Air pollutants are harmful to objects on display. This has long been recognised and even Homer described the damage to metals indoors when Odysseus returned to his home on Ithaca and found his weapons damaged:
(My son! we must remove and safe dispose
All these my well-forged implements of war;
And should the suitors, missing them, enquire
Where are they? thou shalt answer smoothly thus--
I have convey'd them from the reach of smoke,
For they appear no more the same which erst
Ulysses, going hence to Ilium, left,
So smirch'd and sullied by the breath of fire."
Translator: William Cowper
This classical example illustrates the impact that combustion derived pollutants have on indoor objects. There has been a long history of concern over these indoors. In coal burning London Michel Faraday helped advised on the problems that paintings in the new National Gallery might face through exposure to sulfur dioxide and coal smoke in the outdoor air.
However, increasingly and especially as concentrations of the classical combustion derived pollutants decline it may not be these that are most critical for objects on display. Furthermore, the pollutants that are harmful to health are not necessarily the same as those that cause damage to art objects. Many indoor pollutants outgas from display materials or the objects themselves. Many objects are typically stored in cases, and some paintings in well-constructed frames, despite the contemporary interest in open display. Thus outgassing can be important as sources of exposure within tightly sealed museums cases.
Of these indoor pollutants, carbonyls (carboxylic acids, aldehydes and carbonyl sulfide are possibly the most critical in damaging objects although alongside these one would have to place particles, hydrogen sulfide (although not especially common), hydrogen peroxide and some larger organic molecules (e.g. Volent and Baer [1]). In addition to causing damage they can sometimes pose health risks especially the residual pesticides [2].
Formic acid and acetic acid are important compounds in the indoor environment because of the potential to degrade calcareous materials (shells, eggs, tiles and geological specimens). In the case of shells the process has long been known and is referred to as Byne's disease. Additionally acetic acid is relevant in damage to paper. The remodelling of Bankside Power Station by Herzog and de Meuron to create the Tate Modern led to considerable criticism in the media. Some regarded the interior with "... raw oak floors, slightly soiled from use" as inspired [3], but there was an outcry that a warning from the Tate's flooring contractor's over the dangers of using untreated oak had been ignored. Amid denials from the Gallery, conservators agreed that "that the floors could create acidic oak dust, one of the most corrosive forms of indoor pollution, and possibly damage some metals and ceramics" [4].
Formaldehyde has long been of concern because of its potential to cause damage as emphasised in the early booklet by Hatchfield and Carpenter [5], although as discussed by Raychaudhuri and Brimblecombe [6] it is possible that the actions of formaldehyde came about largely from its oxidation to formic acid on the surface of metals. A large amount of work has been done by the building industry concerning the emission of formaldehyde from furnishings, fabrics, foams and glues because of the regulatory pressures to keep the formaldehyde content of constructional and decorative materials at a low level [7] given fears that it may be a carcinogen. While not primarily aimed at museums these have helped lower emissions
Another important carbonyl compound is carbonyl sulfide [8]. Hydrogen sulfide has long been of concern in the museum environment because of its ability to tarnish metals, but it is possible that the sulfide emitted from furnishings is carbonyl sulfide, as shown in the case of wool where at ambient temperatures it is carbonyl sulfide rather than hydrogen sulfide that is emitted [9]. It is this that explains the ability of wool to tarnish metals such as copper and silver. Other materials containing sulfur, such as rubber are also important sources of carbonyl sulphide.
The changing climate in the 20th century has been much discussed. However, the changes that are likely go well beyond the temperature increases that features so widely in the media and in public perception. Pressures to reduce the amount of energy used in buildings has not only meant a greater level of insulation (hence the potential increase of emissions of indoor pollutants from novel materials), but also lowered air exchange rates. Lower exchange rates allow pollutants to accumulate indoors [10]. New materials are also in use indoors and within museum cases, but in the heritage sector increasing care is taken in the choice of display materials through the use of the Oddy test [11]. Indoors temperatures are likely to increase, perhaps in some environments due to a greater demand for comfort heating [12], although this may be countered by regulations over energy efficiency. In historic properties without active climate control temperatures are likely to rise because of the exchange of outside air with interiors [13, 14].
The carbonyls relevant to the heritage environments are likely to respond in various ways to the changing temperature.
Carboxylic acids
Changing thermohygrometric climate is likely to affect the carboxylic acid loss from wood indoors. In the simplest sense this might be related to their increasing vapour pressure with temperature, but to fully account for this it would be important to take the water content, pH and the activity coefficient into account. However, these are likely to be second order effects on the vapour pressure in unheated historic interiors.
Formaldehyde
Changing temperatures are also likely to affect aldehyde concentrations. Xiong and Zhang [15] have examined the dependence of formaldehyde concentration on time in the static chamber measurements between 25°C and 50°C. These experiments suggest that the flux rate and extent of loss of formaldehyde from building materials increases with temperature. However, the damage caused by formaldehyde is not necessarily dependent on its concentration, as much as its ability to be oxidized to formic acid [6]. It has been argued that increased concentrations of volatile organic compounds in outdoor air and longer sunlight hours might enhance ozone concentrations [16]. Were this ozone to be advected indoors it might lead to a more rapid production of formic acid from formaldehyde at metal surfaces.
Carbonyl sulfide
The release of carbonyl sulfide from wool is strongly dependent on temperature, probably because higher temperatures encourage degradation of cystine. Experiments with a range of woolen yarns suggest that changes between ambient (~20°C) and 50°C cause one to two orders of magnitude increase in the rate of loss of carbonyl sulfide [9]. Thus, increases in room temperature would have the potential to enhance indoor concentrations.