Ventilation rate in office buildings and sick building syndrome.

OBJECTIVE--To examine the relation between ventilation rate and occurrence of symptoms of the eyes, nose, throat, and skin as well as general symptoms such as lethargy and headache, often termed the sick building syndrome. METHODS--A cross sectional population based study was carried out in 399 workers from 14 mechanically ventilated office buildings without air recirculation or humidification, selected randomly from the Helsinki metropolitan area. The ventilation type and other characteristics of these buildings were recorded on a site visit and the ventilation in the rooms was assessed by measuring the airflow through the exhaust air outlets in the room. A questionnaire directed at workers inquired about the symptoms and perceived air quality and their possible personal and environmental determinants (response rate 81%). The outcomes were weekly work related symptoms experienced during the previous 12 months and symptom groups defined either by their anatomical location or hypothesised mechanism. RESULTS--In logistic regression analysis, the adjusted odds ratio (OR) for any symptom of interest was 3.03 (95% confidence interval (95% CI) 1.13 to 8.10) in the very low ventilation category of below 5 l/s per person and 2.24 (0.89 to 5.65) in the high ventilation category of over 25 l/s per person compared with the reference (15- < 25 l/s). The ORs for ocular (1.27, 1.11 to 1.46), nasal (1.17, 1.06 to 1.29), skin symptoms (1.18, 1.05 to 1.32), and lethargy (1.09, 1.00 to 1.19) increased significantly by a unit decrease in ventilation from 25 to 0 l/s per person. CONCLUSION--The results suggest that outdoor air ventilation rates below the optimal (15 to 25 l/s per person) increase the risk of the symptoms of sick building syndrome with the sources of pollutants present in mechanically ventilated office buildings. The Finnish guideline value is 10 l/s per person.     

Multiple logistic regression modelling substantiates multifactor contributions associated with sick building syndrome in residential interiors in Mauritius

This paper presents a mathematical model that depicts the relationship between the possibility of occurrence of common health problems and factors leading to Sick Building Syndrome symptoms in domestic interiors in Mauritius. The prevalence of upper respiratory symptoms (dry eyes, runny nose), central nervous system symptoms (headache, nervousness), and musculoskeletal symptoms (pain/stiffness in shoulders/neck) were found to be elevated when responses were statistically regressed to type of building and age of respondents. The model presented here will be useful in helping to identify and quantify the relative role of factors that contribute to Sick Building Syndrome. Thus it may be possible to evaluate the effectiveness of current building operation practices and to prioritise allocations of resources for reduction of risk associated with Indoor Environmental Air Quality.     

Prevalence of objective eye manifestations in people working in office buildings with different prevalences of the sick building syndrome compared with the general population

Summary A cross-sectional clinical epidemiological study was carried out among 169 office workers in four Copenhagen town halls with different prevalences of the sick building syndrome. The results were compared with those in 112 subjects randomly selected from the general population. Biomicroscopic eye manifestations, such as premature break-up of the precorneal tear film, absence of foam at the inner eye canthus and epithelial damage of the bulbar conjunctiva, were investigated together with self-reported eye complaints. Although intercorrelated, the objective eye manifestations independently were statistically associated with self-reported eye complaints in office workers. The prevalence of the objective eye manifestations was significantly elevated in office workers compared with the general population and most pronounced for the buildings with a high prevalence of the sick building syndrome (P < 0.001). In the general population, subjects with a non-industrial occupation, including office workers, had a significantly higher prevalence of objective eye manifestations than those with an industrial occupation (P = 0.03), but the prevalence was still significantly lower than that among the office workers in buildings with a high prevalence of the sick building syndrome (P<0.001). Since possible confounders were found not to explain the difference in prevalence of objective eye manifestations and complaints among the two populations, it is concluded that the office environment (buildings and/or type of office work) promotes these objective changes accompanied by self-reported complaints.     

Dampness,food habits,and sick building syndrome symptoms in elementary school pupils

Objectives  

We investigated dampness/mold in schools and dwellings, and food habits and subjective symptoms in elementary school pupils, in order to clarify the effect of dampness and food habits on subjective symptoms in elementary school pupils.     

