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Abstract
Objective Byssinosis remains a significant problem among textile workers in low/middle-income countries. Here we share our experience of using different prediction equations for assessing ‘chronic’ byssinosis according to the standard WHO classification using measurements of forced expiratory volume in 1 s (FEV1).
Methods We enrolled 1910 workers in a randomised controlled trial of an intervention to improve the health of textile workers in Pakistan. We included in analyses the 1724 (90%) men who performed pre-bronchodilator spirometry tests of acceptable quality. We compared four different equations for deriving lung function percentage predicted values among those with symptoms-based byssinosis: the third US National Health and Nutrition Examination Survey (NHANES-III, with ‘North Indian and Pakistani’ conversion factor); the Global Lung Function Initiative (GLI, ‘other or mixed ethnicities’); a recent equation derived from survey of a western Indian population; and one based on an older and smaller survey of Karachi residents.
Results 58 men (3.4%) had symptoms-based byssinosis according to WHO criteria. Of these, the proportions with a reduced FEV1 (<80% predicted) identified using NHANES and GLI; Indian and Pakistani reference equations were 40%, 41%, 14% and 12%, respectively. Much of this variation was eliminated when we substituted FEV1/forced vital capacity (FVC) ratio (<lower limit of normality) as a measure of airway obstruction.
Conclusion Accurate measures of occupational disease frequency and distribution require approaches that are both standardised and meaningful. We should reconsider the WHO definition of ‘chronic’ byssinosis based on changes in FEV1, and instead use the FEV1/FVC.
- occupational health
- respiratory system
- air pollution
- indoor
Data availability statement
Data are available upon reasonable request.
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Key messages
What is already known about this subject?
Byssinosis is a significant problem among textile workers in low/middle-income countries.
It is primarily identified by its characteristic symptoms, but more severe, chronic disease is accompanied by airway obstruction.
The current WHO classification suggests that obstruction is measured through forced expiratory volume in 1 s (FEV1).
What are the new findings?
In the course of a large trial of a workplace intervention in Pakistani textile mills, the use of different prediction equations for measuring reductions in FEV1 led to widely varying estimates for the prevalence of chronic byssinosis.
Substitution of FEV1 by FEV1/forced vital capacity (FVC) substantially reduced the variation.
How might this impact on policy or clinical practice in the foreseeable future?
Since accurate measures of occupational disease frequency and distribution require approaches that are both standardised and meaningful, we should reconsider the WHO definition of ‘chronic’ byssinosis based on changes in FEV1 and replace it with FEV1/FVC.
Introduction
Byssinosis remains a significant problem among textile workers in low/middle-income countries (LMICs) where much global production is now located.1 2 The disease is characterised by work-related symptoms of chest tightness and dyspnoea and, in its chronic form, by reductions in forced expiratory volume in 1 s (FEV1). There are two classification systems currently in place for byssinosis: that produced by the WHO and the ‘Schilling criteria’.3 4 The most severe grade in the latter includes decrements in FEV1 in addition to presence of symptoms. The WHO approach, in contrast, recommends that respiratory symptoms and chronic changes in lung function are considered ‘together’, although as distinct health outcomes in epidemiological surveys, and that measurements of FEV1 should be compared with ‘data obtained from local populations or similar ethnic and social class groups’.3 Such data, however, are seldom readily available. Here we share our experience of using different prediction equations for assessing ‘chronic’ byssinosis in Pakistani textile workers.
Methods
We recently completed the baseline survey of a cluster randomised trial of a multifaceted intervention to reduce the incidence of byssinosis among textile manufacturers in Karachi, Pakistan.5 We enrolled 1910 workers from 38 textile mills. Following the European Respiratory Society (ERS) guidelines, trained technicians undertook pre-bronchodilator and post-bronchodilator spirometry using EasyOne spirometers (ndd Medizintechnik) and recorded up to eight measurements of FEV1, forced vital capacity (FVC) and their ratio.6 We reviewed all spirograms; the analyses below include the 1724 (90%) men who performed pre-bronchodilator spirometry tests of acceptable quality.
