Study by Klas Nordström, Dan Norbäck, Roland Akselsson
The sick building syndrome (SBS) involves non-specific symptoms such as irritation to the eyes, skin, and upper airways, headache, and fatigue.
1 In recent years, several epidemiological or experimental investigations on such symptoms have been published.
2–14 Various factors such as the age of the building,
2,3 the type of ventilation system,
4–6 room temperature,
7, static electricity,
8,9volatile organic compounds,
3,8,10 and microbial growth
11 have been shown to influence the prevalence of symptoms of SBS. A sensation of dryness and irritation is essential in the SBS,
1 and such symptoms are common in both office
2,4,5,12 and hospital employees.
13 In Scandinavia, the indoor relative humidity is usually in the range of 10-35% during wintertime in workplaces,
8,14 and air humidification is rare.
8
There is conflicting information in the scientific
literature on the effects of air humidification.
In experimental chamber studies with
clean air, no significant relation between
measured and perceived air humidity was
found.
15,16 There may also be negative health
aspects of air humidification. In a large study in
Great Britain it was shown that buildings with
air conditioning have a higher prevalence of
SBS, the possible cause being microbial
growth in humidifiers or chilling units.
5
In contrast with these findings, controlled experimental field studies from Finland have showed beneficial effects of moderate air humidification during the heating season on the sensation of dryness and symptoms of SBS.
14,17 In one cross over study, steam air humidification relative humidity (of 30-40%) significantly reduced the prevalence of symptoms of SBS compared with no air humidification relative humidity (of 20-30%).
14 There
, however, few longitudinal studies available where the influence of air humidification on both symptoms and other indoor exposures has been studied.
Our study had two aims. The main aim of this study was to evaluate the effect of steam air humidification on SBS and perceived air quality during the heating season. The second aim was to study possible effects of air humidification on selected physical, chemical, and microbiological exposures in the buildings.
Material and methods
Subjects
In an earlier survey (unpublished data), the age of the building and supply air flow was determined for all nine geriatric hospital units in one hospital district in southern Sweden. To increase the homogeneity of the study population, we selected the four newest and best ventilated hospital units situated in two hospitals for this investigation. At the beginning of the study in December 1991, 100 subjects were employed in these units. The subjects were invited to participate in the indoor climate investigation, but were not informed that it included air humidification in some departments. The study was restricted to those 90 employees who worked during the daytime and always in the same unit. The study base was dynamic, and was made up of employees working in the four selected departments during the study period (December 199 1-April 1992). Steam air humidification was applied in two randomly selected units, one in each hospital during a four month period. The steam humidification was introduced in December 1991, after the first questionnaire was completed. The other two units, one in each hospital, served as controls. No type of air humidification had previously been used in any of the four departments.
Assessment of symptoms and perceived
To evaluate the perception of symptoms of SBS and perceived indoor air quality (dry air, stuffy air, odour, and static electricity) the personnel were asked to answer a standardised self administered questionnaire. The questionnaire has been used in Sweden for some years, and most of the questions have been validated previously. The current version, with the designation MM-040-NA, was developed by the Department of Occupational Health in Orebro, Sweden.
18 It contains questions on perceived air quality, symptoms included in the SBS, personal factors, and the psychosocial climate at the workplace. A recall period of three months was used in the questionnaire. Work related symptoms were not assessed in the questionnaire. There was one question asking whether the respondents attributed perceived symptoms to indoor climatic factors. This information,
, was not used in this study, which covers symptoms regardless of the subjects' opinion on causes.
For each symptom or air quality perception, an answer could be given according to one of three options: "no, never", "yes, sometimes" and "yes, often". Often means every week. Each of the symptoms was assigned an index value, 2, 1, or 0, according to the answer. Twelve questions on medical symptoms, one on eye irritation, three on airway symptoms, three on dermal symptoms, and five on general symptoms were included in the questionnaire. The prevalence of subjects with at least one weekly symptom from eyes, airway, skin, or general symptoms was calculated. Also, an overall score of symptoms of SBS that ranged from 0 to 24 was constructed, by summing the individual symptom scores. Changes of symptoms or perceptions, ranging from -2 to 2, were calculated for each individual, by subtracting each index (0-2) at the beginning of the study period, from the score at the end of the period. The same questionnaire was distributed to all hospital employees before and after the study period. The subjects responded anonymously, and were instructed to answer all questions without paying any attention to the experience of indoor air quality expressed by colleagues.
Assessment of personal factors
Information on age, sex, job category, years of employment, numbers of working hours, smoking habits, hay fever, and asthmatic symptoms was obtained from the questionnaire. The current version of the questionnaire contained four questions covering different aspects of the psychosocial work conditions. The question that involved "interesting/ stimulating work" measured work satisfaction. The question "too much work to do" covered work stress. The question "do you get help from your colleagues when you have a problem at work" measured the degree of social support, and the question on "opportunity to influence working conditions" measured the degree of personal influence experienced by the subjects. The questions on psychosocial conditions had four possible answers: "yes, often", "yes, sometimes", "no, seldom", and "no, never". Each of the variables was assigned an index value, 3, 2, 1, or 0 according to the answer; and a psychosocial dissatisfaction index was calculated by forming a total sum of the three indices (0-12).
Assessment of exposure
The technical investigation comprised a
building survey. Measurements of temperature,
indoor air humidity, static electricity,
exhaust air flow/person, microorganisms,
volatile organic compounds, and particle concentration
were performed by an experienced
safety engineer. The room temperature, supply
air temperature, exhaust air flow, and ventilation
noise were measured to ensure that
these conditions did not change during the
study period. The other factors were measured
to investigate if they were influenced by
the air humidification.
In the building survey, information was
gathered on the age of the building, the type
of ventilation system used in it, the type of
floor covering, signs of building dampness,
and smoking restriction in the building.
Information on work related exposures,
including use of biocides, bulk dispensing of
Ispagula, and use of gloves was also gathered.
Measurements of room temperature and relative air humidity were performed in one room in each unit during the whole four months period by a termohygrograph (CASELLA T 9420). Also, temperature was measured with a four channel instrument (MITEG MTM 20) every hour in one week in each unit, and recorded in a datalogger. The sites of measurement for the four channel instrument were outdoor air, supply air, and locations in two different rooms. Exhaust air flows were measured in each patients's room by a thermoanemometer (ALNOR GGA 65 P). The personal exposure to static electricity was measured by a field meter (EMF 57) coupled to a metal bracelet, and the charge was recorded on a pen plotter. The charge was measured during one minute of standardized walk by five test people in all units. The particle concentration was measured in the breathing zone and in the supply air ducts by a particle counter (NET 2000). This instrument registers particles within the range of 0 5-5 pum. In total, 40 three minute average measurements were performed.
Volatile organic compounds were sampled in one unit with humidification, and one control unit, on charcoal sorbent tubes (SKC- 226-01). The air sampling rate was one 1/min for over 90 minutes. The charcoal tubes were desorbed with 1 ml of carbon disulphide before analysis, which was performed by gas chromatography within two weeks from the sampling day.8 Both the concentrations of 16 common compounds, and the total concentration of the identified and unidentified volatile organic compounds (jg/M3) were calculated. Viable moulds and bacteria were analysed in settled dust from the ventilation. Bacteria were cultivated on trypton glucose agar extract. Moulds were cultivated on malt agar. The number of colonies was counted, and compared in the laboratory with normal values for dust from ventilation ducts, obtained from the Swedish Building Research Institute.
19 Also, the water supplies of the humidification units were analysed for presence of legionella bacteria by the Microbiological Laboratory, University Hospital in Lund, Sweden.
All measuring instruments were calibrated before measurements were taken. The temperature instruments were calibrated before and during the measuring period by comparison with an accurate mercury thermometer. The termohygrograph was calibrated by comparison with a sling psychrometer. Temperature, relative humidity, volatile organic compounds, and particle concentration were measured 1-5 m above the floor, during the study period. No information on the result of the technical measurements was delivered to the hospital units before the questionnaire study was completed.
Statistical methods
Differences in symptom index between the humidified group and controls at the beginning of the study period were determined by X2 analysis for 2 x 3 tables, or by the Mann- Whitney U test if the number in any of the cells was zero. For weekly symptoms, crude odds ratios (OR), with a 95% confidence interval (95% CI) were calculated. Also, multiple logistic regression was applied to calculated adjusted ORs. Differences in scores of changes in symptoms between exposed people and controls were analysed by Mann-Whitney U test. Differences in exposures between humidified and control units were analysed by Student's t test, or by Wilcoxon matched
signed rank test. In all statistical analyses, two tailed tests and 5% level of significance were used.
