The effect of the indoor environment on museum objects has received an ever increasing awareness from museum and conservation staff during the 20th century. Effects caused by variations in the relative humidity of air have been observed and described for at least the last one hundred years, and so has the effect of temperature, and light exposure. But pollutants in the indoor museum environment receive much less attention. This is a bit odd, as the deterioration such pollutants causes can be just as destroying for a museum object as e.g. exposure to high light levels.
One reason to this could be that the effect of indoor air pollutants is not always obvious. Some deterioration types are easily recognized, like corrosion. But other decay processes are more hidden and harder to detect, such as loss of fiber strength in a material. And as a pollutant rarely is the only factor in a deterioration process, but interacts with relative humidity, temperature, and even other pollution compounds, the situation is rather complex.
Indoor air pollution is not a uniform problem set, as there are literally hundreds of compounds in average indoor air generally considered as being "pollutants". These compounds origins from just as many sources, and while a compound may be very aggressive towards one type of material it may be harmless toward other materials.
Much of today's terminology and the approaches towards fighting indoor air pollutants have been adapted from the human health and comfort field of science. While the technology from that field, such as air measuring methods, will be a useful tool, other approaches are not necessarily adaptable when dealing with the "health" of museum objects instead of humans. In opposition to people, museum objects are intended to last for centuries or even millennia. And opposite the human body, which, to a certain extent, will heal again if exposed to small doses of poisonous substances, materials in an object will accumulate deterioration from any attack, slowly decaying more and more. Therefore even small exposures to pollutants will have an effect in the large perspective, a problem, which, in principle is not much different from that of accumulated light fading of dyes.
Ammunition sample from the Danish Royal Arsenal Museum. Cartridges with lead bullets had evolved white corrosion on the lead. The ammunition was mounted on cardboard and had been placed in a wooden enclosure for many years. Photo: Roberto Fortuna, National Museum of Denmark
Museums pose special indoor air quality situations
In museums we tend to store objects in airtight and confined boxes like display cases or storage containers. If such a case is made of a pollution emitting material, the pollutants will be released and kept within the case volume together with the museum objects. An example of this could be a display cabinet made of oak wood, which is known to release formic and acetic acid vapors. The main factors which controls the concentration of compounds in the air space within this cabinet are the volume of the air, the air exchange rate of the cabinet, the surface area of the wood, and the rate of which the pollutant is released from the wood ("emission rate"). Generally, cases and cabinets have a high inner surface-to-volume ratio, why the total emission from the wood will have a large effect on the air quality. Furthermore, if the emission rate in itself is high (oak releases much acetic acid) and if the cabinet is airtight, then the carboxylic acid concentration inside such a cabinet can be extremely high.
In this context, most indoor air pollution problems seems to be found in such smaller, confined spaces, like cases, boxes, and crates. In open galleries or storage rooms these problems are less common, except when large surfaces acts as pollution sources, like newly painted walls or new floorings.
Indoor air quality's effect on metals
One of the most well known examples of air pollution attacks in museums is that of lead corroding because of exposure to carboxylic acid vapours (fig. 1). Already in the 18th century observations were made of lead roofs corroding where in contact with oak planks1. Lead objects like 'beggars badges' and stained glass window cames2, or coins3, has been reported to corrode heavily if stored in wooden surroundings. Corrosion initiated by formic and acetic acid vapours will result in lead formate and basic lead acetate respectively.
Also bronze artifacts have shown to develop blue corrosion products if exposed to a carboxylic acid containing atmosphere. In some instances such a 'sky blue' corrosion product was found to be hydrated sodium acetate, where the sodium could be residues of former conservation treatments4,5.
Silver objects are known to tarnish if left unprotected, this black tarnish is caused by sulphur compounds which may origin from both outdoors and indoors. Sources of the latter can e.g. be sulphur-containing materials like wool or rubber found in carpets.
'Modern' metals like aluminium, zinc, and magnesium has also shown to be sensitive to attack from pollutants originating from common construction materials, like wood, paints, and adhesives6.
