Case studies

Indoor environment and conservation in the Royal Museum of Fine Arts,

Antwerp, Belgium

Kristin Gysels a, Filip Delalieux a, Felix Deutsch a,1, René Van Grieken a,*, Dario Camuffo b,

Adriana Bernardi b, Giovanni Sturaro b, Hans-Jürgen Busse c, Monika Wieser c

a Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgiumb CNR-ISAC Corso Stati Uniti 4, 35127 Padova, Italy

c Institute of Bacteriology, Mycology and Hygiene, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210 Vienna, Austria

Received 20 June 2003; accepted 16 February 2004

Abstract

Indoor and outdoor atmospheres of the ‘Koninklijk Museum voor Schone Kunsten’ (KMSK, Royal Museum of Fine Arts) in Antwerp,

Belgium, were thoroughly characterised to determine the air quality inside the museum and the factors controlling it. During a winter and a

summer campaign aerosol particles, pollutant gases, bacteria and fungi were sampled and different indoors microclimatic parameters were

measured. The chemical composition of particulates suspended in indoor and outdoor air was analysed, both with reference to bulk aerosol

matter and to individual particles. Outdoor sources largely determined the composition of indoor aerosol. The main particle types identified in

winter were Ca-rich, Ca–Si and sea salt particles. In summer, S-rich particles were most abundant. Dry deposition was sampled in order to

determine the amount of particulate matter that could potentially deposit onto the works of art. The concentrations of NO2 and SO2 amounted

to 12 and 5–6 ppb, respectively, both in winter and in the summer. The microclimates inside the exhibition rooms were affected by poorly

balanced heating and air-conditioning, free-standing humidifiers, ventilating and lighting systems and the daily flux of visitors, which

produced rapid changes and marked thermo-hygrometric gradients. Based on these results, suggestions for the improvement of the heating and

air-conditioning system could be made. Microbial loads were higher in summer than in winter. However, the proportion of microorganisms

capable of degrading proteins or hydrolysing fats, and thus pernicious to works of art, was not significantly increased inside the museum.

© 2004 Elsevier SAS. All rights reserved.

Keywords: Aerosol particles; Gases; Microbiology; Microclimate; Conservation; Indoor air quality; Museum

1. Research aims

The conservation of works of art exhibited inside muse-

ums is influenced considerably by the indoor environment,

i.e. microclimate and air pollutants. Temperature and relative

humidity (RH) variations, metabolic actions and excretions

of microorganisms and gaseous and particulate pollution all

play an important role in the deterioration of works of art.

Daily temperature and RH cycles are induced by heating and

air-conditioning devices and cause mechanical stress. More-

over, crystallisation of salts in the micropores of the paintings

can cause further damage [1].

Atmospheric particles are potentially harmful because

they can cause significant soiling. Apart from the mere cos-

metic effect, dust deposited on works of art contains variable

amounts of moisture attracting compounds. Further damage

can be caused at the moist dust/surface interface by chemical

reactions with gases or by harmful compounds present in the

deposited particles. The following compounds can be consid-

ered threatening to the preservation of works of art. Organic

material, especially soot, can cause significant visual degra-

dation by soiling the surface [2] and it can constitute a

medium for SO2 adsorption [3]. CaSO4 can enhance these

effects by adsorption of soot [4]. (NH4)2SO4 and S-rich

material are threatening to the preservation of paintings,

because (NH4)2SO4 can induce bloom on varnish [5] and

oxidation of S-rich particles to H2SO4 can cause discolouring

of the paint (e.g. [3,6]). This process can be catalysed by

Fe-rich particles [7]. Therefore, a detailed characterisation of

the particulate and gaseous phases of the environment inside

museums can lead to improvements in the conservation of

works of art.

* Corresponding author.

E-mail address: rene.vangrieken@ua.ac.be (R. Van Grieken).1 Present address: VITO, Centre for Remote Sensing and Atmospheric

Processes, Boeretang 200, 2400 Mol, Belgium.