Results
The response rate was 89%, both in the initial questionnaire and in the second study at the end of the study period. Due to changes of the dynamic population during the study period, however, a total number of 104 subjects participated in the study. Twenty four subjects responded in the first study only. Another twenty four subjects responded in the second study only, while 56 subjects participated in both studies. No significant differences in age or duration of employment were found between the units during the study period (table 1). The prevalence of current smokers was 45% in the humidified units, and 39% in the control units in December 1991. No significant difference in proportion of smokers, men, or job categories was found between the humidified and the control units during the study period. In the control units, however, the proportion of subjects with asthma or hayfever was significantly higher (P < 0 05, table 2).
|
Humidified units |
Control units |
| Characteristic |
December 1991
(n = 42) |
April 1992
(n = 38) |
December 1991
(n = 42) |
April 1992
(n = 38) |
| Age |
39 (12) |
40 (13) |
36 (10) |
38 (9) |
| Employment (y) |
4,2 (4,0) |
3,9 (2,8) |
4,3 (2,6) |
5,6 (5,7) |
| All results are NS |
Table 1
Mean (SD) age andyears of employment, in a dynamic population of hospital employees in two humidified two control units, before and after a four month study period
|
Humidified units |
Control units |
| Characteristic |
December 1991
(n = 42) |
April 1992
(n = 38) |
December 1991
(n = 42) |
April 1992
(n = 38) |
| Women |
100 |
100 |
93 |
92 |
| Current tobacco smoker |
45 |
35 |
39 |
50 |
| Asthma or hayfever* |
17 |
13 |
43 |
31 |
| Nurse |
12 |
11 |
12 |
13 |
| Auxiliary nurse |
81 |
79 |
79 |
72 |
| Other job category |
7 |
10 |
9 |
15 |
| *p < 0,05 Humidified v control units |
Table 2
Selected demographic data (%) of a dynamic population of hospital employees in two humidified and two control units before and after a four month study period
Building characteritics and occupational exposures
All hospital units were situated in small towns (5000-10 000 inhabitants), not near to heavy traffic or polluting industries. The age of the buildings ranged from five to eight years. All buildings were built of concrete or bricks, with slanting tile roofs, and sun protection devices above the windows, to minimise temperature variation. Two of the units had signs of dampness in the concrete floor, which was verified by humidity measurements. The floor coatings in all units consisted of polyvinyl chloride (PVC) material, and no wall to wall carpets were found. All buildings were equipped with mechanical ventilation with both supply and exhaust air (mixed system). The ventilation systems were equipped with rotary air to air heat exchangers without return air devices. The heating system was always water borne central heating with radiators, in combination with air heating by way of the supply air. General smoking indoors was allowed in one humidified and one control unit, and restricted to certain rooms in the other two units. The cleaning procedures of the floors included daily mopping with water. No use of glutaraldehyde or other biocides, either as cleaning materials or in the air conditioning systems, occurred. Bulk dispensing of Ispagula was not used in any of the four departments, and the degree of use of latex gloves was similar in all units.
physical measurements
As expected, the relative air humidity was significantly higher in the humidified departments (table 3): the relative humidity ranged from 35-45% in one and 40-45% in the other. In both control departments, the relative humidity ranged from 28-38%. Also, the air humidification resulted in a significant decrease in personal exposure to static electricity (P < 0-05). The average indoor temperature during the day did not differ significantly between the humidified and the control units. The supply air temperature ranged from 21 0-22-O0C in all hospital units. No significant differences in degree of ventilation, or particle concentration in the air between the humidified and the control units were found.
| Variable |
Humidified units
Arithmetic
mean (range) |
Control units
Arithmetic
mean (range) |
| Room temperature (°C) |
22,4 (21,5–23,5) |
21,7 (20,5–23,0) |
| Relative air humidity (%) |
40 (35–45) |
31 (28–38) |
| Exhaust air flow (1/s/person) |
14 (7–22) |
14 (6–22) |
| Static electricity (V) |
183 (25–400) |
305 (50–500)* |
| Particles µm (103/f3): |
| 0,5–0,7 |
29,9 (20–51) |
29,8 (19–53) |
| 0,7–1,0 |
6,9 (2–13) |
8,1 (2–17) |
| 1,0–5,0 |
5,1 (1–11) |
5,2 (1–12) |
| > 5,0 |
0,10 (0,01–0,6) |
0,07 (0,01–0,2) |
| *p < 0,05 By Wilcoxon matched paired signed rank test. |
Table 3
Temperature, air humidity, ventilation flow, static electricity, and air concentration ofparticles in two humidified and two control units
Chemical and biological measurements
Volatile organic compounds in the breathing zone were measured in the two hospital units with increased moisture content in the concrete floor. Table 4 shows that 2-ethyl-ihexanol was detected in all rooms, but no significant differences in the chemical composition of the air between the humidified and the control unit were found. The biological investigations showed that no viable moulds or bacteria were present in dust from the supply air ducts in any of the units. No legionella bacteria were found in the supply water of the humidification systems.
