Molecular pollutants
Although there are natural sources of airborne contamination such as hot water springs and volcanoes, atmospheric molecular pollutants can predominantly be attributed to human activity such as power generation and transport. They are normally associated with high population density, e.g. cities.
In terms of damage to artefacts, molecular pollutants fall into two broad categories:
1. those with acidic chemical properties
2. those with oxidizing chemical properties
Acids cause corrosive damage 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 mainly cause damage in organic materials, with effects that 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 artefact 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 tyres 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 of rooms and display cabinets. Here, surface deposition will lead to soiling or discolouration.
Many particles, particularly those arising from combustion processes, are oily or sooty in nature and have acidic properties. These particles are particularly harmful 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 artefacts such as paintings and textile fibres.
The solution for particulate pollution
For effective preservation, artefacts 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 mandatory to use a final stage filter in the class F9, and to ensure that the filter provides high initial efficiency as well as 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.
