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Inappropriate environmental conditions may cause irreversible damage to vulnerable artifacts. Critical parameters include; temperature, relative humidity, lighting, particulate pollution (dust), molecular (gaseous) pollutants and pests.

Molecular pollutants

Boy in MuseumAlthough there are natural sources of airborne contamination such as hot water springs and volcanoes, atmospheric molecular pollutants can be predominantly attributed to human activity such as power generation and transport. Normally they are associated with high population density, e.g. cities.

In terms of damage to artifacts, molecular pollutants fall into two broad categories

1. those with acidic chemical properties
2. those with oxidizing chemical properties

Acids cause damage by corrosion to materials such as metals and marble. Other materials susceptible to damage include leather, wool, silk, paper and photographic items.

The predominant oxidizing gases are ozone, nitric acid and other oxygen/nitrogen compounds. These gases will cause damage mainly in organic materials and the effects can be likened to premature ageing. Typical visual changes resulting from oxidation include; yellowing, brittleness, fading and tarnishing of metals.

Molecular pollutants are specified in terms of the concentration of individual chemicals or groups of chemicals.

The solution to molecular pollutants

Molecular filtration provides a cost effective method of controlling the harmful pollutants, thereby ensuring safe storage and display conditions. Various types of solution are available depending on the types and concentrations of gaseous pollutants, the type of artifact to be protected and the layout of the ventilation system.

Molecular filtration may be applied in either the fresh air make-up or recirculation air units. Solutions for make-up applications need to reflect the high external concentrations and one-pass operation. Solutions for recirculation applications reflect lower ambient concentrations and multi-pass operation.

Particulate pollutants

Particulate pollutants arise from multiple sources including combustion processes (industrial, power generation, vehicle exhausts, cigarette smoke), vehicle tires running on roads, building activity and human beings. Heavy particles with metallic content are abrasive and may settle on surfaces and cause scratching. Smaller particles may remain suspended and be transported by air movement to even the remotest corners in rooms and display cabinets. Here surface deposition will lead to soiling or discoloration.

Many particles, particularly those arising from combustion processes will be oily or sooty in nature and have acidic properties. These particles are particularly damaging since they are very sticky and can cause corrosion in many materials.

Particles arising from building works (concrete) have both alkaline and abrasive properties and are harmful to artifacts such as paintings, and textile fibres.

The solution for particulate pollution

For effective preservation, artifacts must be protected from small, aggressive particles . Such particles, which are often also acidic in nature originate from combustion processes and are in the sub-micron size range. Molecular filters must therefore be used in conjunction with high efficiency particle filters. According to European standard EN 779:2002, it is required to use a final stage filter in the class F9. It is also required to ensure that the filter provides a high initial efficiency and also a high efficiency throughout its lifetime. See “discharged efficiency” in the EN 779:2002 protocols. Note that the penetration of fine particles through an F9 class filter is less than half the penetration through an F7 class filters.


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