Author: Ben Knox-Brown

The small airways, quiet no more

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:




Using spirometry to measure small airways obstruction, time for a rethink?

The airways in our lungs are like the branches of a tree. At their widest they resemble the sturdy tree trunk and at their most narrow, the fragile twigs. Due to their size, these twig-like structures are called the small airways and they are easily damaged by noxious particles or gases we inhale, for example cigarette smoke. When they are damaged, our small airways become inflamed, which stops air moving freely and leads to something called small airways obstruction. In conditions like asthma and chronic obstructive pulmonary disease (COPD), small airways obstruction is common. However, evidence also suggests that this condition can occur on its own and may be a sign of future and more serious lung disease. A test called spirometry is often used to investigate whether the small airways are obstructed or not, however, there is no agreement on how best to do this.

We conducted a systematic review of the scientific literature to identify and summarise studies that measured small airways obstruction in the general population using spirometry. We extracted information from suitable studies on the choice of spirometry parameter used to measure small airways obstruction, criteria used to diagnose this condition, the proportion of people affected by it, and any factors that could be increase the odds of having this type of obstruction.

We found that only 25 studies have measured small airways obstruction in general populations around the world. Across these studies, 16 different spirometry parameters were used to measure it, along with 8 different diagnostic criteria. The proportion of people with small airways obstruction ranged from 7.5% to 45.9% and varied depending on the choice of spirometry parameter and world region. Just two studies identified potential risk factors for this type of obstruction, with cigarette smoking, passive smoking, increasing age, being women, low education level, and exposure to high levels of air pollution being the most promising.

Small airways obstruction is a common but understudied condition in general populations around the world. Risk factors for this condition appear to be similar to more established lung diseases, such as COPD. However, there is no consensus on the best spirometry parameter or diagnostic criteria to use when measuring this type of obstruction. For this reason, more research is required to identify the best measure of small airways obstruction and to assess whether having it increases the risk of future lung disease.

The findings of this study were published in the peer-reviewed journal Respiratory Research. The article can be read here: