The objective of this study was to investigate the effects of occupational exposure during firefighting and oxygen diving on ventilatory capacity while also examining the potential benefit of levels of physical activity and cardiorespiratory fitness on the same ventilatory capacity values. Specifically, study #1 compared firefighter ventilatory capacity values with that of controls and external reference values from non-smoking segments of populations. In addition, the study sought to assess the relationships between levels of physical activity and levels of firefighting exposure with ventilatory capacity. Study #2 was designed to compare ventilatory capacity values of divers with those measured in firefighters and external reference values from non-smoking segments of populations. The study also sought to examine the relationship between levels of physical activity, cardiorespiratory fitness, ventilatory threshold and levels of diving exposure with ventilatory capacity values. (Abstract shortened by UMI.