Diving using compressed air involves the potential for changing lung function through several mechanisms, including bronchial responsiveness. Of particular interest is the effect self-contained underwater breathing apparatus (SCUBA) may have on airflow limitation following a dive such as a possible result of aerosolization of seawater within the regulator, breathing cold dry gas from the tank, or some other putative mechanisms. This cross-sectional observational study was conducted in the field and investigated the hypothesis that bronchoconstriction may occur during a SCUBA dive in some individuals. Measurements included forced vital capacity (FVC), forced expiratory volume in one second (FEV1) and peak expiratory flow. Spirometry instrumentation provided immediate feedback regarding test quality. Data were collected from May 2007-May 2009, at which time we calculated the data had an 88% statistical power to detect the clinically significant mean change in FEV1 of 200mls. Post-dive lung function values showed no clinically important impairment (FVC fell by 80 mls on average, 95% confidential interval CI 20mls to 140 mls, t(131)=2.57, p=0.01) and the hypothesis was not supported. Of 209 eligible divers who participated in the study, 63% provided valid spirometry data to ERS/ATS standards for acceptability and reproducibility. This study was conducted in the field as opposed to the more commonly used laboratory setting. Although spirometry is a simple test, the ease of data collection was made more difficult by environmental conditions and variables outside the researchers control.