Monday, September 02, 2013


Ventilation, as defined here, is the supply of outdoor air to a building. Ventilation rates vary considerably from building to building and over time within individual buildings. Throughout the normal range of ventilation rates encountered in buildings, increased ventilation rates are, on average, associated, with fewer adverse health effects and with superior work and school performance. There is also some limited evidence that occupants of buildings with higher ventilation rates have lower rates of absence from work or school. The main findings of related scientific research are as follows:
Ventilation Rates and Office Work Performance: On average, performance (speed and accuracy) of typical office tasks improves with increased ventilation rate. For initial ventilation rates between 14 and 30 cfm per person, the average performance increases by approximately 0.8% per 10 cfm per person increase in ventilation rate. At higher ventilation rates, the average performance increase is smaller, approximately 0.3% per 10 cfm per person increase in ventilation rate. For ventilation rates less than 14 cfm per person, performance increases with ventilation rate seem likely; however, sufficient data are not yet available to confirm this hypothesis.
Ventilation Rates and School Performance: Increases of 5% to 10% in aspects of student performance may be associated with doubling the ventilation rate when rates are at or below minimum ventilation standards (15 cfm per student). However, data relating ventilation rate with school performance are not extensive.
Ventilation Rates and Absences in Offices and Schools: In offices, a 35% decrease in short term absence was associated with a doubling of ventilation rate from 25 to 50 cfm per person. In an elementary grade classroom study, on average, for each 100 ppm decrease in the difference between indoor and outdoor CO2 concentrations there was a 1% to 2% relative decrease in the absence rate. Given the relationship of CO2 concentrations with ventilation rates, for each 1 cfm per person increase in ventilation rate, it is estimated that the relative decrease in absence rates is approximately 0.5% to 2%. This relationship applies over an estimated ventilation rate range of 5 to 30 cfm per person, and should not be applied outside those limits. Data relating building ventilation rates and absence rates are very limited.
Ventilation Rates and Sick Building Syndrome Symptoms: Many studies have found that occupants of office buildings with above-average ventilation rates (up to 40 cfm per person) have 10% to 80% fewer sick building syndrome (SBS) symptoms at work. A statistical analysis of existing data has provided a central estimate of the average relationship between SBS symptom prevalence in office workers and building ventilation rate. This analysis indicates a 15% increase in symptom prevalence as the ventilation rate drops from 17 to 10 cfm per person and a 33% decrease in symptom prevalence rates as ventilation rate increases from 17 to 50 cfm per person. The uncertainty in these central estimates is considerable.
Ventilation Rates and Respiratory Illness: Substantially higher rates of respiratory illness (e.g., 50% - 370%) in high density buildings (barracks, jails, nursing homes, and health care facilities) have been associated with very low ventilation rates, presumably because lower ventilation rates are likely to result in higher airborne concentrations of infectious viruses and bacteria. Only a few studies have been performed.
Ventilation Rates and Health in Homes: Very little research has been conducted on the relationship of ventilation rates in homes with the health of the occupants of the homes. The results of a few studies suggest that children in homes with low ventilation rates have more allergic or respiratory symptoms compared to children in homes with high ventilation rates. There is also indirect evidence that ventilation rates of homes will affect health by modifying the indoor concentrations of a broad range of indoor-generated air pollutants.