Sick building syndrome in relation to building dampness in multi-family residential buildings in Stockholm

Objectives: The aim was to study relationships between symptoms compatible with sick building syndrome (SBS) on one hand, and different indicators of building dampness in Swedish multi-family buildings on the other. Methods: In Stockholm, 609 multi-family buildings with 14,235 dwellings were identified, and selected by stratified random sampling. The response rate was 77%. Information on weekly symptoms, age, gender, population density in the apartment, water leakage during the past 5 years, mouldy odour, condensation on windows, and high air humidity in the bathroom was assessed by a postal questionnaire. In addition, independent information on building characteristics was gathered from the building owners, and the central building register in Stockholm. Multiple logistic regression analysis was applied, and adjusted odds ratios (OR) were calculated, adjusted for age and gender, population density, and selected building characteristics. Results: Condensation on windows, high air humidity in the bathroom, mouldy odour, and water leakage was reported from 9.0%, 12.4%, 7.7% and 12.7% of the dwellings, respectively. In total 28.5% reported at least one sign of dampness. All indicators of dampness were related to an increase of all types of symptoms, significant even when adjusted for age, gender, population density, type of ventilation system, and ownership of the building. A combination of mouldy odour and signs of high air humidity was related to an increased occurrence of all types of symptoms (OR=3.7–6.0). Similar findings were observed for a combination of mouldy odour and structural building dampness (water leakage) (OR=2.9–5.2). In addition, a dose-response relationship between symptoms and number of signs of dampness was observed. In dwellings with all four dampness indicators, OR was 6.5, 7.1, 19.9, 5.8, 6.1, 9.4, 15.0 for ocular, nasal, throat, dermal symptoms, cough, headache and tiredness, respectively. Conclusion: Signs of high air humidity, as well as of structural building dampness, are common in multi-family buildings in Stockholm. Reports of building dampness in dwellings is related to a pronounced increase of symptoms compatible with the SBS, even when adjusted for possible confounding by age, gender, population density, and building-related risk factors. Received: 10 April 2000 / Accepted: 10 November 2000     

Prevalence of irritative symptoms in a nonproblem air-conditioned office building

Objectives: To assess the prevalence of work-related complaints and symptoms in employees in an air-conditioned office building (Building AC) in a mild climatic area (Italy). No discomfort had previously been reported. Methods: A total of 198 employees in Building AC and 281 controls working in three naturally ventilated buildings (Building NV) answered a questionnaire investigating work-related complaints and symptoms. Results: A significantly higher percentage of workers in Building AC reported a lack of comfort in the working environment as compared with the controls (30.6% versus 18.9%). The most common complaints were strong lighting, high temperature, and dry, dusty, and/or stuffy air. The prevalence of ocular, upper airway, and cutaneous symptoms was significantly higher (29.8% versus 14.9%, 25.3% versus 9.6%, and 14.1% versus 3.6%, respectively). No significant difference was observed in respiratory or general symptoms. Logistic regression analysis showed that working with video display units and photocopiers influenced ocular symptoms; upper airway and cutaneous symptoms were influenced by female gender and working in the air-conditioned building. Conclusions: In an apparently healthy air-conditioned office building, complaints and symptoms are reported more often than in a naturally ventilated edifice, but the prevalence is lower than that usually observed in sick buildings. Symptoms are influenced by individual and work-related characteristics. Received: 17 September 1997 / Accepted: 26 February 1998     

Relation of dampness to sick building syndrome in Japanese public apartment houses

Objectives  The effect of dampness on sick building syndrome (SBS) symptoms has not been fully investigated in Japan. The purpose of this study is to elucidate the possible effects of dampness on SBS symptoms among residents in Japanese public apartment houses. Methods  A questionnaire was used to investigate the degree of dampness in public apartment houses in Asahikawa, Japan, and its effect on SBS symptoms, involving 480 residents in 64 buildings. Dampness indicators were as follows: condensation on the windowpanes, condensation on the walls and/or closets, visible mold in the bathrooms, visible mold on the walls, window frames, and/or closet, moldy odor, slow drying of the wet towels in bathrooms, water leakage, and bad drainage in bathrooms. Results  All dampness indicators except for visible mold in bathrooms had significantly higher odds ratios (ORs) for all or any SBS symptoms after adjustment. The dampness index, the number of positive dampness indicators, was significantly related to all SBS symptoms after adjustment. Conclusions  There are serious problems relating to dampness in Japanese public housing, which affects the health of residents. There is a need to educate the residents about the relationship between dampness and SBS, and building problems should be rectified.     