| Compounds |
Humidified unit
(n = 6)
Arithmetic
mean (range) |
Control unit
(n=5)
Arithmetic
mean (range) |
| Aromatics1 |
12 (3–29) |
23 (4–63) |
| n-Alkanes2 |
3 (3–6) |
4 (3–6) |
| Terpenes3 |
5 (2–11) |
6 (2–9) |
| Butanols4 |
7 (4–8) |
9 (1–12) |
| 2-etyl-1-hexanol |
10 (6–17) |
9 (1–12) |
| Unidentified low boiling compounds |
6,9 (2–13) |
8,1 (2–17) |
| Unidentified high boiling compounds |
5,1 (1–11) |
5,2 (1–12) |
| Total concentration of volatile organic compounds |
0,10 (0,01–0,6) |
0,07 (0,01–0,2) |
1Sum of toluene, m-xylene, o-xylene, p-xylene, and
ethylbenzene.
2tSum of noctane, n-nonane, n-decane,
Aand n-undecane.
3Sum of a-pinene, fi-carene, and limonene.
4Sum of n-butanol and
iso-butanol.
Sum of unidentified organic compounds with a retention time < n-dodecane.
lSum of unidentified organic compounds with a retention time > n-dodecane |
Table 4
Indoor concentration of volatile organic compounds (uglm3) in one humidified
and one control unit
Reception of indoor air quality
In December 1991, before air humidification,
no significant differences in any type of perception
between the control units and the
humidified units, were found. The most
prevalent discomforts in all four units were
complaints on dry air, static electricity, and
stuffy air. On average, 73% of the 84 responders
in 1991 reported weekly sensations of air
dryness, 39% perceived static electricity every
week, and 35% reported weekly perceptions
of stuffy air.
Humidification greatly affected the perception
of dry air and static electricity. In April
1992, after four months of air humidification,
the prevalence of weekly complaints of dry air
was 24% in the humidified group (n = 38),
but still 73% in the controls (n = 38) (OR =
0 1; 95%CI 0 04-0 3, P < 0001). Weekly
complaints on static electricity were reported
by 26% in the humidified group, compared
with a 55% prevalence in controls (OR = 0 3;
95% CI 0-1-0-8, P < 0-05). The reductions of
these discomforts were similar in both humidified
departments. No significant difference
for other types of perception of air quality
were found in April 1992. Similar results were
obtained if the analysis was restricted to those
56 subjects who responded both in December
1992 and April 1992.
The incidence of both
reduced and increased perceptions were high
for many complaints about physical factors
such as draught, temperature, and inadequate
light, suggesting that perceptions of indoor
climate may have large normal fluctuations.
Despite these fluctuations, significant differences
between humidified units and controls
were found for both the change of sensation of
air dryness (P < 0-01) and static electricity (P
< 0-05, table 5).
Table 5
| Type ofperception |
Humidified units
(incidence (%) of) |
Control units
(incidence (0%) of) |
|
Decreased*
perception |
Increasedt**
perception
|
Decreased*
perception
|
Increasedt**
perception
|
p-value*** |
| Draught |
11 |
54 |
4 |
30 |
NS |
| Variable temperature |
17 |
26 |
42 |
17 |
NS |
| Too high temperature |
33 |
11 |
22 |
13 |
NS |
| Too low temperature |
22 |
44 |
25 |
33 |
NS |
| Stuffy "bad" air |
25 |
18 |
28 |
20 |
NS |
| Dry air |
62 |
0 |
21 |
13 |
< 0,01 |
Unpleasant odour
|
22 |
15 |
32 |
20 |
NS |
| Static electricity |
27 |
8 |
4 |
21 |
< 0,01 |
| Passive smoking |
7 |
15 |
4 |
28 |
NS |
| Noise |
28 |
14 |
12 |
8 |
NS |
| Inadequate light |
37 |
33 |
32 |
28 |
NS |
| Dustiness |
24 |
17 |
29 |
17 |
NS |
Calculated as difference in perception index (0-2) before and after the study period.