Another 'classic' case history is of calcareous objects such as limestone, or sea shells, which evolves efflorescence on their surface in a carboxylic acid containing atmosphere (fig. 2). It has been observed for over 100 years that collections of sea shells stored in oak cabinets could develop a white powdery surface efflorescence. One of the first reports of this phenomenon was of L. St. Byne7 who in 1899 described shell corrosion on specimens from the National Collection of South Kensington in England. Byne thought this to be caused by remains of the dead Mollusca inside the shells, as well as bacteria growth. He suspected that this kind of deterioration could spread like a disease, and to this day the phenomenon is still commonly called "Bynes Disease" even though this name was based on much erroneous information.
Efflorescence has been found on other calcareous materials. Birds egg shells, limestone reliefs, terra cotta and clay potsherds, and Cuneiform tablets has all been reported to develop long needle-shaped crystals on the surface during storage in new wooden cabinets8,9,10. Analysis has determined these salts to be various forms of hydrated calcium acetate combined with chloride and nitrate.
Mollusca shells from a private Danish collection. White efflorescence was flaking of the shells, and found in the bottom of all the oak-wood drawers. The efflorscence is very likely caused by acetic acid given of by the oak wood. Photo: Morten Ryhl-Svendsen, National Museum of Denmark
The presence of oxidizing gases in air may cause a type of deterioration on photographic image silver which is commonly known as "micro blemishes" or "red spots" 11. This type of damage gives a reddish hue to the middle tone areas of the image, and may even cause an overall image bleaching out. These tiny red spots are seen on photographs in direct contact with low quality paper used for sleeves or envelopes, but the problem can also be airborne. Fresh paint is a major source of peroxides, why spotting on prints exhibited in newly painted galleries is a common kind of damage12,13. Even peroxides given of from wood-fibre notice boards are capable to bleach out or to discolour photographs pinned to the board (fig. 3).
The effect of peroxides, given of by a wood-fiber noticeboard, on a contemporary Black-and-White resin-coated photographic print. Of the two test photographs shown in the figure, the one to the right has been pinned to a noticeboard for half a year, the one to the right has been kept as a reference (stored in an acid free sleeve in an inert environment). Middletones in the noticeboard photo has changed to a yellowish/reddish hue, caused by thousands of microscopic spots due to oxidation of the image silver.
Test-photos produced by: John Lee, National Museum of Denmark
Finally, a number of materials will, during their own deterioration processes, emit corrosive vapours, which will then either re-attack the material or attack other near-by objects. This is especially a problem for certain plastics, and is a well known problem in archives. Examples of this are cellulose acetate, which gives of acetic acid as a break down product, or cellulose nitrate, which gives of nitrogen oxides. Both plastics has been used extensively in the manufacture of photographic films and both the instability of the material as well as the deterioration products which is emitted are big problems in archives (fig 4). Also collodion glass plate negatives are known to emit nitrogen oxides, and it is in general recommended that these unstable types of photographic materials must be archived separately from other photographs in order not to harm them. Both types of plastics has also been used for producing other kinds of objects, such as buttons, combs, spectacle frames, etc., and are very common materials in contemporary museum collections14.
A special case of indoor air pollution is of objects, which has been treated with pesticides or other conservation substances against biological attacks. Just recently this author has seen an example of ethnographical museum objects, which in the past were treated with, among other substances, DDT. Today, the pesticides are re-entering the surrounding air, released from the object, and are then condensing out on new surfaces. In this instance the display case in which the objects are exhibited was completely covered on the inside with crystals containing pesticides. While the objects may be well preserved, this poses - needless to say - a serious health risk for museum staff.
A reel of motion picture film on cellulose nitrate base. The film base has compleately melted into one solid lump during its decomposition. This has caused a heavy release of nitrogene oxides to the sorrounding environment which has attacked the cardboard box the film was stored in and compleately destroyed it. Other nearby objects would also be attacked by the corrosive vapours.
Photo: Morten Ryhl-Svendsen, National Museum of Denmark
Monitoring the museum environment
In general, two approaches are used for monitoring and control of the indoor air quality in museums: measuring the air quality in galleries and storage areas, and testing of materials before use near museum objects.
Since the 1960s film strips with a colloidal silver grain emulsion (AGFA Gevaeret Gelbfilm) has been used in photographic archives for monitoring the concentration of oxidizing gases. The film strips will fade if oxidized, and monitoring is performed by regularly densitometric measurements of the film strips. This method is still in use today15,16.
The corrosivity of air can, in its most simple form, be monitored by placing sample metals in the environment in question, e.g. inside a display case. Small coupons of lead will, if they corrode, inform that carbonyl compounds are present. Likewise silver coupons will, if they tarnish, inform of the presence of sulphuric compounds in the air.
