Airborne Infection: Theoretical Limits of Protection Achievable by Building Ventilation

Abstract

Of 67 office workers 27 (40%) had documented tuberculin skin test conversions after an estimated 4-wk exposure to a coworker with cavitary tuberculosis. Worker complaints for more than 2 yr before the tuberculosis exposure prompted investigations of air quality in the building before and after the tuberculosis exposure. Carbon dioxide concentrations in many parts of the building were found to be above recommended levels, indicating suboptimal ventilation with outdoor air. We applied a mathematical model of airborne transmission to the data to assess the role of building ventilation and other transmission factors. We estimated that ventilation with outside air averaged about 15 feet 3/min (cfm) per occupant, the low end of acceptable ventilation, corresponding to CO2 levels of about 1,000 ppm. The model predicted that at 25 cfm per person 18 workers would have been infected (a 33% reduction) and at 35 cfm, a level considered optimal for comfort, that 13 workers would have been infected (an additional 19% reduction). Further increases in outdoor air ventilation would be impractical and would have resulted in progressively smaller increments in protection. According to the model, the index case added approximately 13 infectious doses (quanta) per hour (qph) to the office air during the exposure period, 10 times the average infectiousness reported in a large series of tuberculosis cases. Further modeling predicted that as infectiousness rises, ventilation would offer progressively less protection. We conclude that outdoor air ventilation that is inadequate for comfort may contribute to airborne infection but that the protection afforded to building occupants by ventilation above comfort levels may be inherently limited, especially when the level of exposure to infection is high.

Transmission and Institutional Infection Control of Tuberculosis

“…In an office building TB exposure, in which building ventilation rates were known, one infectious source infected 27 of 67 susceptible occupants (40%) over a 1 mo period, and the calculated q was 13 quanta per hour. Other exposures have shown q values as high as 250 quanta per hour compared with Riley's ward average figure of 1.25 quanta per hour (Riley et al 1962;Catanzaro 1982;Nardell et al 1991). This leads to one of the clearest facts about TB transmission-that patients vary greatly in infectiousness.…”

“…Although molecular methods have detected Mtb nucleic acid, advanced methods are required to estimate viability, and no molecular methods are able to determine infectivity, which requires a vulnerable host (Nardell 1999). However, good estimates of the concentration of infectious doses, what Wells called "infectious quanta," have been made epidemiologically in a few outbreak situations in which it was assumed that each infected person represented one infectious dose or quanta (Catanzaro 1982;Nardell et al 1991). It has also been measured experimentally using large numbers of guinea pigs as quantitative air samplers (Riley et al 1962).…”

“…In other less hospitable climates, especially in resource-rich settings, mechanical ventilation of hospitals and clinics may be the norm, although many (World Health Organization 2009) older buildings still depend on radiator heat and operable windows. Most ventilation systems are designed for comfort, not airborne infection control (Nardell et al 1991). The difference is that levels of ventilation adequate for odor control are generally not adequate for infection control, depending on source strength and duration of exposure.…”

“…Doubling ventilation again reduces risk by half again, and so on for each doubling. Clearly, there are practical limits to increasing ventilation rates because of system capacity, comfort, cost, and noise (Nardell et al 1991). Although not true outside air ventilation, two additional interventions can provide the equivalent of air changes per hour (EqACH) from an infection control perspective-upper-room ultraviolet germicidal irradiation (UVGI) and room air cleaners.…”

The Influence Ultraviolet Light and Scent Therapy for Reducing Germs Pulmonary Tuberculosis

“…Environmental control measures such as negative pressure mechanical ventilation are expensive to install and maintain and offer limited protection [7]. Natural ventilation may provide greater protection for little cost [8], but is climate dependent.…”