Journal of Cultural Heritage 5 (2004) 221–230

www.elsevier.com/locate/culher

© 2004 Elsevier SAS. All rights reserved.

doi:10.1016/j.culher.2004.02.002

In this study, the Koninklijk Museum voor Schone Kun-

sten (KMSK) in Antwerp, was chosen for a closer investiga-

tion of indoor particulate and gaseous pollution, microbiol-

ogy and microclimate. Indoor climate and thermo-

hygrometric parameters had to be determined for different

rooms, including the Rubens room and the rooms in which a

popular exhibition was held about the (baroque) painter An-

toon Van Dyck. Horizontal and vertical distributions of tem-

perature, relative and specific humidity (SH) were recorded.

Together with the results of the aerosol, gaseous pollution

and microbiology investigations, an integrated view on the

indoor environment could be obtained. The results should be

interpreted with reference to the conservation of works of art.

This research was performed in the framework of a Euro-

pean project that studied four museums, having different

climate and pollution conditions: The Correr Museum, Ven-

ice, Italy [8]; Kunsthistorisches Museum, Vienna, Austria

[9]; KMSK and Sainsbury Centre for Visual Arts, Norwich,

UK [10]. The first three museums are of traditional design,

housed in historic buildings; the last one has a modern de-

sign, with use of new materials (glass and metal).

2. Experimental

2.1. Methodologies

2.1.1. Aerosol sampling and analysis

The KMSK is located just south of the city centre of

Antwerp near major traffic axes and the river Scheldt. The

surroundings can be characterised as urban with a moderate

maritime climate. Antwerp is situated about 100 km away

from the North Sea, in a heavily industrialised area. The

museum is housed in a historic limestone building, con-

structed between 1873 and 1890. A heating, ventilation and

air-conditioning (HVAC) system is operational throughout

the building. All the rooms have a parquet floor and painted

textile walls.

During two campaigns, organised in February 1999 and

July/August 1999, time-resolved aerosol samples were taken

for single particle analysis using streakers. The indoor sam-

pling site was situated on the first floor in room K (see Fig. 1).

Restoration works were carried out during the winter sam-

pling campaign. During the summer campaign a special

exhibition about Van Dyck, which attracted a lot of visitors

was held on the ground floor of the museum.

Aerosol samples for single particle analysis were taken

indoor and outdoor both with May cascade impactors (theo-

retical cut-offs 20, 8, 4, 2, 1 and 0.5 µm) on Apiezon-coated

Nuclepore filters and without size-segregation on Nuclepore

filters. The samples were analysed automatically by electron

probe X-ray micro-analysis (EPMA; JEOL Superprobe 733).

The resulting data was subjected to Hierarchical Cluster

Analysis (HCA) in order to identify different particle types

present in the samples (e.g. aluminosilicates, sea salt, Ca-

and S-rich).

Total particulate number concentrations were determined

by means of an optical particle counter (based on light scat-

tering by aerosols). The combination of these data with the

relative abundances of the different particle types gives the

absolute abundances of these particle types.

In order to measure dry deposition, Apiezon-coated

Nuclepore filters were attached vertically to the museum

walls at 17 different locations, at a height of approximately

2 m for a period of 6 months. The samples were analysed by

energy dispersive X-ray fluorescence spectrometry

(EDXRF; Tracor 5000).

Time-resolved aerosol samples were collected by means

of a streaker sampler. The streaker consists of two rotating

stages: an impaction stage with a theoretical cut-off of 2.5 µm

and a filter stage consisting of a 0.4 µm pore-size Nuclepore

filter. Particles >10 µm dae (aerodynamic diameter) impact on

the pre-impactor stage. The samples were analysed by

particle-induced X-ray emission (PIXE) spectrometry. Data

about (outdoor) wind speed, wind direction, temperature and

rainfall, were obtained from the Belgian Royal Meteorologi-

cal Institute.

2.1.2. Microclimate

The (indoor) microclimate was also investigated during

the winter and the summer of 1999, particularly in the rooms

I and K on the first floor and in some of the rooms on the

ground floor (Antwerpen I and Genova), where the much-

frequented Van Dyck exhibition was held. The main thermo-

hygrometric parameters analysed (day and night) were air

temperature (T), SH and RH measured with automatic data

loggers and with manually operated psychrometers during

specific surveys in winter (9–13 February) and summer (15–

17 July). The hygrometric parameters were obtained by us-

ing a high resolution (0.1 °C), fast response (5 s) psychrom-

eter.

