In an indoor environment the risk to a susceptible individual of acquiring a new TB infection in the presence of an infectious source is proportional to the fraction of contaminated exhaled air and the duration of the exposure ( Riley et al . , 1978 ) . A systematic approach to identifying transmission sites must be based on both these factors . By equipping our study participants with unobtrusive personal CO 2 /GIS monitoring devices we were able to integrate data regarding both the level of risk at different sites visited and the length of time an individual remained exposed to that risk . Applying a Getis - Ord - GI * cluster analysis ( Getis and Ord , 1992 ) primarily located the community school as a potential TB hot spot .