For making quantitative measurements of specific pollutants more sophisticated methods has been developed. Basically two approaches exist: passive and active sampling. The principle of pollution sampling is to allow air to pass through a sampling media, which specifically reacts with the target pollutant, e.g. potassium hydroxide for sampling carboxylic acids, or dinitrophenyl hydrazine for sampling aldehydes. Analysis is then carried out by liquid or gas chromatography. Passive samplers are for easy handling packed in small badges or tubes, which can be posted back and forth from the analytical laboratory to the measuring point by normal mail. They are small and user friendly, but requires long exposure times, up to two weeks, as the sampling is done by allowing air to diffuse slowly into the sampler17,18. A faster alternative to this is active sampling, where the air is dragged through the sampling media by a pump. Active sampling is a more sensitive method than passive sampling, but needs also more equipment and a experienced operator. As both of these techniques demands a line of equipment for analysis not commonly found in conservation workshops, it's advisable for the average museum to buy this expert-help elsewhere. One institution, which has specialized in pollution sampling in museum environments, is The Nederlands Institute of Cultural Heritage.
Other approaches for monitoring air pollutants (and other climate factors) effect on museum objects exists, e.g. sensors of glass19, colour-changing pH-test papers20, and tempera paint based dosimeters21.
There is today a fast development in technology in the direction of gaining more and more sensitive methods for air quality analysis. Traditional analytical methods like gas chromatography are constantly developed, being able to detect compounds in lower and lower quantities. But a new generation of instrument, the 'electronic nose', is a promising heir. Based on advanced chip technology, an array of chemical sensors is mimicking the olfactory receptors of the human nose. Today electronic noses are already used in food industry, for detection of narcotics and explosives, and a number of similar tasks.
In the museum and conservation world, a handful of quite simple material test methods have been suggested for evaluation of construction materials for display cases and the like. For measuring the release of volatile pollutants like aldehydes or acids, tests exists where a material sample should be enclosed in a flask with a reagent or a monitor, which then will react upon the presence of pollutants. Examples are the 'chromotropic acid test', where a solution of chromotropic acid and sulphuric acid will turn blue if aldehydes are released from the material sample22, or the 'iodide-iodate test', where a solution of potassium iodide and potassium iodate will turn blue on the presence of volatile acids22.
A popular test method is the accelerated corrosion test ('the Oddy test') where the monitors are small coupons of lead, copper, and silver. Enclosed in separate test tubes, each with a material sample and some water for maintaining a high relative humidity, the coupons are exposed to the possible emission from the material sample for 28 days at 60°C. The material is then evaluated on the formation of corrosion is may cause on the coupons, compared to a 'blind test' coupon22.
The above mentioned test methods are good guidelines, but gives only indications of which types of compounds a material may release. For a detailed identification of the pollutants and for a quantification of the emission, climate chamber investigations are suitable methods. Emitted vapours can be collected on absorbent samplers like activated charcoal pellets, and analysed by chromatography. These are standardized methods used e.g. in investigations of construction materials effect on human comfort. Depending on the chosen sampler and analytical method, the screening can aim towards one single pollutant, or the total mass of released volatile organic compounds. As for the pollution sampling techniques described above, these test methods must be carried out at specialized laboratories.
Commercial passive sampler for formaldehyde
How to deal with air pollution indoors
Avoiding polluting sources is the key issue here. It is always better to avoid air pollution in the first place than to be forced to make all sorts of mitigating arrangements later - with the possibility of losing objects in the meantime due to deterioration, or having to take on costly conservation treatments. Therefore a careful evaluation of all materials intended for storage or display of museum objects should always be carried out before use. This evaluation should be on ground of knowledge of the museum objects composition, and the possible compounds one can expect the inquisitive construction material to give off. This can be backed up by tests like the accelerated corrosion test, or for more detailed analysis of the pollution emission by a chromatographic analysis. The latter involves quite advanced methods and equipment, which only specialized laboratories will be able to offer. But literature offers a number of construction material guidelines with the basic do's and don'ts in regard to museum environment, and by following these one should be well prepared23,24,25.