2.1.3. Gaseous pollutants

SO2 and NO2 concentrations were determined using pas-

sive diffusion tubes (Gradko International Ltd.). SO2-tubes

were analysed with ion chromatography (Dionex 4000i),

NO2-tubes with photometry (UVIKON 930). Three diffusion

tubes for either gas were placed at each of the 20 sampling

sites.

2.1.4. Microbiology

Fungal attack on works of art displayed in a museum

environment with elevated humidity has long been a problem

and some conservators believe that bacteria may also present

risks (e.g. [11,12]). Airborne bacteria were sampled onto

agar strips (using a Biotest Hycon RCS Plus air sampler) and

subsequently cultured in the laboratory to count the number

of colony forming units (cfu), and to characterise the strains.

Airborne microorganisms were collected in February and

July 1999, both inside and outside the museum. The same

methodology was used as previously in other museums

[8,10]. Different growth media were used; CasMM agar and

222 K. Gysels et al. / Journal of Cultural Heritage 5 (2004) 221–230

commercial total count agar (TCA) for bacteria, and com-

mercial yeast/mould agar for fungi. TCA especially supports

the growth of bacteria with high nutritional requirements

such as bacteria associated with humans, animals and plants.

In contrast CasMM agar favours the growth of environmental

strains (bacteria with high nutritional requirements grow at

significantly reduced rates). In order to determine some as-

pects of the hazardous potential of the airborne microorgan-

isms, additional measurements were carried out. Hydrolysis

of Tween 80 would demonstrate the presence of lipolytic

activity and thus the microorganisms’ potential to destroy oil

paintings. Hydrolysis of Casein would demonstrate the abil-

ity to degrade proteins and thus all works of art containing

proteins [13].

Fig. 1. Map of the ground (top) and first (bottom) floor of the Museum voor Schone Kunsten. H, dry deposition sampling site.

223K. Gysels et al. / Journal of Cultural Heritage 5 (2004) 221–230

2.2. Results

2.2.1. Chemical characterisation of aerosol particles

2.2.1.1. Bulk concentrations and dry deposition. Especially

for Na and Cl, but also for other elements, the average bulk

indoor concentrations were higher in winter than in summer.

Only the concentrations of soil-derived elements as Al, Si

and Ti were higher in summer (Table 1). This can be ex-

plained by the more effective resuspension of particles and

the larger number of visitors during summer, which repre-

sented an important source of indoor soil dust.

The highest contributions of all elements (except Ti and

Si) occurred during winter in the fine size range, which was

dominated by Na and Cl. Cl was also the main element in the

coarse fraction. During summer, the coarse fraction (>10 µm)

was much larger. Si, Cl, K, Ca, Ti, Cu and Zn mainly oc-

curred in this fraction. Na, S, Fe and Pb were predominantly

present in the fine size range (<2.5 µm), while Al was mainly

detected in the intermediate fraction (2.5–10 µm). This indi-

cates that in summer, the relative importance of resuspension

of coarse particles was larger than in winter.

Because most elements detected in the time-resolved

samples were assumed to have an outdoor origin, the fluctua-

tions in their abundances were compared with meteorologi-

cal data. The fine aerosol concentration inside the museum

appeared to be anti-correlated with the outdoor wind speed.

During periods with low wind speed, fine aerosol concentra-

tions could build up and decrease again due to increased

ventilation and wash-out processes. The positive correlation

between the contribution of coarse particles and wind speed

can be explained by the increased resuspension of these

particles [14]. Variations in the weather conditions (wind

speed and precipitation) were directly reflected in the indoor

aerosol concentration, which indicates the strong outdoor

influence.

For the fine size fraction elemental concentrations (espe-

cially S) seemed to be anti-correlated with wind speed, while

for the coarse fraction, a clear positive correlation with wind

speed could be observed (especially for Cl and Fe). This was

the case during both summer and winter campaigns. From

this observation it is clear that these elements have an out-

door origin. Also Ca exhibited a clear positive correlation

with wind speed in summer and thus originated from an

outdoor source. In winter, the construction and restoration

works provided a strong indoor Ca-source.

