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Exposure and risk assessment for infectious aerosols.

NIOSH 1999 Nov; :1-94
The general aim was to develop a quantitative framework for estimating M. tb infection and tuberculosis (TB) incidence among health care workers (HCW) in a hospital setting, such that the framework could inform risk management decisions. A stochastic simulation model was developed to describe M. tb infection and disease incidence among a hospital HCW cohort. The model provided results that reasonably adhered to published observations. Application of the model showed that a binary type of patient infectivity generates substantial variability in infection incidence. A binary patient infectivity adheres to the common view that most TB patients are not infectious, but that a few are dangerous disseminators. Application of the simulation model also showed that infection among immunocompromised HCWs causes only a slight increase in incidence of secondary M. tb infection and TB disease at the HCW cohort level, although TB disease in immunocompromised individuals has severe personal consequences. To provide an analytical framework for investigating cost-efficacy, a component reliability analysis of a hospital TB control program was performed to identify the most important control measures. For hospital staff as a whole, the rapid identification of suspected TB disease in presenting patients is the most important measure, but once TB disease in a presenting patient is identified, applying highly efficient environmental controls is the most important measure. With regard to the infection surveillance component of the TB control program, an analytical risk model was developed to examine the cost-efficacy of alternative screening intervals for new infection among HCW's. Efficacy was the reduction in TB disease incidence among HCW's, with the U.S. population TB disease risk taken as the target goal. It was shown that among HCW's subject to a relatively "low" annual infection risk of 0.5%, an infection screening interval of 6 months rather than 12 months is necessary to meet the target risk level, yet this interval is only slightly less cost effective than using a 12-month interval. A source-pathway-receptor engineering construct was developed to assess the efficacy of alternative control measures (primarily environmental controls) for preventing M tb infection. Similar to the finding of the component reliabiity analysis, it was shown that for HCW's who do not attend TB patients, the rapid identification of suspected TB disease in a presenting patient is substantially more effective than increasing the effective ventilation rate via increased mechanical ventilation, air filtration of recirculating air, and ultraviolet disinfection of air. However, for those HCW's who directly attend TB patients, personal respiratory protection is substantially more effective than increasing effective ventilation. A related engineering model showed that a previous analysis of upper-room air ultraviolet germicidal irradiation likely overestimated the efficiacy of this control technique. Markov models of contaminant dispersion in indoor air were developed. It was shown that Markov models provide a relatively simple and explicitly probabilistic description of the transport and fate of airborne particles such as M tb aerosol. These models indicate that increasing mechanical dilution ventilation has a limited ability to decrease a HCW's infection risk if most of that risk occurs while the HCW is in close proximity to the TB patient. The Markov modeling approach has broad applications to describing concentrations of indoor air contaminants other than infectious aerosols.
Infectious-diseases; Infection-control; Aerosol-particles; Aerosol-sampling; Disease-transmission; Disease-incidence; Disease-control; Risk-analysis; Risk-factors; Health-care-facilities; Health-care-personnel; Simulation-methods; Analytical-models
University of California-Berkeley, School of Public Health, Center for Occupational and Environmental Health, Berkeley, CA 94720
Publication Date
Document Type
Final Grant Report
Email Address
Funding Amount
Funding Type
Fiscal Year
NTIS Accession No.
NTIS Price
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NIOSH Division
Priority Area
Research Tools and Approaches: Risk Assessment Methods
Source Name
National Institute for Occupational Safety and Health
Performing Organization
University of California-Berkeley, School of Public Health, Center for Occupational and Environmental Health, Berkeley, CA 94720