Effects of Temperature and Humidification in the Office Environment

In this investigation, the authors evaluated the relationship between temperature and (a) Sick Building Syndrome symptoms and (b) workers' perceptions of air dryness in environments with and without humidification. The authors studied the average intensity of symptoms and perceptions of dry air relative to room temperature in humidified and nonhumidified conditions. During the 6 wk of the experiment, 2 wings of the building were humidified one-by-one for 1 wk, followed by a week without humidification. A total of 230 daily questionnaires were completed during the nonhumidified period, and 233 were completed during the humidified period. The results were analyzed with linear regression analysis, and the average intensity of dryness symptoms and sensations of dryness increased with each unit increase in temperature above 22 °C, both in the humidified and nonhumidified conditions. Sick Building Syndrome symptoms increased relative only to temperature during the period of no humidification. In conclusion, temperatures above 22 °C caused increased dryness symptoms and a sensation of dryness, independent of humidification. The overall intensity of Sick Building Syndrome symptoms increased only when indoor air was not humidified.     

A case of sick building syndrome in a Japanese office worker

The adverse health effects caused by indoor air pollution are termed "sick building syndrome". We report such a patient whose symptoms appeared in the workplace. A 36-year-old female office worker developed nausea and headache during working hours in a refurbished office. After eight months of seeking help at other clinics or hospitals without improvement, she was referred to our hospital. At that time she reacted to the smells of various chemicals outside of the office building. Biochemical findings were all within normal ranges. Specific IgE antibody to cedar pollen was positive and the ratio of TH1/TH2 was 4.5. In the Eye Tracking Test (ETT), vertical eye movement was saccadic. Her anxiety level was very high according to the State-Trait Anxiety Inventory (STAI) questionnaire. Subjective symptoms, ETT findings and anxiety levels on STAI gradually improved during two years of follow-up. One year after the onset of her illness, the formaldehyde concentrations in the building air ranged from 0.017-0.053 ppm. Even though relatively low, chemical exposure from building materials such as formaldehyde induced a range of symptoms. Also, lack of recognition by superiors and doctors that sick building syndrome might have been the source of her illness coupled with her high state of anxiety may have exacerbated her symptoms and led to the onset of multiple chemical sensitivity. Thus psychosocial factors may contribute to sick building syndrome in the workplace.     

No consistent risk factor pattern for symptoms related to the sick building syndrome: a prospective population based study

Objectives: To examine associations between perceived indoor environment at work and the non-specific symptoms that are often referred to as the sick building syndrome (SBS), using cross-sectional and prospective analyses for a large cohort from the general population. Methods: The sample comprised 2,164 adults in employment, who completed a postal questionnaire in April 2001. Of these 1,402, who were still working and living in the same place, completed a second questionnaire a year later. The outcome measures were the prevalence of mucous membrane symptoms and general symptoms at baseline and the incidence and persistence of these symptoms at follow-up. Self-reports of the indoor environment from the baseline questionnaire were used as predictors in the analyses. Results: Inconsistent results were found between the cross-sectional and the longitudinal analyses for the associations between perceived indoor environment factors at work and symptoms. Whereas mucous membrane symptoms in the cross-sectional analysis were significantly associated with self-reported high temperature and dry air, the prospective analyses showed that onset of mucous membrane symptoms was associated with the sensation of draught, dry air, and noise. Persistent mucous membrane symptoms were associated only with stuffy air. General symptoms were associated with self-reported stuffy air and dry air in the cross-sectional analysis, while draught was the only predictor of onset of general symptoms. We found no predictors in the indoor environment for the persistence of general symptoms. Conclusions: The symptoms that are often connected with SBS are very common symptoms in the general population among manual workers as well as non-manual workers. Our study gives only limited support to the hypothesis of causal relationships between the indoor environment and these symptoms. We found no evidence of persistent mucous membrane symptoms and general symptoms related to specific factors in the indoor environment.     

Exposure to varying levels of contaminants and symptoms among workers in two office buildings.