In the dry deposition the highest elemental concentrations

were found for S; concentrations of Ca were rather low. In

general, the dry deposition turned out to be very homoge-

neous throughout the entire investigated area of the museum

(Fig. 1). Results of the analyses performed on dry deposition,

which in view of conservation are very important, will be

discussed in Chapter 3.

2.2.1.2. Single particle analysis. During the winter cam-

paign large particles (size range ≥4 µm) were characterised

by high abundances of Ca-rich, Ca–Si and aluminosilicate

particles. The same particle types were identified in the

intermediate size range (2–4 µm), but sea salt gained impor-

tance as well. The particles <2 µm were completely domi-

nated by sea salt and aged sea salt. Concerning potentially

dangerous aerosols (cf. Section 1); Fe-rich particles were

present in every investigated size range, while low-Z and

S-rich particles were only identified in the smallest size

fraction.

During the summer campaign, however, aluminosilicate

and low-Z particles were the most abundant types in the size

range ≥2 µm. Na- and K-containing particles were present as

well. The small particles (<2 µm) contained significant

amounts of sulphur. Outdoor samples contained higher rela-

tive amounts of sea salt particles (especially in the size

fraction ≥4 µm), but these were still lower than in winter. The

Table 1

Bulk aerosol concentrations (µg/m3)

Element February 1999 August 1999

Na 0.61 ± 0.55 0.004 ± 0.009

Si 0.005 ± 0.004 0.008 ± 0.002

Al <DL 0.006 ± 0.001

S 0.27 ± 0.14 0.05 ± 0.03

Cl 0.54 ± 0.76 0.009 ± 0.003

K 0.10 ± 0.11 0.005 ± 0.001

Ca 0.13 ± 0.06 0.016 ± 0.002

Ti 0.0003 ± 0.0003 0.0008 ± 0.0005

Mn 0.002 ± 0.002 0.0003 ± 0.0004

Fe 0.24 ± 0.26 0.016 ± 0.008

Cu 0.002 ± 0.002 0.0003 ± 0.0001

Zn 0.021 ± 0.016 0.004 ± 0.001

Pb 0.015 ± 0.028 0.0011 ± 0.0008

0.001

0.01

0.1

1

10

100

1000

10000

100000

0 3 6 9 12 15 18 21 0

Hours

Dg

old/

Nd

mc/1(

3)

0,1 µm

0,3 µm

0,5 µm

1 µm

2 µm

5 µm

10 µm

Fig. 2. Suspended particle density distribution dN/dlogD versus time for

February 10th 1999.

224 K. Gysels et al. / Journal of Cultural Heritage 5 (2004) 221–230

fraction 1–4 µm was mainly characterised by S-rich, low-Z,

Na–S and Na-rich particles, while in the smallest size range

(0.5–1 µm), S-rich and K–S particles were predominant. The

outdoor influence appeared to be most significant in the

smallest size range.

Both for the summer and winter campaigns, the results of

the filter samples were in very good agreement with those of

the impactor samples.

2.2.1.3. Comparison indoor/outdoor. The outdoor environ-

ment appeared to have a very significant influence on the

atmosphere inside the KMSK, especially in summer. Abso-

lute particle number concentrations were calculated and

compared for the indoor and outdoor environments.

Most of the time absolute sea salt concentrations were

higher in outdoor air compared to indoor air. The concentra-

tions of Ca-rich and Ca–Si particles were found to be highest

indoors in winter, as a consequence of the restoration works.

In summer, however, indoor concentrations were comparable

to or even lower than outdoor concentrations.

The average indoor/outdoor (I/O) ratios were clearly high-

est during the winter campaign. Especially Ca–Si particles

exhibited very high I/O ratios in winter (10.7), and thus

originate from indoor sources. Ca-rich and CaSO4 particles

also showed higher indoor concentrations in winter. These

particles (similar to the Ca–Si particles) originate from the

construction works, although outdoor sources might be

present as well.

It can be concluded that, for both seasons, the outdoor

influence was significant, i.e. the indoor aerosol composition

depended largely on the outdoor composition. In summer,

indoor concentrations were about 40% lower compared to

the outdoor concentrations. In winter, an extra indoor source

of mainly Ca-containing particles was present. This led to

higher I/O ratios for the total aerosol number concentration

in winter (0.9 vs. 0.6 in summer).

