Engineering environments Waste treatment Agriculture Urban environments [12, 16] Ventilation & air conditioning systems [8] Water/Soil/ Biosphere [16] Desert storms [14] Landfills [15] Composting [10, 15] Flooded buildings [13] Sewage treatment plants [7] Disposal of biosolids [17] Crop and livestock production [9, 15] Land application of biosolids [17] Can we identify at-risk groups in relation to their activities and environments? It is important to better understand the risk factors associated with the outdoor air transmission of infections, but the best approach is not always straightforward. Epidemiology is the study of the link between exposure, outcome and confounders, but the measurement of exposure to outdoor pathogens in the air is difficult, as it is problematic to conduct controlled laboratory experiments, for example, to expose people to pathogens within the laboratory. In addition, the technology to obtain accurate organism counts may not yet be available. Moreover, it will be a challenge to evaluate the possible environmental influences such as climatic conditions and proximity to a source, when we assess exposure levels, especially retrospectively [6] [7] [8] . Furthermore, it is not always clear how the outcomes should be measured [9, 10] . Many respiratory infections do not have a definitive causal organism [3] . Infections may be asymptomatic and most infections have numerous subtypes [3, 8] . Age, gender, social class, health, exposure to pollution, and a variety of other factors are the potential confounders that must be addressed in future epidemiology studies.