OBJECTIVES: We hypothesized that exposure to contaminants would be associated with symptoms reported by office workers. METHODS: In two mechanically ventilated office buildings in. Montreal, the outdoor air supply was manipulated for 6 weeks, while symptoms were reported and environmental parameters were measured at multiple sites. RESULTS: Contaminant concentrations varied considerably, in part related to experimental changes in outdoor air supply. Eye symptoms were reported with higher dust and with higher concentrations of nitrogen dioxide. Mucosal symptoms were increased with higher TVOCs, higher nitrogen dioxide, and higher total contaminant load. Systemic symptoms were associated with higher dust levels. CONCLUSIONS: Symptoms reported by the workers were associated with increased concentrations of several contaminants and a summary measure of all contaminants.     

Relationship between sick building syndrome and indoor environmental factors in newly built Japanese dwellings

Objectives  Indoor air contaminants and dampness in dwellings have become important environmental health issues. The aim of this study is to clarify which factors are related to sick building syndrome (SBS) in newly built dwellings at Hokkaido, Japan, through a comprehensive evaluation of the indoor environment and validated sick building symptom questionnaires. Methods  The symptoms of 343 residents in 104 detached houses were surveyed by standardized questionnaires, and the concentrations of formaldehyde, acetaldehyde, volatile organic compounds (VOCs), airborne fungi, and dust mite allergen in their living rooms were measured. By summing the presence or absence of the five dampness indicators (condensations, mold growth, moldy odor, high air humidity of the bathroom, water leakage), a dampness index was calculated. Results  SBS symptoms were found in 21.6% of surveyed individuals. In a fully adjusted multivariate logistic regression analysis, the dampness index [odds ratio (OR) = 1.50; 95% confidence interval (CI): 1.06–1.11], log formaldehyde (OR = 23.79, 95% CI: 2.49–277.65), and log alpha-pinene (OR = 2.87, 95% CI: 1.36–6.03) had significantly higher ORs for SBS symptoms. However, other VOCs, airborne fungi, and dust mite allergen did not have significantly higher ORs. Conclusion  Dampness, formaldehyde, and alpha-pinene were significantly related to SBS symptoms in newly built dwellings. We should, therefore, take measures to reduce the chemicals and dampness in dwellings.     

Influence of indoor climate on the sick building syndrome in an office environment. The Danish Indoor Climate Study Group

The influence of indoor climate factors on symptoms of the sick building syndrome was investigated in Greater Copenhagen, Denmark. Altogether, 2369 office workers in 14 buildings, where indoor climate measurements were made, filled out a questionnaire. Multivariate logistic regression analyses of the multifactorial effects on the prevalence of work-related mucosal irritation and work-related general symptoms among the office workers showed that the concentration of macromolecular organic floor dust, the floor covering, the number of workplaces in the office, the age of the building, the type of ventilation, and two easily recognizable factors, the shelf factor and the fleece factor, were associated with the prevalence of symptoms.     

Symptom definitions for SBS (sick building syndrome) in residential dwellings

The potential risk factors for sick building syndrome (SBS) in newly built dwellings were investigated. Two different definitions for SBS were used, a narrow definition (symptoms related to home environment and continuously occurring in the last 3 months were regarded as positive) and another relatively broad definition (symptoms related to home environment and either continuously or sporadically occurring in the last 3 months were regarded as positive). With both definitions indoor air chemicals, especially TVOC, and high stress during work were found to be significantly associated with SBS symptoms. Allergic history was more associated with narrow-sense symptoms and odor perception with broad-sense symptoms. The results indicate that the broad definition be preferred to find more potential risk factors.     

Possible Sources of Sick Building Syndrome in a Tennessee Middle School

Sick Building Syndrome has been reported with increasing frequency during recent years. Buildings that have sustained water damage harbor various molds, some of which may be toxic. Students and staff at Central Middle School in Murfreesboro, Tennessee, reported symptoms similar to those associated with Sick Building Syndrome. Upon investigation, investigators noted that a black fungal growth occurred throughout the building on wet cellulose ceiling tiles. Fungal growth of this type is consistent with the genus Stachybotrys. Stachybotrys spores contain macrocyclic trichothecenes, which may cause harm when inhaled or ingested. Bulk sampling of the black mold was initiated, and the samples were cultured in a moist chamber. Testing of the samples confirmed the presence of Stachybotrys spp., a finding that implies that air sampling procedures should ensue. Professional remediation of this potentially hazardous fungal contaminant is therefore recommended.     

Prevalence of the sick building syndrome symptoms in office workers before and after being exposed to a building with an improved ventilation system.