* Calculated as difference in perception index (0-2) before and after the study period.
** Proportion of subjects (%) with a positive value of perception change (range -2 to 2).
*** Test for difference in perception change in humidified, compared with control units, by Mann- Whitney U-test.
|
Table 5
Changes of indoor climate perceptions* among employees in two humidified hospital units (n = 30) and two control units (n = 26) during a four month study period
Sick-Building-Syndrome
No significant differences in any symptom scores were found between the control units and the experimental units in December 1991, before air humidification. The prevalence of weekly symptoms was high in all units. The average prevalence in all subjects (n = 84) were 28% for eye symptoms, 34% for airway symptoms, 39% for dermal symptoms, and 37% for general symptoms. No significant differences between experimental and control subjects were found in 1991 (table 6).
|
Type symptom |
|
Eye |
Airway |
Derma |
Generall |
| Before humidification: |
|
|
|
|
| Symptom prevalence (%) in |
|
|
|
|
| Pre-exposed (n = 42) |
35 |
35 |
46 |
35 |
| Controls (n = 42) |
21 |
32 |
32 |
38 |
| Crude OR (95% CI) |
2,0 (0,7 - 5,3) |
1,2 (0,5–3,0) |
1,8 (0,7–4,6) |
0,9 (0,3–2,2) |
| Adjusted ORII (95% CI) |
1,4 (0,4–4,5) |
1,4 (0,4–4,9) |
2,8 (0,8–9,8) |
1,1 (0,3–3,6) |
| After humidification: |
|
|
|
|
| Symptom prevalence (%) in |
|
|
|
|
| Post-exposed (n = 38) |
19 |
11 |
39 |
32 |
| Controls (n =38) |
28 |
39 |
36 |
27 |
| Crude OR (95 % CI) |
0,6 (0,2–1,8) |
0,2 (0,06–0,6) |
1,1 (0,4–2,9) |
1,3 (0,5–3,6) |
| Adjusted ORII (95 % CI) |
0,5 (0,1–1,9) |
0,2 (0,04–0,7) |
1,5 (0,5–5,1) |
1,3 (0,4–4,5) |
Table 6
Adjusted ORsfor weekly symptom* (0-1) among employees in two units exposed to air humidification and two control units before and after afour month study period
In April 1992, the prevalence of airway symptoms was significance lower (P < 0-001) in humidified units than in the control units (adjusted OR = 0-2; 95% CI 004-007, table 6). For other weekly symptoms, no significant differences were found in April 1992. When analysing changes of scores of symptoms among subjects responding both times (n = 56), similar results were obtained. The humidified units reported a significant reduction in the score for throat dryness compared with changes in controls (P < 0-001). A borderline significance was also found for the reduction of mental fatigue (P = 005) and cough (P = 0-05, table 7). No significant effect of air humidification on the change of the overall score for SBS symptoms, incidence of asthma, or the psychosocial score, were detected in those 56 subjects responding both times. The overall incidence of new asthmatic symptoms was 4%, and the incidence of disappearance of asthma symptoms was 16%.
|
Humidified units (incidence (%) of) |
Control units (incidence (%) of) |
|
| Type of symptom |
Decreased symptom** |
Increased symptom*** |
Decreasedt symtom** |
Increasedf symtom*** |
p-value § |
| General fatigue |
20 |
13 |
35 |
8 |
NS |
Feeling heavy-headed
(mental fatigue) |
34 |
7 |
12 |
16 |
0,05 |
| Headache |
24 |
14 |
26 |
17 |
NS |
| Nausea or dizziness |
18 |
4 |
8 |
19 |
NS |
Difficulties in
concentrating |
14 |
14 |
15 |
12 |
NS |
Itching, burning, or
irritation of the eyes |
29 |
7 |
12 |
11 |
NS |
Irritated, stuffy,
or runny nose |
20 |
10 |
12 |
16 |
NS |
| Hoarse, dry throat |
61 |
4 |
16 |
32 |
< 0,01 |
| Cough |
28 |
4 |
17 |
25 |
0,05 |
| Dryfacial skin |
21 |
10 |
19 |
15 |
NS |
| Itch of scalp or ears |
7 |
14 |
12 |
12 |
NS |
| Dry skin on the hands |
11 |
18 |
16 |
36 |
NS |
* Calculated as difference in symptom index (0-2) before and after the study period.
