could lead to further unpredictable reactions. Many organic materials contain anti-oxidant compounds which fight the oxidation reactions, but once the anti-oxidants are exhausted, deterioration processes can become more rapid. 

Oxidants can have the following effects on museum objects: 

• Dyes and pigments fade or are altered. 
• Rubbers and plastics crack.
• Textiles become brittle. 
• Paint binder resins are attacked. 
• Shell, marble and some geological specimens suffer surface alteration. 
• The tarnish rates of metals such as silver, copper, and iron are increased.

Particulate pollution can be big and abrasive, or it can be small enough to enter display cases through even the tiniest cracks. Large particles can scratch during cleaning, whereas small particles will only settle if they are trapped or held down by electrostatic attraction. 

Deterioration risks with particles 

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

Burning fuel in vehicle engines, power stations and heating systems produces black soot and tar particles which are then dispersed into the air and often cause soiling in urban museums. This is particularly noticeable in the build-up of grime around window frames. 

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

Museums located near the coast face the danger 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 can also support the growth of fungi and micro-organisms, even when the surrounding appears quite dry. 

Externally-generated particles such as soil grains, pollen, and fungal spores can combine with internally-generated particles such as textile fibres and skin fragments to form an attractive food source for insects and fungi which may then affect museum objects. 

Although the outside air can carry any number of harmful pollutants, museums can take steps to control and even eliminate many dangers posed by air pollution. Find out what the pollution levels are in your area from The Environmental Health Department of the Local Council to know what you’re up against. Then, devise a plan to protect and care for your collections. 

Monitoring 

The Scottish and UK Governments release information about pollutants in a variety of ways including publicly-accessible websites. If you think there’s a specific problem in your museum, an environmental scientist with experience in monitoring Indoor Air Pollution (IAP) should be invited to monitor the museum’s air for the suspected pollutants. 

Museum staff can also play a role. See our guide on monitoring museum environments. 

Prevention 

The ideal situation is to stop any external pollutants from entering the building by controlling the airflow in and out of the building. Ensure that windows and doors close properly and open them as infrequently as possible. Add seals to the bottom of doors and the edges of windows to create a more controlled environment. While keeping your doors open can make your museum feel more welcoming, this can increase the amount of pollutants entering your building. If possible, create a porch or lobby with a separate set of external glass doors that can be kept closed. 

Considerations for museum design 

• The building should be free of leaks and drafts. Leak-test a building to identify problems and control any ventilation required for the building. 
• Place stores deep within the structure of a building to ensure minimal levels of exposure to outdoor air. 
• Internal lobbies are safe places for the deposit of active pollutants. 
• Use internal doors to control the airflow between exhibits.
• Apply HEPA high-performance particle filters on ventilation systems. 
• Use a water spray system or activated carbon filters on any ventilation from exterior air to filter out gaseous pollutants. 
• Control the airspeed in ventilation systems, keeping it slow to trap more pollutants. 

Ventilation systems with adequate filters can be expensive to install and maintain, often requiring contractors. You can save money by recirculating filtered air and taking regular performance reports of the systems, although ventilation systems are more effective. 

Preventive Conservators can offer further advice for your museum. In some cases, negotiations with the Planning Authority can lead to a relaxation of the ventilation requirements for public spaces in new or refurbished buildings.

It’s unlikely that your museum will be totally free of airborne pollutants, especially given limitations in budgeting and building consents. The key is stopping these pollutants from reaching the objects. 

You can protect your collections in several different ways: 

• Place the objects in an enclosure such as cupboard, storage box or display case. 
• Choose the materials of enclosures carefully, using conservation-grade materials that don’t give off pollutants themselves.
• Avoid open display and storage. Use dust covers for items that cannot be stored in enclosures. 
• Use pollutant absorbers such as buffered, acid-free paper, and board. Don’t use buffered material near items that are affected by buffering agents such as photographic materials and textiles. 
• Activated carbon, found in products such as carbon cloths, is a powerful absorber of pollutants and can be used in showcases and storage boxes. Be aware that it can become saturated with pollutants. 
• Use conservation storage materials that have been designed to protect from materials. Read our advice guide for tips on which cases to use. 
• Maintain a regular housekeeping program to prevent the build-up of dust, dirt, and fibres.
• Use air cleaners, which consist of filters attached to a fan that draws in air from the room. Some models can remove both particulate and gaseous pollution. Room air cleaners are particularly useful in listed buildings where work that can be done to the building is limited. 
• Monitor and control both temperature and relative humidity to slow reactions. Aim for cooler, less humid conditions. 
• Minimise or block off air ventilation in storage rooms which don’t require the same standards as public areas. 

These actions help in protecting items from external pollution. For internal pollution, read our advice guide on storage and display materials.

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