OBJECTIVE: To find if the prevalence of symptoms associated with sick building syndrome decreased among office workers after moving to a building with improved ventilation (after controlling for potential confounders). METHODS: Workers in five buildings in 1991 all moved in 1992 into a single building with improved design, operation, and maintenance of the ventilation system. All buildings had sealed windows with mechanical ventilation, air conditioning, and humidification. Workers completed a self administered questionnaire during normal working hours in February 1991 and February 1992. The questionnaire encompassed symptoms of the eyes, nose and throat, respiratory system, skin, fatigue, headache, and difficulty concentrating, personal, psychosocial, and work related factors. During normal office hours of the same week environmental variables were measured. RESULTS: The study population comprised 1390 workers in 1991 and 1371 workers in 1992 who represented more than 80% of the eligible population. The prevalence of most symptoms decreased when workers moved to the new building: skin (54%), respiratory system (53%), nose and throat (46%), fatigue (44%), headache (37%), eyes (23%). These findings were all significant and remained generally similar after controlling for personal, psychosocial, and work related factors. Furthermore, more than 60% of workers symptomatic in 1991 were asymptomatic in 1992 for all types of symptoms. In contrast, less than 15% of workers were asymptomatic in 1991 but symptomatic in 1992 for all types of symptoms. CONCLUSION: In this study, the prevalence of most symptoms usually associated with the sick building syndrome decreased by 40% to 50% after workers were transferred to a building with an improved ventilation system. The results show that it is possible to diminish the prevalence of symptoms associated with the sick building syndrome among office workers occupying a building with mechanical ventilation, air conditioning, and sealed windows.     

Evaluation of the relationship between sick building syndrome complaints among hospital employees and indoor environmental quality

Background:Sick building syndrome (SBS) is defined as a condition occurring in people who live or work in a modern building and who suffer from complaints such as headache, fatigue, lack of concentration, and irritation of the skin and mucous membranes.Objectives:The aim of this study is to examine the complaints associated with SBS in the employees of our hospital and evaluate the relationship between the characteristics of the work environment and the complaints of SBS.Methods:890 workers participated in the study. The complaints of the participants were rated and the sum of all complaints was recorded as the Total Complaint Score (TCS). The mean TCS of the participants was compared according to demographic characteristics and work environment characteristics.Results:The most common complaints among the employees were fatigue (40%), and general muscle and joint pain (31.4%). There was a statistically significant difference in TCS according to the position (p < 0.001). The mean TCS was significantly higher in females [13 (0–81)] than in males [6 (0–59)] (p < 0.001). The mean TCS increased with the presence of odor, new wall paint, the presence of fungus/mold on walls, and the presence of rotting/mold smell (p < 0.001, for all). TCS positively correlated with stress level, social relationship, noise level, comfort, cleanliness, number of employees in the same room, presence of odor, new wall paint, presence of rotting/mold, and use of chemical materials for cleaning in the room (p < 0.001, for all), and negatively correlated with room size and number of windows (p = 0.006, p < 0.001, respectively).Discussion:The present study found that the female gender, a high level of education, a high level of stress, a low level of social relationships and work environment characteristics were associated with the complaints of SBS among the employees. Accordingly, we believe that hospital management should be informed in order for the managers to take precautions and make new regulations.     

Nasal and ocular symptoms, tear film stability and biomarkers in nasal lavage, in relation to building-dampness and building design in hospitals