2.2.2. Microclimate

2.2.2.1. Winter campaign. The very large room I (Rubens

room, first floor) with several communicating doors is

equipped with an air-conditioning system. Nevertheless, the

thermo-hygrometrical gradients often exhibited perturba-

tions generated within the room by the entrance of condi-

tioned air or air from the neighbouring rooms (Fig. 3). Colder

and more humid air always penetrated especially from the

adjacent rooms into room I. In addition, the construction

works carried out during the winter campaign enhanced the

flow of external air into the museum. The microclimate was

relatively constant throughout the whole room, except in the

vicinity of the doors, where local gradients were always

present. The most pronounced effects were situated around

the main entrance. Fluctuations in SH amounted 2 g/kg

within 3–4 m. RH changed according to the same pattern,

with changes reaching levels as low as 34%. In general, less

steep gradients between rooms would be recommendable.

Fig. 3. Horizontal cross section of the distribution of temperature (°C), RH (%) and SH (g/kg) in the Rubens room on February 11th 1999, 10:00 h.

225K. Gysels et al. / Journal of Cultural Heritage 5 (2004) 221–230

The construction works might have aggravated the extent of

these gradients.

In room K (Fig. 4) on the first floor, marked gradients of T

(up to 1 °C within a few meters), SH (up to 1.5–2 g/kg within

a few meters) and RH (up to 10–15% within a few meters)

were measured. Airflow was observed originating from the

Rubens room (with its very different microclimate) crossing

room K to reach room L. In this way a significant difference

in humidity content could build up between the air in the

centre and along the walls of room K. When the glass door

between rooms K and L was closed, exchange of air was

prevented and the situation improved. The average RH level

(45–55%) was acceptable, but fluctuations could reach up to

15% in the course of the morning and the afternoon.

In room T (Fig. 5) on the first floor, very strong gradients

of T (differences reaching up to 8–9 °C within 1–2 m) and

RH were generated when the radiators switched on. At first

sight, this perturbation seemed acceptable as it was confined

to the centre of the room, far from the paintings. However, the

hot radiators caused a convective air current in the room with

hot air rising in the centre and cold air descending along the

walls, which led to increased inertial deposition of suspended

particles on to the paintings. In addition, discontinuous op-

eration of the radiators led to RH fluctuations that amounted

to 25–30%.

In general, several perturbations in microclimatic homo-

geneity were present on the first floor. Although the museum

is equipped with a central heating system that supplies hot

and moistened air to every room temperature fluctuations

amounting to 2–3 °C between neighbouring rooms could be

observed. The SH was not well balanced either; it was higher

in the outermost rooms, in particular in the rooms G, M, L, R,

S and T. As a consequence of the uneven distributions of

temperature and SH, RH fluctuations between rooms

amounted to 10–15% and RH levels in the inmost rooms

19

20

21

22

23

24

25

0 6 12 18 0 6 12 18 0

Time

erut

arep

meT

)C°(

30

40

50

60

70

ytidi

mu

H evit

aleR

)%(

T(0.1m) T(1m) T(2m)

T(4m) RH(1m) RH(4m)

Fig. 4. Profiles of vertical temperature (°C) and RH (%) on February 10th

and 11th 1999 in room K.

Fig. 5. Horizontal cross section of the distribution of temperature (°C), RH (%) and SH (g/kg) in room T on February 13th 1999, 10:30 h.

226 K. Gysels et al. / Journal of Cultural Heritage 5 (2004) 221–230

were too low (37–38%). These considerable differences in

thermo-hygrometrical conditions between rooms in combi-

nation with the free exchange of air caused unacceptable air

currents and spatial and temporal gradients.

The forced circulation caused by heating, the circulation

of air between adjacent rooms and the movement of visitors

generated turbulences that increased the deposition rate of

pollutants onto the works of art. The highest concentrations

of particles were consistently found during the visiting hours

of the museum (Fig. 2). Especially the largest particles (i.e.

between 5 and 10 µm), which settle during the night, were

resuspended during the day.

