Lung Function Over Time: Monitoring and Interpretation in Occupational Health

Author: Charlene Mhangami, V-Core Senior Product Specialist

Spirometry is a valuable and commonly used lung function test. It measures the volume and flow of air a person can inhale and exhale in one breath. Measurements obtained from spirometry facilitate interpretation of lung function and assess a worker's respiratory health. Commonly used reported values are Vital Capacity (VC), Forced Expiratory Volume in one second (FEV1 ) , Forced Vital Capacity (FVC), and FEV1/FVC ratio. These measurements are essential for monitoring lung function and identifying relevant early changes. This is particularly important in occupational settings where workers may be exposed to various substances and chemicals that affect respiratory health.

Monitoring lung function in occupational health

Regular monitoring of lung function using spirometry is essential for early identification of changes in lung function, which could suggest onset of disease or worsening of already confirmed disease, such as occupational asthma.

Spirometry is conducted at pre-employment or within the first six weeks of the worker beginning the job role to establish a baseline measurement. This baseline result will be used as a comparison point for tracking changes over time. Serial spirometry measurements are repeated at intervals that are in line with the individual's level of risk due to the nature of their work. In the UK, the Health and Safety Executive (HSE) provides guidance on substances and job roles which are known to be linked to respiratory risks. By comparing serial measurements to the baseline measurement, occupational health practitioners can determine whether a worker’s lung function remains stable or shows signs of decline. An example set of serial measurements is shown in Figure 1.

Understanding natural changes over time

Like other organs within the human body, the lungs will naturally change with age. Lung function will increase from birth, reaching a peak in early adulthood around the age of 25, and then enter a gradual declining aging phase. In healthy, non-smoking individuals an average annual reduction is estimated to be 30ml in both FEV1 and FVC according to Ponce et al, 2023.

It is crucial that occupational health professionals can distinguish between this normal age-related decline, and decline related to developing disease due to workplace exposure. Figure 2 shows this natural aging process of the lungs. Awareness of this pattern can aid with the interpretation of serial measurements over time.

Interpreting lung function results

In occupational health, spirometry results have traditionally been assessed using percent predicted values. Thresholds such as an FEV₁/FVC ratio below 70% or FEV₁ or FVC values falling below 80% of predicted have historically prompted referrals. However, this method is increasingly viewed as statistically flawed. The percent predicted approach assumes homoscedasticity, meaning equal variance across all ages and body sizes, which is not supported by current evidence.

Lung function values vary significantly with height, age, sex, and ethnicity. Taller or younger individuals typically exhibit higher absolute lung volumes than shorter or older individuals. This variability, known as heteroscedasticity, undermines the reliability of percent predicted values across diverse populations.

To address this, the use of z-scores is now recommended. A z-score reflects how many standard deviations a measurement lies from the mean for a healthy reference population. A z-score of -1.645 (5th percentile) is generally used as the lower limit of normal. Unlike percent predicted values, z-scores adjust for demographic differences, providing a more standardised and statistically valid method for assessing lung function.

Assessing Meaningful Changes in Lung Function

When evaluating whether changes in lung function are clinically significant, it is essential to consider several factors. Practitioners should first determine if previous spirometry results are available. Next, they should assess how the current measurements compare within the normal distribution, evaluate the variability in past results and analyse the overall rate of change.

Assessing variability is important, as in patients with low variation in measurements, smaller changes in function are meaningful and for those with already variable lung function at baseline, much larger levels of decline are required to detect change.

 It is also vital to account for clinical factors such as symptom development, changes in medication, test quality, and the inherent limitations of spirometry. The presence of respiratory symptoms or other clinical signs may necessitate action even if spirometry changes do not meet statistical thresholds.

Quantifying Change Over Time

Lung function decline can be assessed through various methods. One common approach is the slope method, which involves plotting serial measurements to calculate a trend line over time. This requires multiple data points and may be affected by variability in test performance.

Appointment-to-appointment comparisons can also be informative, allowing for the observation of absolute changes in millilitres or percentage between visits. However, to determine whether such changes are meaningful, the coefficient of variation (CoV) should be calculated. This represents the extent of variability relative to the mean baseline measurement. In healthy individuals, short-term CoV is typically around 5%. Over longer periods, such as one year or more, a change of 330 mL or 11–15% in FEV₁ or FVC may be expected.

A study by Thomas (2018) observed annual FEV₁ declines of 43.5 mL in men and 30.5 mL in women over periods exceeding three years, supporting the need to consider sex-specific norms in long-term surveillance.

The American Thoracic Society and European Respiratory Society (ATS/ERS) 2020 statement for Bonini et al states FEV1 changes greater than 20%≥ 20% over short-term trials of weeks in duration is clinically significant and FEV1 changes ≥ 15% in long-term measurements ≥1 year are clinically significant.

In the UK, the Association for Respiratory Technology and Physiology (ARTP) released guidance in 2020 stating that an FEV₁ decline of 20% or more in short-term trials, and 15% or more over periods exceeding one year, should be regarded as clinically meaningful. Nonetheless, if lung function declines alongside relevant symptoms or clinical changes, practitioners should act promptly, even if these thresholds have not yet been met.

The Importance of Lung Function Monitoring in Occupational Asthma

Spirometry plays a vital role in the early detection and monitoring of occupational asthma. In affected individuals, FEV₁ may decline rapidly, by approximately 100 mL per year, while exposure to the causative agent continues. Upon removal from exposure, the rate of decline typically slows and returns to a pattern more consistent with normal aging.

Frequent and accurate monitoring improves the precision with which decline rates are estimated. For slowly progressive diseases such as chronic obstructive pulmonary disease (COPD) or pneumoconiosis, spirometry every two to three years may suffice. However, for conditions that can progress more rapidly, such as occupational asthma, more frequent testing—every six to twelve months—is appropriate.

Regular spirometry is essential for assessing and protecting respiratory health in the workplace. Detecting clinically meaningful changes requires consideration of Spirometry interpretation and clinical context. With ongoing surveillance and accurate interpretation, occupational health practitioners can intervene early, improving outcomes and reducing the burden of occupational respiratory disease.

References

• Bonini M, Di Paolo M, Bagnasco D, Baiardini I, Braido F, Caminati M, Carpagnano E, Contoli M, Corsico A, Del Giacco S, Heffler E, Lombardi C, Menichini I, Milanese M, Scichilone N, Senna G, Canonica GW. Minimal clinically important difference for asthma endpoints: an expert consensus report. Eur Respir Rev. 2020 Jun 3;29(156):190137

• Wise, R.A., 1995. Changing methods of assessing lung function and their relation to changes in symptoms. American Journal of Respiratory and Critical Care Medicine, 152(6), pp. S77–S83.

• Thomas, P., 2018. Normal decline in lung function: long-term cohort data for spirometric reference. European Respiratory Journal, 52(4), p.1800646.

• Association for Respiratory Technology and Physiology (ARTP), 2020. Interpretation of Spirometry in the Occupational Health Setting: ARTP Statement. [Accessed 1st May 2025].

• Pellegrino, R., Viegi, G., Brusasco, V., Crapo, R.O., Burgos, F., Casaburi, R., Coates, A., van der Grinten, C.P., Gustafsson, P., Hankinson, J., Jensen, R., Johnson, D.C.,

This article originally featured in OH Today, Volume 32 Issue 2.