tection protocols unchanged.6 The controls included installation of four isolation rooms, improved general-area ventilation, and installation of Plexiglas shields for registration personnel. The researchers compared skin test conversion rates for emergency department personnel and other hospital personnel. (The two groups did not differ significantly in age, ethnicity, foreign birth, county of residence, BCG vaccination status, or initial tuberculin skin test status.) The department implemented tuberculosis control measures at the end of the first year for which conversion rates were compared. In that year, emergency department personnel had significantly higher conversion rates than other hospital personnel (12 versus 2 percent). In the year after the measures were adopted, the rates did not differ significantly (0.0 percent for emergency department personnel compared with 1.2 percent for other workers).

In one of the few studies in a correctional facility, Puisis and colleagues (1996) reported on the introduction of radiographic screening in the Cook County Jail as part of the intake medical evaluation process. The new technology reduced the time from jail entry to isolation to 2.3 days from 17.5 days. During the period from March 1992 through December 1997, jail health care staff screened more than 445,000 inmates and found 206 cases of active tuberculosis (46.2 per 100,000 population) from the radiographic screening alone (McAuley, 2000). Staff concluded that screening had been cost-effective but noted that decreasing tuberculosis case rates could change the picture in the future.

Finally, a few studies have attempted to model the contributions of engineering controls and respiratory protections to preventing transmission of tuberculosis.7 Summarizing the results of one such effort, Fennelly and Nardell (1998) suggest that the “risk of occupational tuberculosis probably can be lowered considerably by using relatively simple respirators combined with modest room ventilation rates for the infectious aero-


Menzies and colleagues, in an article published after the committee concluded its analyses, reported a cross-sectional study of 17 Canadian hospitals. It showed skin test conversions “strongly associated with inadequate ventilation in general patient rooms [and bronchoscopy rooms] and with type and duration of work, but not with ventilation of isolation rooms” (Menzies et al., 2000, p. 779). The authors suggest that this association reflects the “exposure in nonisolation rooms of undiagnosed patients…[who] are known to pose the greatest risk to hospital workers” (p. 788). In the higher risk hospitals, the room changes per hour in the negative-pressure isolation rooms averaged between 6.1 and 9.4. The 1994 CDC guidelines recommend a minimum of 6 air changes per hour for negative-pressure isolation rooms and 12 air changes per hour where feasible (CDC, 1994b).


Modeling studies are important for the assessment of risk in a number of situations, for example, when levels of a hazard are low, slow to produce observable effects, or difficult to measure directly. They may likewise be useful when the effect of an intervention is expected to be small. In such situations, clinical studies may be impractical or ethically dubious, and epidemiologic studies may be of limited use because they can not detect effects of intervention without very large numbers of subjects or very long periods of time, or both.

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