** Proportion of subjects (%) with a negative value of symptom change (range -2 to 2).
*** Proportion of subjects (%) with a positive value of symptom change (range -2 to 2).
§Test for difference in symptom change in humidified, compared with control units, by Mann-Whitney U-test. |
Table 7
Changes ofsymptoms* among employees in two humidified hospital units (n =30) and two control units (n = 26) during a four month study period
Discussion
This experimental study has shown that air humidification up to a relative humidity of 40-45% decreases the perception of dry air and malodour. Also, air humidification reduced the prevalence of symptoms compatible with the SBS.
Many problems are inherited in a cross sectional epidemiological study, particularly if the studied phenomena have a multifactorial cause. Some of these problems can be reduced if an experimental and longitudinal design is selected. A selection bias can occur, both because of an incorrect study design and as a result of a low response rate. This study was designed to include the four newest geriatric hospital units within one geographical area, and the humidification exposure was randomly distributed in two of the units, while the other two served as controls. Also, the participation rate was high. Because of the dynamic population structure, changes of subjects during the study period could have influenced the results. When applying paired comparisons, however, similar results for the dynamic population were obtained. Thus it is less likely that selection bias had any important influence on the result of our study.
Another problem of validity is response bias due to the awareness of the exposure. In this study, however, no information on the purpose of the study was given to the participants during the study period. Also, the results of the measurements or the questionnaire study were not reported to the units until the questionnaire investigation was completed. Only specific exposures and perceptions were related to air humidification. Finally, in studies where multiple effects of exposure are analysed, mass significance may influence the results. In our study, however, most results were significant and < 1% level, and the total pattern of the results was consistent with the hypothesis. We do not think that the internal validity of our results have been unduly affected by multiple statistical tests, response bias or selection bias.
The perception of poor indoor air quality, particularly air dryness and irritation is essential in the SBS as defined by a working group of the World Health Organisation.
1 This syndrome comprises irritative symptoms from eyes, skin, and upper airways as well as general symptoms, including headache and fatigue. Other investigators have mainly studied complaints in office workers. In the Danish Town Hall Study,
2 52% of the women complained of dry air and 16% complained of static electricity. Similar figures were obtained in a study from northern Sweden,
12 where 43% of the women complained of dry air. There are, however, indications that symptoms of SBS are common also in hospital workers.
9,13
Common complaints in our study were dry air and static electricity. We showed that these perceptions were drastically reduced even with moderate air humidification. Also, a significant decrease of airway symptoms, particularly throat dryness, was found. Our results agree with other recent studies from Finland,
14,17 Sweden,
13 i and the United Kingdom.
20 In one crossover experimental field study, steam air humidification (relative humidity of 30-40%) significantly reduced the prevalence of symptoms of dryness compared with no air humidification (relative humidity of 20-30%).
14 In another experimental study in a large office building, humidification (relative humidity of 45-55%) significantly reduced the prevalence of skin and airway symptoms and perception of dry air compared with no air humidification (relative humidity of 10-20%).
17 Similar results were obtained in an experimental field study in a Swedish hospital with indoor air complaints. Finally, in a non-experimental longitudinal study from the United Kingdom, higher air humidity was associated with less symptoms.
20 There are also indications that air humidification may result in a decrease of sickness absence in office workers.
21
Our results do not contradict other studies where adverse health effects due to microbial growth in humidifiers or air conditioning units have been found.
5,22 These studies show the importance of avoiding spread or growth of airborne microorganisms indoors, and therefore a moderate steam humidification under safe conditions is the best choice. It is still unclear which mechanisms are involved in the reduction of symptoms, air dryness, and malodour in humidified buildings, but it has been recommended that the relative humidity should be between 40 and 60%.
23 The possible influence of air humidification on air quality seems not to have been measured in most other experimental field studies. We found no effect of air humidification on the concentration of volatile organic compounds or particles, but a significant reduction of measured static electricity. Our results indicate that further research is needed to understand the mechanism of the positive health effects of air humidification in Nordic countries, where air humidity can be extremely low during cold winters.
This study was supported by funding from the National Swedish Institute for Building Research and the Malmöhus County Council.
