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Identifying and reducing air pollution

Overview

Creating the right environment in a museum presents two challenges. Firstly, you need to make it welcoming, healthy and comfortable for all your visitors. The second, larger challenge is creating a climate that protects collections from the risks of airborne pollution.

Museum designers and conservators have to work out a way that allows fresh air into museums - the law requires it and people need it - while still protecting vulnerable and unstable items. This is particularly challenging in towns and built up areas where the outdoor air carries car fumes and pollutants from construction sites and industrial produce. Visitors themselves carry pollutants, from the fibres of their clothes to loose skin particles.

This guide will provide a plan for combatting all of these pollutants coming in from outdoors. For internal pollution, read our guide on storage and display materials.

Gaseous pollutants

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 is such that it 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.

Acidic substances

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 are naturally acidic they are 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

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 pollutants

Particulate pollution can be big and abrasive, like the dust which will settle on anything if the air is still, 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, in turn, 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.

Reducing pollution damage

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

Monitoring

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

That said, museum staff can do a lot of the work themselves when it comes to monitoring museum environments.

Urban and industrial areas, or locations downwind of such places, are likely to suffer the most from externally generated pollution. However, even in rural areas cars can produce significant amounts of sulphur dioxide. Marine locations come with the added risk of salt in the air.

Prevention

For museums and galleries, prevention is always better than a cure.

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 better create a controlled interior environment.

Many museums like to keep their doors open to appear welcoming, but this can let in far too much pollution. If possible, create a porch or lobby with a separate set of external glass doors that can be kept closed while still appearing welcoming.

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. Save money by recirculating filtered air and taking regular performance reports of the systems. Ultimately the cost is worth it as it can protect your collections from requiring expensive conservation work.

If in doubt, advice should be sought from Preventive Conservators with experience of managing ventilation systems, and/or from Mechanical and Electrical Engineers with an understanding of the needs of museums with regard to air quality. 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.

Protecting individual items

In many cases it is impossible to fully stop airborne pollutants entering your museum from outside, 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 do not 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. Do not 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 room 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 stop up air ventilation in storage rooms, which do not require the same standards as public areas.

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

Further information and advice

Museums Galleries Scotland are here to help with comprehensive guides to collections care and a contact page for any further questions.

We also recommend the advice given by the Collections Trust and these books for more information:

  • The National Trust Manual of Housekeeping (Butterworth-Heinemann, The National Trust 2006 (Revised 2011), ISBN 978-1907892189)
  • Pollutants in the Museum Environment (Hatchfield, P. Archetype, 2002, ISBN 1873132964)