2.2.2.2. Summer campaign. Temperature gradients in room I

(Rubens room, first floor) appeared to be quite strong during

the day (differences of around 1 °C within 4–5 m) and

attenuated in the evening. A marked thermal minimum was

present in front of the door between rooms I and H. The SH

and RH distributions were good with weak gradients. Only

sometimes in front of open doors connecting adjacent rooms,

in particular H and K, weak maxima of RH were found

(coinciding with temperature minima). The average RH

(around 50–55%) was acceptable. In general, the microcli-

mate in summer was better than in winter.

The horizontal temperature distribution in room K on the

first floor exhibited differences reaching up to 1.5 °C within

2–3 m. Probably some of them were due to the air-

conditioning system. Colder air came in from room I and

strong gradients were found in front of the door. The glass

door between rooms K and L limited the exchange of air

masses between the two rooms. On the other hand, the

horizontal SH and RH distributions were quite homoge-

neous, showing only weak gradients. The average horizontal

RH level varied between 45% and 50% with week temporal

changes that were acceptable for an effective conservation.

The vertical distribution showed a stable thermal gradient

characterised by a temperature difference of 2 °C within 4 m.

This gradient tended to suppress turbulence and reduced the

deposition of suspended particles on the paintings.

In general, strong differences in the temperature distribu-

tion have been found within the rooms on the first floor. In

particular, the inmost rooms were colder compared to the

outermost rooms with differences reaching up to 4 °C. The

air inside the inmost rooms was characterised by a higher

humidity, compared to the outermost rooms, with differences

reaching up to 2 g/kg (SH) or 10% (RH).

In the large Antwerpen I room (ground floor), where the

Van Dyck exhibition was held, the horizontal temperature

distribution was uneven with strong gradients (2–3 °C within

2–3 m) that became less pronounced in the afternoon (about

1 °C within 2–3 m). This was due to the large number of

visitors and to different air masses that came from neighbour-

ing rooms. A marked thermal minimum was found, which

coincided with the presence of two humidifiers close to the

entrance. At this location, strong gradients of SH (1.2 g/kg

within 1–2 m) and RH were observed. The humidifiers were

operated at maximum power in the morning and were attenu-

ated in the afternoon. In the evening their effect disappeared.

A Van Dyck picture on the wall behind the humidifiers was

exposed to a flow of air, which changed its moisture content

by about 2 g/kg during the day. In the morning, the RH at this

place was 8–10% higher than the average level of the room

(about 57–58%), reaching equilibrium with the room only in

the evening when the humidifiers were turned off. Hence, the

humidifiers were placed too close to paintings that suffer

from the humidity changes. This was very evident in the

morning before opening hours. Later, the visitors were an-

other cause of internal perturbation.

In the Genova room (ground floor), the airflow of the

air-conditioning system is released from outlets located in

the ceiling and is extracted through grids all around the floor.

Thermal and hygrometric gradients were less marked than in

the large Antwerpen I room. Temperature proved to be the

most variable parameter (1–2 °C within 1–2 m). SH and RH

gradients were weak and the average RH was acceptable

(55–60%). Emission of air from the ceiling, with a moisture

content very different from the rest of the room, resulted in

relatively stable local gradients.

2.2.3. Gaseous pollution

The concentrations of NO2 and SO2 appeared to be dis-

tributed very homogeneously throughout the museum [15].

Indoor winter and summer concentrations were comparable

(12 ppb NO2 and 5–6 ppb SO2). However, the summer I/O

ratios were found to be about 1.3 times higher than the winter

I/O ratios for both compounds. This indicates a better air

exchange in summer. I/O ratios for NO2 were about twice as

high as the I/O ratios for SO2, reflecting the higher deposition

velocity of the latter. For particle types originating from an

outdoor source, winter and summer I/O ratios were compa-

rable. This suggests that the ventilation rate is not the only

factor determining I/O ratios of aerosol particles. Deposition,

as suggested by Tatcher and Layton [16], probably plays an

important role.

2.2.4. Microbiology

Generally, in summer higher bacterial counts were ob-

served on CasMM agar than on TCA while in winter the

opposite was observed (Tables 2 and 3). This might be

explained by the reduced growth of environmental strains

during the wintertime resulting in their low contribution to

counts of collected airborne bacteria. In contrast, the propor-

tion of human associated bacteria, such as staphylococci [17]

is higher. This assumption is supported by the observation

that after 9 days of incubation the bacterial counts on both

TCA and CasMM agar were almost identical. Since the

human associated bacteria grow more slowly on CasMM

agar, their colonies (which are counted for enumeration of

bacterial counts) were only visible after a prolonged incuba-

tion on the growth medium.