In situations where polluting construction materials can't be completely avoided, problems can be minimized by a number of mitigative measures. The concentration of pollutants in air can be kept low by diluting, e.g. by ventilation of display cases. Ventilation may cause other problems like dust entering the case or fluctuations of the case climate. However, these problems are mostly a question of design, and besides this does exactly make the point that it would have been better to avoid the pollution source in the first place. Also the use of barriers between the pollution source and the museum objects is one way of control, like applying foils or coatings to wooden boards in order to stop emission26. Such coatings can be quite effective, but care should be taken not to puncture the coats. Also one should consider the ageing effect on a coating, which may become more permeable over time.
Air filters like the absorbent materials activated charcoal or molecular sieves can be used both in large scale for cleaning the air stream of a complete building ventilation system, or for more local applications like cleaning the inside air of a cabinet. Most often absorbents in small cases is used passively, simply by hiding a tray of activated charcoal somewhere in the case. Again, design is here really important as the charcoal in it selves doesn't prevent pollutants to attack the museum objects if they are positioned in the path between the pollution source and the absorbent. And actually there exist almost no data to evidence that simply placing a tray of absorbent in a display case ever improved the conditions for the museum objects.
To sum up, the most effective approach to avoid corrosive vapours in the near-environment around museum objects is not to use polluting construction materials at all. For example; if carboxylic acids poses a threat to the objects, wood products should not be used, end of story! In general care should be taken with all newly applied "wet" products like paints, sealants and adhesives. And a large range of rubber products will emit sulphur compounds. Completely inert materials include metal; glass; "baked" paints; some types of cardboard, polymer foam or aluminium cell boards; ceramics and terra cotta.
Wood and wooden boards are always the big problem in exhibition designs; being probably the most common house construction material it is every workman's first choice. Furthermore these types of materials are cheap, and easy to work into shape. It is complicated to find a good substitute, this is really the core problem in the majority of all museum interior designs. Here is a real challenge for museum exhibition architects!
Thresholds for air pollution exposure
A much debated issue is if it is possible to define thresholds of museum objects' exposure to air pollutants, and if; what they should be. One can argue that, as pollution initiated deterioration can be accumulative, only a zero pollution level is safe. However, this will in practice show to be an almost impossible level of air purity to reach for the average museum, and the argument is debatable; with the very same argument one should ban any light at all for museum exhibits!
Actually, there may be low pollution concentration levels where material deterioration reactions cannot happen, relying on kinetics effects and thermodynamics27. But only little work on such thresholds has been done, especially with regard to museum object preservation
The atmospheric background level of air pollutants in a clean outdoor environment has been suggested as guideline for acceptable indoor air pollution levels28. From this philosophy one can argue that the museum building and its environment does not add any un-wanted compounds to the indoor climate. This kind of threshold is political based rather than based on research into material deterioration patterns, however, the average natural background levels for most pollutants are often low that a good preventive environment actually is provided29.
Much research is still needed in order to establish safe air pollution limits with regard to cultural heritage objects. It may be that the most realistic approach is to agree on pollution levels, which doesn't causes more than an 'acceptable' degree of deterioration over a certain time span, e.g. 100 years. A few materials has been investigated in that extent, examples are calcareous shells30, pure lead31, and cellulose32. However, the amount of data available is still low, and some of it only provisional laboratory results.
The Indoor Air Pollution Working Group
Pollution thresholds is one of the topics that several work groups around the world are debating. One such group is the international Indoor Air Pollution (IAP) Working Group. This group has during the last couple of years held annual meetings, with presentations of research from group members, and with discussion sessions on air pollution subjects. Abstracts from all meetings are found on-line at the working group's homepage33.
In the years to come defining pollution thresholds towards common materials in museum collections will be a major task for this working group and others. Furthermore will the development of more refined material test methods be primary importance. But just as important is it to raise the awareness on the consequences of using the wrong construction materials in the museum environment, and to develop a general understanding of indoor air quality problems between museum staff.
Notes and references:
1: Watson, R. (1800): "Essay X: Of Red and White Lead". In: Chemical Essays, vol. III (Seventh Edition), London, pp. 337-376.
2: Tennent, N.H. & Cannon, L. (1993): "The corrosion of lead artifacts in wooden storage cabinets", Scottish Society for Conservation & Restoration (SSCR) Journal, 4,1, pp. 8-11.