Objectives: To study the relationships between dampness in concrete floors and building design on the one hand, and symptoms and medical signs of the eyes and nose in hospital workers, on the other. Methods: Four hospitals for geriatrics were selected to represent buildings with different ages and design, irrespective of symptom prevalence. The first building was built in 1925.The second, built in 1985, was known to have dampness in the floor. Conventional building techniques were used in the third building, built in 1993, and the last building was built in 1994, and was specially designed to include high ceilings, and minimal use of fluorescent lighting and interior plastic materials. The interior surfaces were painted with water-based beeswax glazing. All staff (n=95) working day shifts were invited to take part in a medical examination of the eyes and nose including acoustic rhinometry and nasal lavage, and a medical questionnaire, and 93% participated. Measurements of temperature, relative air humidity, air flow, illumination, volatile organic compounds (VOCs), molds, and bacteria were carried out in all buildings, together with measurements of formaldehyde, respirable dust, carbon monoxide (CO), carbon dioxide (CO₂), nitrogen dioxide (NO₂) and ozone. Statistical analyses were performed by bivariate analysis, and linear, ordinal, and logistic multiple regressions, adjusting for age, gender, tobacco smoking, atopy, and the perceived psychosocial work environment. Results: Dampness in the upper concrete floor surface (75–84%), ammonia under the floor [3 parts per million (ppm)], and 2-ethyl-1-hexanol in the air were detected in the two buildings built in 1985 and 1993. Increased occurrences of ocular and nasal symptoms, an increased concentration of lysozyme in nasal lavage, and decreased tear film stability were found in the subjects working in the damp buildings. Those in the specially designed building had fewer ocular and nasal symptoms, and increased tear film stability. All buildings had low levels of formaldehyde, molds, bacteria, ozone, and NO₂. The lowest total concentration of VOCs, and the highest concentration of specific VOCs of microbial origin, were found in the building with special design. Conclusion: The study provides new evidence of the role of dampness-related alkaline degradation of di-(2-ethylhexyl) phthalate (DEHP) in polyvinyl chloride (PVC) building material. Emissions related to degradation of DEHP due to dampness in the floor, indicated by increased 2-ethyl-1-hexanol in the air, seem to increase both the secretion of lysozyme from the nasal mucosa and the occurrence of ocular and nasal symptoms. The indoor environment of the specially designed building with high ceilings and no fluorescent lighting or interior plastics seemed to have a positive influence on the nasal and ocular mucous membranes. Received: 5 February 1999 / Accepted: 25 May 1999     

Trichothecene mycotoxins in the dust of ventilation systems in office buildings

Summary Analysis of trichothecene mycotoxins in dust samples from ventilation systems of office buildings was applied as a rapid and inexpensive method for the detection of mycotoxins. Dust samples from three different office spaces of the Montreal urban area, reportedly affected by the sick buildings syndrome, were analysed by thin-layer chromatography (TLC). Positive colour reaction on TLC plates with 4-(p-nitrobenzyl) pyridine, specific for the 12,13-epoxy group in the trichothecene nucleus, was obtained for the extracts of 0.5- to 50-g dust samples. The dust samples contained at least four trichothecenes: T-2 toxin, diacetoxyscirpenol, roridine A and T-2 tetraol. The results were confirmed by high-performance liquid chromatography analysis. Screening of dust samples from air ventilation systems of reportedly affected buildings provided direct evidence of trichothecene mycotoxins, with the detection limit estimated as 0.4-4 ng/mg dust. Thus, the dust sample analysis is suggested as a rapid technique for detecting the presence of mycotoxins in the dust of ventilation systems.     

Prevalence of the sick building syndrome symptoms in office workers before and six months and three years after being exposed to a building with an improved ventilation system.

OBJECTIVE: The prevalence of symptoms associated with the sick building syndrome (SBS) has recently been shown to decrease by 40% to 50% among office workers six months after they were exposed to a building with an improved ventilation system. The objective of the present study was to find whether the decrease in the prevalence of symptoms was maintained three years later. METHODS: Workers from the same organisation occupied five buildings in 1991 and moved during that year to a single building with an improved ventilation system. All buildings had sealed windows with mechanical ventilation, air conditioning, and humidification. Workers completed a self administered questionnaire during normal working hours in February 1991 before moving, in February 1992 six months after moving, and in February 1995, three years after moving. The questionnaire encompassed symptoms of the eyes, nose and throat, respiratory system, skin, fatigue, and headache, as well as difficulty concentrating, personal, psychosocial, and workstation factors. During normal office hours of the same weeks, environmental variables were measured. RESULTS: The study population comprised 1390 workers in 1991, 1371 in 1993, and 1359 in 1995, which represents 80% of the population eligible each year. The prevalence of most symptoms decreased by 40% to 50% in 1992 compared with 1991. This was similar in 1995. These findings were significant and remained generally similar after controlling for personal, psychosocial, and work related factors. CONCLUSION: In this study, the decrease of 40% to 50% in the prevalence of most symptoms investigated six months after workers were exposed to a new building with an improved ventilation system was maintained three years later. The results of the present follow up study provide further support for a real effect of exposure to a new building with an improved ventilation system on the prevalence of symptoms associated with the SBS.