The small airways of our lungs are less than 2mm in diameter. Since the 1960s, these airways have been considered are a “quiet zone” where diseases progress unnoticed. That is until they cause symptoms and can be detected using specialist diagnostic equipment. Dysfunction or more appropriately obstruction of the small airways is a key feature of lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). A lung function test called spirometry is the most widely used method of assessing the small airways. However, it is largely unknown how common small airways obstruction is in the general population and what its likely determinants are.
We conducted a study to estimate the prevalence and associated risk factors for small airways obstruction across several world regions. We used data from the multinational Burden of Obstructive Lung Disease (BOLD) study, which included 26,500 participants from 41 study sites, across 34 countries. All participants performed spirometry before and after inhalation of salbutamol, which is a medication designed to open the airways. Participants also had their height and weight taken and completed a questionnaire which enabled us to collect information on exposures such as tobacco smoking. Due to the lack of agreement as to which spirometry parameter is best for assessing small airways obstruction, we compared results for two different parameters, the mean forced expiratory flow rate between 25% and 75% of the forced vital capacity (FEF25-75) and the forced expiratory volume in 3 seconds as a ratio of the forced vital capacity (FEV3/FVC).
We found that approximately one in five study participants have small airways obstruction. Prevalence estimates for individual sites varied from 5% to 33%. We identified several preventable risk factors for small airways obstruction including tobacco smoking, including passive smoking, low education level, low body mass index, working a dusty job for over ten years, family history of COPD and past tuberculosis infection. Results were similar for both FEF25-75 and FEV3/FVC parameters.
Small airways obstruction is relatively common in general populations around the world, with prevalence estimates greater than those of both asthma and COPD. Risk factors for small airways obstruction are like those of COPD, however, further studies are needed to corroborate our findings. It is especially important for future research to investigate whether those with SAO are more likely to develop established lung disease in later life, which could have significant implications for public health policy making.
The findings of this study were published in the open access, peer-reviewed journal The Lancet Global Health. The article can be read here: https://doi.org/10.1016/S2214-109X(22)00456-9