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| Image source: iaqscience.lbl.org |
Many studies have investigated the linkage between ventilation rates and the proportion of the occupants reporting sick building syndrome (SBS) symptoms. Most studies have collected symptom and ventilation rate data from sets of five to 160 buildings or building spaces and assessed how symptom reporting rates were correlated with ventilation rates, using statistical models to control for other factors. A few studies experimentally manipulated ventilation rates in one or a few buildings and assessed how symptom reporting rates or symptom severity varied with changes in ventilation rate. Occupants were unaware of the ventilation rates during the various experimental periods. Considering the full set of studies, ventilation rates spanned a very broad range from zero mechanical ventilation and an uncharacterized amount of infiltration to total ventilation of more than 100 cfm per person. In many individual studies, building ventilation rates spanned a smaller range from approximately 10 to 35 cfm per person.
Three critical reviews of this literature have been performed. In the review by Seppänen et al. [24], 20 out of 27 studies found a statistically significant increase in reporting of at least one SBS symptom among occupants of the study buildings with lower ventilation rates. In nine of these studies, there was at least an 80% increase in the fraction of occupants reporting at least one of the types of SBS symptoms in the buildings with lower ventilation rates. The second critical review [32] was performed by an interdisciplinary group from Europe and concluded that "increasing the ventilation rate .....decreases the prevalence rates of SBS symptoms." This interdisciplinary group also concluded that "ventilation rates below 25 liters per second per person [53 cfm per person] in offices can increase the risk of health and comfort problems". The third critical review [33] by another interdisciplinary group concluded that "higher ventilation rates in offices, up to about 25 L/s [53 cfm] per person are associated with reduced prevalence of sick building syndrome symptoms".
The largest U.S. study of building characteristics and occupant symptoms is the EPA Building Assessment Survey and Evaluation (BASE) Study of 100 representative office buildings. Analyses of data from this study [34] indicate a general decrease in SBS symptoms as study space ventilation rates increase from as low as 10 cfm per person up to approximately 25 to 35 cfm per person, while current U.S. building codes and professional standards generally require a minimum of 15 to 20 cfm per person in offices. Twenty percent to 30% fewer occupants reported SBS symptoms in study spaces with ventilation rates above 20 to 25 cfm per person, compared to study spaces with lower rates typically ranging between 10 and 20 cfm per person.
In work performed for this IAQ Scientific Findings Resource Bank, the published data relating ventilation rates in office buildings with prevalence rates of SBS symptoms were analyzed statistically [35]. This analysis yielded a quantitative estimate of how, on average, the prevalence of SBS symptoms varies with ventilation rate per person. The key results are provided as Figure 4 which shows the estimated curve of relative SBS symptom prevalence (RP) plotted versus ventilation rate with RP equal to unity at a ventilation rate of 17 cfm per person, which is the minimum required ventilation rate for offices (based on an assumed default occupant density of 5 occupants per 1000 ft2) in the applicable ventilation standard of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers [36]. The central estimate of RP is indicated by the bold red line and the 95% confidence band is indicated by the shading. The RP increases from 1.0 to approximately 1.15 (95% confidence interval: 1.09 - 1.22) as the ventilation rate drops from 17 to 10 cfm per person. RP decreases from 1.0 to approximately 0.67 (95% confidence interval: 0.55 - 0.81) as ventilation rate increases from 17 to 50 cfm per person. At higher ventilation rates, the statistical uncertainty in RP becomes large relative to the estimated deviation from unity.
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| Figure 4: Estimated relative SBS symptom prevalence in office workers versus ventilation rate. Image source: iaqscience.lbl.gov |
Interpretation of Figure 4 may be facilitated by the following application example. Consider a situation in which the SBS symptom prevalence is 20% when the ventilation rate is 17 cfm per person. Based on the RP curve in Figure 4, one would predict an SBS symptom prevalence of 23% (20% times 1.15) with 10 cfm per person of ventilation and a symptom prevalence of 13% (20% times 0.67) with 50 cfm per person of ventilation. The findings also indicate that, on average, increases in ventilation rates above 50 cfm per person will not substantially reduce SBS symptom prevalence although the uncertainty of the estimates are high in this ventilation-rate range.
The analyses underlying Figure 4 have several important limitations. The amount of original data available in a useable form was quite limited. Only eight useable studies with 43 data points were identified. Because of the limited data, distinct analyses for different types of SBS symptoms were not performed. In actuality, the relationship of ventilation rates with SBS symptoms may vary with symptom type. In addition, one would expect the relationship of SBS symptom prevalence with ventilation rate to vary depending on the strength of indoor pollutant sources, the levels of outdoor air pollution, and other factors. For example, in buildings with high indoor pollutant source strengths ventilation rates may have a larger impact on SBS symptom prevalence rates than in buildings with weak indoor pollutant sources. Thus, Figure 4 provides only an estimate of the average relationship, based on the available data. The equation shown on Figure 4 should not be used for ventilation rates below 10 cfm per person or above 70 cfm per person.
With respect to the impact of ventilation rates in schools on SBS symptoms in students, only two studies were identified. The study resulting in Figure 3, relating ventilation rates in two elementary classrooms with school work performance, included surveys of SBS symptoms in students. No statistically significant relationship of ventilation rate with SBS symptoms was identified [37]. The second study intentionally changed ventilation rates in four university computer classrooms [38]. When carbon dioxide concentrations were higher, corresponding to less ventilation per student, there were statistically significant increases in four of nine SBS symptoms (throat, breathing, headache, and tiredness symptoms increased). However, there were statistically significant increases in only two of these symptoms (headache and tiredness) with decreased air exchange rates (rates of outdoor air supply per unit indoor air volume).
Many of the studies of how ventilation rate in affects SBS symptoms have measured indoor CO2 concentrations. In the majority of these studies, a larger fraction of occupants reported SBS symptoms in buildings with higher CO2 concentrations.
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