3: Gottlieb, B.; Jakobsen, T. & Jensen, J. S. (1993): "Triste blymønter fra Trankebar". In: Nationalmuseets Arbejdsmark 1993, National Museum of Denmark, Copenhagen, pp. 112-123. (In Danish, with English abstract).
4: Tennent, N.H. & Baird, T. (1992): "The identification of acetate efflorescence on bronze antiquities stored in wooden cabinets", The Conservator, 16, pp. 39-47.
5: Thickett, D. & Odlyha, M. (2000): "Note on the identification of an unusual pale blue corrosion product from Egyptian copper alloy artifacts". Studies in Conservation, 45,1, pp. 63-67.
6: Green, L. R. & Thickett, D. (1993): "Modern Metals in Museum Collections". In: Saving the Twentieth Century: The Conservation of Modern Materials, (Proceedings: 1991), Canadian Conservation Institute, Ottawa, pp. 261-272.
7: Byne, L.St.G. (1899): "The Corrosion of Shells in Cabinets". Journal of Conchology, 9,6, pp 172-178.
8: Agnew, N. (1981): "The corrosion of egg shells by acetic acid vapour", Australian Institute for the Conservation of Cultural Material (ICCM) Bulletin, 7,4, pp. 3-9.
9: FitzHugh, E.W. & Gettens, R.J. (1971): "Calclacite and other efflorescent salts on objects stored in wooden museum cases". In: Science and Archaeology. The MIT Press, Cambridge MA., pp. 91-102.
10: Gibson, L.T.; Cooksey, B.G.; Littlejohn, D. & Tennent, N.H. (1997): "Investigation of the composition of a unique efflorescence on calcareous museum artifacts", Analytica Chimica Acta, 337, pp. 253-264.
11: McCrady, E. (1984): "The History of Microfilm Blemishes". Restaurator, 6, pp. 191-204.
12: Feldman, L.H. (1981): "Discoloration of Black-and-White Photographic Prints". Journal of Applied Photographic Engineering, 7,1, pp 1-9.
13: Ryhl-Svendsen, M. (1999): "Pollution in the photographic archive - a practical approach to the problem". In: Preprints, 9th IADA Congress 1999, The Royal Academy of Fine Arts, Copenhagen.
On-line version: http://iaq.dk/papers/iada1999.htm
14: Quye, A. & Williamson, C. (eds.) (1999): Plastics, Collecting and Conserving, NMS Publishing, Edinburgh, ISBN 1-901663-12-4, 152 pp.
15: Weyde, E. (1972): "A simple test to identify gases which destroy silver images". Photographic Science and Engineering, 15,4, pp. 283-286.
Löbach, W. (1999): "Kollodiale Silbergelb-Filteremulsionen als Schadstoffindikatoren für Raumluft und den PAT - Test (Teil I)", Rundbrief Fotografie, 21, pp. 12-14.
Löbach, W. (1999): "Kollodiale Silbergelb-Filteremulsionen als Schadstoffindikatoren für Raumluft und den PAT - Test (Teil II)", Rundbrief Fotografie, 22, pp. 11-13.
17: Gibson, L.T.; Cooksey, B.G.; Littlejohn, D. & Tennent, N.H. (1997): "A diffusion tube sampler for the determination of acetic acid and formic acid vapours in museum cabinets", Analytica Chimica Acta, 341, pp. 11-19.
18: Shooter, D; Watts, S.F. & Hayes, A.J. (1995): "A passive sampler for hydrogene sulphide", Environmental Monitoring and Assessment, 38, pp. 11-23.
19: Martin, G. (1996): "Study points to use of glass sensors for museum monitoring". Museum Practice, 1,1, Museums Association, London, p. 6.
20: Sano, C. (2000): "Indoor Air Quality in Museums: Their Existing Levels, Desirable Conditions and Countermeasures", Journal of Japan Air Cleaning Association, 38,1, Tokyo. (In Japanese)
21: Bacci M.,Picollo M.,Porcinai S.,Radicati B. (2000): "Evaluation of the museum environmental risk by means of tempera-painted dosimeters", Thermochimica Acta, 29, pp. 25-34
22: Lee, L. R. & Thickett, D. (1996): Selection of Materials for the Storage or Display of Museum Objects. Occasional Paper no. 111, The British Museum, London, ISBN 0-86159-111-9, 54 pp.
