This paper [1] presents an interesting and novel approach to examining the variation in TB risk across different environments. However, we have concerns over the modelling approach, specifically with using the minimum daily value as an estimate of the outdoor concentration of CO2.

In our experience, there can be a large spatial and temporal variation in outdoor CO2 levels, which would not be captured when using a single minimum value from a 24 hour period. Furthermore, different environments are likely to have sources of CO2 present, such as those from combustion engines during transport, or gas cooking in the home environment. While the authors mention that they asked participants to record if there were any alternative sources of CO2 present, they do not mention how they accounted for this in their calculations.

In interpreting these results, it is also important to consider likely heterogeneity in infectivity. Substantial heterogeneity in the infectivity of individuals with TB has been demonstrated in a number of studies [2-6]. Were a limited number of individuals the source of many new infections in a community, this might be expected to increase the importance of indoor public spaces as sites of TB transmission. Thus, whilst much rebreathing of air might occur at home (48% in this study), school and, potentially, other public spaces, such as clinics, might be the main sites of TB transmission.

Molecular epidemiology suggests that transmission between members of the same household makes a limited contribution to TB disease in high burden settings [7-9].

Jonathon Taylor
The Bartlett, Faculty of the Built Environment, University College London

Tom A. Yates
Research Department of Infection and Population Health, University College London

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