Both on the first floor (Table 2) and on the ground floor

(Table 3) of KMSK, microbial loads were significantly

227K. Gysels et al. / Journal of Cultural Heritage 5 (2004) 221–230

higher (at least threefold) in summer than in winter. This was

also noted for the outdoor samples (Table 4). This observa-

tion is in accordance with previous studies [8,10] and can be

explained by the fact that the growth of the majority of

bacteria is favoured during summertime due to the higher

temperature. Both in summer and in winter the bacterial

counts were higher on the ground floor than on the first floor.

In summer, the bacterial counts on the ground floor were

similar to those measured outdoors but significantly higher

(at least twofold) than on the first floor. This does not seem to

be caused by the exchange of air through the nearby main

entrance because the fungal counts measured outdoors were

threefold higher than those measured indoors. Even the

higher number of visitors on the ground floor during the

summer measurements should not have contributed signifi-

cantly to the higher counts of airborne bacteria. The growth

of human related bacteria is favoured on the TCA on which

lower counts were observed during the summer than on

CasMM agar. Thus, it has to be assumed that the ecological

niche(s) from where these airborne bacteria were suspended

to the air is (are) located somewhere on the ground floor

(inside the museum). Sampling on numerous surfaces on the

ground floor would be needed to pinpoint the source of these

bacteria. No significant counts of fungi were found indicat-

ing that the fungal load indoors cannot be considered as

harmful [18].

Counts of microorganisms able to hydrolyse Tween 80 or

Casein were not significantly increased (Tables 2 and 3). A

higher portion of either type of microorganisms would have

indicated that there is a source of microorganisms within the

confines of the museum, which are able to destroy oil paint-

ings or all works of art containing proteins, respectively.

Thus an increased hazard for the works of art inside the

KMSK by lipolytic or proteolytic airborne bacteria should

not be assumed. However, caution is advisable with respect

to unexplained high counts of airborne bacteria during the

summer on the ground floor. Since it is assumed that their

ecological niche is situated somewhere on the ground floor

Table 2

Microbial counts collected in February and in July in room K (first floor) expressed in cfu obtained after 3, 5 and 9 days of incubation at room temperature (*

expressed as positive colonies per total counts of cfu)

Medium Volume [I]

collected

Day 3 Day 5 Day 9

February July February July February July

Mould agar 100 0 0 1 0 1 2

500 0 0 0 0 0 2

TCA 100 2 22 5 22 6 25

500 20 44 27 51 30 49

CasMM agar 100 2 43 3 44 5 46

500 5 125 16 134 33 168

Tween 80 100 2/15 0/9

Hydrolysis* 300 4/24 2/71

Casein 100 2/8 9/29

Hydrolysis* 300 0/6 11/29

Table 3

Microbial counts collected in February (Catalogue reading room) and in July (Antwerpen I room, both ground floor) expressed in cfu obtained after 3, 5 and

9 days of incubation at room temperature (* expressed as positive colonies per total counts of cfu)

Medium Volume [I]

collected

Day 3 Day 5 Day 9

February July February July February July

Mould agar 100 1 1 2 1 2 4

500 3 5 4 8 5 11

TCA 100 2 65 4 65 7 74

500 35 259 109 >300 145 >300

CasMM agar 100 6 74 18 89 23 99

500 18 306 53 >300 101 >300

Tween 80 100 3/25 2/117

Hydrolysis* 300 4/70 5/>300

Casein 100 7/25 2/82

Hydrolysis* 300 5/24 6/>300

Table 4

Microbial counts collected in February and in July outdoors near the back entrance of the museum obtained after 3, 5 and 9 days of incubation

Medium Volume [I]

collected

Day 3 Day 5 Day 9

February July February July February July

Mould agar 500 6 34 19 34 25 34

TCA 500 6 197 25 197 44 201

CasMM agar 500 38 290 152 >300 >200 >300

228 K. Gysels et al. / Journal of Cultural Heritage 5 (2004) 221–230

they must also have a source of nutrients (carbon, ammonia,

phosphate, etc.) in order to grow there. Since it is most likely

(based on the kind of growth media employed) that these

nutrients are not supplied via the air they have to come from

somewhere else. Hence, it is thought expedient to carefully

investigate all works of art for any onset of decay. In order to

study the contribution of such bacteria to any decay, bacteria

would have to be isolated from these surfaces and they would

have to be studied for their potential for biodecay.