23: Craddock, A. B. (1992): "Construction Materials for Storage and Exhibition". In: Conservation Concerns. A Guide for Collectors and Curators. Smithsonian Institution Press, Washington, ISBN 1-56098-174-1, pp. 23-28.
24: Tetreault, J. (1992): "Materiaux de construction, materiaux de destruction". In: La Conservation Preventive. 3e colleque international de l'ARAAFU, Association des Restaurateurs d'Art et d'Archeologie de Formation Universitaire, Paris, pp. 163-176. (In French)
25: Tetreault, J. (1994): "Display Materials: The Good, The Bad and The Ugly". In: Exhibitions and Conservation. Preprints of the Conference held at The Royal College of Physicans, Edinburg. SSCR, Edinburg, pp. 79-87.
26: Tetreault, J. (1999): Coatings for Display and Storage in Museums. Technical Bulletin 21, Canadian Conservation Institute, Ottawa, ISBN 0-662-27955-7, 46 pp.
27: Brimblecombe, P. (1994): "The Balance of Environmental Factors Attacking Artifacts", In: Durability and Change: The Science, Responsibility, and Cost of Sustaining Cultural Heritage, John Wiley & Sons Ltd, London, pp. 67-80.
28: Tetreault, J. (1999): "Standards for Levels of Indoor Pollutants in Museums". In: Indoor Air Pollution: Detection and Mitigation of Carbonyls. Presentation abstracts and additional notes: Glasgow 1998, ICN, pp. 15-17.
29: This may not be the case in certain local areas e.g. large cities where the outdoor pollution load is high.
30: Brokerhof, A.W. & van Bommel, M. (1996): "Deterioration of Calcareous Materials by Acetic Acid Vapour: A Model Study". In: 11th Triennial Meeting of the ICOM Committee for Conservation, Preprints: Edinburgh, James & James, pp. 769-775.
31: Tetreault, J.; Sirois, J. & Stamatopoulou, E. (1998): "Studies of Lead Corrosion in Acetic Acid Environments". Studies in Conservation, vol. 43, pp. 17-32.
32: Dupont, A.-L. & Tetreault, J. (2000): "Cellulose degradation in an acetic acid environment". Studies in Conservation, 45, 3, pp. 201-210.
33: IAP Homepage [Internet]: http://iaq.dk/iap.htm
For a general introduction to indoor air pollution in museums, these references are a good starting point:
- Blades, N; Oreszczyn, T; Bordass, B. & Cassar, M. (2000): Guidelines on Pollution Control in Museum Buildings, Museum Practice, 15 (supplement), London, ISBN 0-902102-81-8, 27 pp.
Brimblecombe, P. (1990): "The Composition of Museum Atmospheres", Atmospheric Environment, 24B, no. 1, pp. 1-8.
Craddock, A.B. (1992): "Construction Materials for Storage and Exhibition", In: Bachmann (ed.): Conservation Concerns. A Guide for Collectors and Curators, Smithsonian Institution Press, Washington, ISBN 1-56098-174-1, pp. 23-28.
Grzywacz, C. & Tennent, N. (1997): "The Threat of Organic Carbonyl Pollutants to Museum Collections", European Cultural Heritage Newsletter on Research, 10, European Commission - Environment and Climate Research Programme, Brussels, pp. 98-104.
Hatchfield, P. (2002): Pollutants in the Museum Environment -Practical Strategies for Problem Solving in Design, Exhibition and Storage, Archetype Publications, London, ISBN 1873132964
Lavedrine, B. (1997): "An Assessment of Pollution and its Effects on Photographic Collections", European Cultural Heritage Newsletter on Research, 10, European Commission - Environment and Climate Research Programme, Brussels, pp. 87-92.
Padfield, T; Erhardt, D. & Hopwood, W. (1982): "Trouble in Store", Science and Technology in the Service of Conservation. Preprints of the Contributions to the Washington Congress, 3-9 September 1982, IIC, pp. 24-27.
Tetreault, J. (1994): "Display Materials: The Good, The Bad and The Ugly". In: Sarge (ed.): Exhibitions and Conservation. Pre-prints of the Conference held at The Royal College of Physicans, Edinburg, The Scottish Society for Conservation & Restoration (SSCR), Edinburg, pp. 79-87.
Indoor Air Quality in Museums and Archives [Internet]: http://IAQ.dk