Airborne infection: theoretical limits of protection achievable by building ventilation.

This study investigates the effects of ventilation on the spread of airborne diseases. After four weeks contact with a colleague suffering from cavitary tuberculosis 27 of 67 office workers had documented tuberculin skin test conversions. Complaints by workers for more than two years previously prompted studies of the air quality in the building before and after the tuberculosis exposure. Concentrations of Carbon Dioxide in many parts of the building exceeded recommended levels, indicating inadequate ventilation with external air. A mathematical model of airborne transmission was applied to the data to determine the role of building ventilation and other possible transmission factors. It was estimated that ventilation with outside air averaged about 15 feet3/min per occupant, a scarcely acceptable figure, corresponding to Carbon Dioxide levels of about 1000 ppm. The model predicted that at 25 cfm per person, 18 workers would have been infected and at 35 cfm, considered the optimal comfort level, that 13 workers would have been infected. Greater increases in outdoor air ventilation would have been impractical and would have given progressively smaller improvements in protection. According to the mathematical model, the index case added approximately 13 infectious doses per hour to the office air during the period of exposure, 10 times the average 'infectiousness' reported in a series of cases of TB. Further modelling predicted an inverse relationship between infectiousness and ventilation - in other words, as infectiousness rises, ventilation would give progressively less protection. The study concludes that outdoor air ventilation below optimal comfort levels may contribute to airborne infection but that the protection given to occupants by higher ventilation levels may be inherently limited, especially when there is a high level of exposure to infection.