3. Conclusions

It was shown that outdoor atmosphere significantly influ-

ences the indoor air of the Royal Museum of Fine Arts. This

impact is in particular revealed by the correlation between

the time-evolution of indoor aerosol concentrations and wind

speed. Fine particulate matter concentrations (<2.5 µm) were

inversely correlated with wind speed, due to the ventilation

effect. In the larger size range (2.5–10 µm), increased resus-

pension induced by higher wind speeds, was more important,

so the large particle concentrations were found to increase

with wind speed. In winter, restoration and construction

works constituted an additional indoor source of Ca-rich and

Ca–Si particles. Along with sea salt, these were the main

particle types identified in this season. In summer, S-rich

particles were most abundant. The summer abundances of

Ca-rich particles remained low, even though the museum is

situated in a limestone building. Interior wall plaster and

construction works apparently have a far greater influence on

indoor Ca-concentration.

Organic, S-rich and Fe-rich particles in the size range

around 0.5 µm can be considered most harmful with respect

to the preservation of works of art. The joint contributions of

low-Z, S-rich and Fe-rich particles to the 0.5–1 µm size range

were 15% in the winter and 90% in the summer. The contri-

butions to the total size range (0.5–20 µm) were 20% in

winter and 49% in summer. Hence, especially during the

summer, high relative abundances of potentially harmful

particles were measured. However, the absolute concentra-

tions of S and Fe (which are more important with respect to

preservation) remained low in the KMSK, i.e. 0.25 µg/m3 or

lower. Dry deposition measurements indicated nevertheless

that the S deposition was rather high (0.029 µg/cm2 in

6 months), probably because this element was mainly present

in small ammonium sulphate particles.

From the point of view of conservation, it became clear

that the works of art are not directly threatened by particulate

pollution. Still, the situation could be improved by employ-

ing filters with higher collection efficiency in the HVAC

system, especially for small particles. The microbiological

measurements have proved that airborne microorganisms do

not seem to represent an increased hazard for the works of

art.

As far as the microclimate is concerned, this study has

evidenced that the central HVAC system causes a very irregu-

lar airflow characterised by sharp changes in temperature and

RH. On the long run, these fluctuations cause ageing and

damage to the exhibits. Winter was shown to be the most

problematic period, because of the cold the building has to be

heated and the moisture content of the external air is very low

in comparison with the air inside the museum. Hence, the RH

drops to unacceptable levels unless it is well controlled.

The spatial distribution of microclimatic parameters

showed significant perturbations due to turbulences caused

by visitors and the outlets and suction grids of the HVAC

system. In addition, heat and moisture were not well distrib-

uted among the rooms on the same floor, causing strong

gradients at communicating doors. When doors were open,

air currents established from one room to another and gener-

ated microclimatic anomalies. On the long run, these cycles

are dangerous for the exhibits. The existing HVAC was

designed mainly to meet the needs of visitors rather than the

requirements to preserve works of art. Now it needs to be

improved to reach a better conservation standard. However, it

should be kept in mind that the construction works required

to make the changes could cause even more damage by

generating numerous particles. This was clearly demon-

strated by the results obtained in this study. Leaving the

situation as it is, even if not ideal, might cause less damage

than the construction works needed to alter it.

Acknowledgements

Special thanks are due to Dr. Paul Huvenne and Mrs.

Yolande Deckers and all the staff of the Koninklijk Museum

voor Schone Kunsten in Antwerp for their sincere interest in

this work and their eager assistance during the measurement

campaigns. The PIXE analyses were performed at PIXE

Analytical Laboratories, Tallahassee, FL, USA. This study

was supported by the European Commission, Programme

Environment, contract ENV4-CT95-088 AER, aimed to

identify the main causes of environmental risk to cultural

heritage due to unsound use of technologies and mass tour-

ism. K.G. and F.D. were supported by the Flemish Fund for

Scientific Research (FWO).

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