We compared four different equations for deriving lung function: those established through the third US National Health and Nutrition Examination Survey (NHANES-III; ‘Caucasian’)7 with a conversion factor of 0.9 recommended for North Indian and Pakistani individuals8 ; the Global Lung Function Initiative (GLI, ‘other or mixed ethnicities’) equations 9; a recent equation derived from a survey of western Indian population (n=1258) aged 19–88 years10 ; and one based on an older and smaller (n=504) survey of Karachi residents aged 16–65 years.11 We classified workers using the WHO recommended FEV1 cut-off for identification of workers at risk of developing permanent pulmonary impairment: FEV1 <80% predicted. We compared results based on this classification with one where we replaced FEV1 by FEV1/FVC ratio considering values below the lower limit of normality (LLN) to be abnormal. We undertook analyses in Microsoft Excel.
Results
Using symptom classification (alone), the prevalence rates of byssinosis were 3.4% (n=58) and 3.9% (n=67) according to WHO and Schilling criteria, respectively—reflecting the grade 0.5 (nine workers) in the latter. Of the 58 men with byssinosis according to WHO criteria, the proportion with a reduced FEV1 (<80% predicted) varied according to which set of predictive equations was used, from 40%–41% with those from NHANES and GLI, to 12%–14% with the more locally derived models (table 1). Much of this variation was eliminated when we substituted FEV1/FVC ratio (<LLN) as a measure of airway obstruction; in particular, the estimates derived from using the GLI and Indian reference equations were very similar. We observed the same patterns when estimating the prevalence of airway obstruction in the total mill population.
Proportion of workers with airway obstruction, by different criteria and reference equations, in those with symptoms of byssinosis and in the total mill population
Discussion
The substantial variation in the prevalence of abnormal FEV1, consistent with standard classifications of ‘chronic’ byssinosis, resulting from the use of different lung function prediction equations in this population, reinforces the WHO recommendation that reference data from ‘local’ populations be used. Whether the stipulation for data from ‘similar social class groups’ is met is more difficult to ascertain since these are seldom reported for the populations from which predicted values are derived. It is evident that the widely used GLI (in which there is a lack of representation of South Asian populations) and NHANES-III equations (even after adjustment) give a very different picture of the prevalence of lung function abnormality when this is expressed by FEV1, and therefore may not be useful in a local/regional South Asian context. Much textile manufacture now takes place in populations that are poorly served by spirometric norms, and the problem we have identified in Pakistan will be reflected in many other LMICs.1 2 In contrast, substitution of FEV1 with FEV1/FVC ratio, with an LLN criterion of abnormality, produces estimates of airway obstruction that are relatively stable across different prediction equations, including the GLI. Since the FEV1 is correlated with the FVC, it is not an unambiguous measure of obstruction, and for this the FEV1/FVC, in which the FEV1 is adjusted for lung size, is preferable for defining obstruction. In the current context, this has the added advantage that it is largely independent of ethnicity, the ethnic differences in FEV1 and FVC largely cancelling each other out.
A potential limitation of our work includes the effect of a ‘healthy worker effect’ and the consequent underestimation of the risk of byssinosis in this context; it is improbable that this will have affected our findings in relation to the relative merits of FEV1 and FEV1/FVC ratio. Reproducible measurements of FVC are more difficult than those of FEV1 and require greater technical skill. Finally, the information provided by the authors of the reference equation for Pakistanis11 was insufficient to calculate an LLN.
Notwithstanding this, all efforts should be made to reduce exposures to cotton dust to identify byssinosis early through periodical workplace surveillance for the presence, nature, and extent of characteristic symptoms and before lung function loss has occurred.
Exposures in the workplace may be important causes of respiratory disease12 and especially so in LMICs where occupational health and safety measures may be poor. The basis for the prevention of the important public health burden of occupational disease is accurate measure of its frequency and distribution which requires approaches to its recognition and classification that are both standardised and meaningful. We should reconsider the WHO definition of ‘chronic’ byssinosis based on changes in FEV1 and substitute it with the use of FEV1/FVC.
Data availability statement
Data are available upon reasonable request.
Ethics statements
Patient consent for publication
Ethics approval
The study was approved by the ethics committees at Aga Khan University, Karachi (2019-0962-3710), the National Bioethics Committee in Pakistan (4-87/NBC-402/19/483) and Imperial College London (19IC4968).
Footnotes
Contributors AAN led this work in conceptualisation, analysis and write-up. MZM supported its statistical analysis. SDM, PB, AA and PC provided supervision throughout this work. All authors read and approved the manuscript before submission.
Funding This study was suppoted by Wellcome Trust (Ref: 206757/Z/17/Z).
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.