Growing disparity in the prevalence of chronic obstructive pulmonary disease between people with and without disabilities: a Korean nationwide serial cross-sectional study

Few studies have examined the association between disability and chronic obstructive pulmonary disease (COPD). We compared the trends in the annual COPD prevalence between people with and without disabilities, and examined the association between disability and COPD. We linked the National Health Information Database (2008–2017) with the National Disability Registration Database, which includes more than 2 million people with disabilities every year. In the 2017 dataset, people with disabilities had a higher prevalence of COPD than those without disabilities (30.6% vs. 12.5%, P < 0.001). The age-standardized prevalence rate of COPD among people without disabilities increased from 4.2 in 2008 to 10.9% in 2017 (change of 6.7%), whereas that among those with disabilities increased from 7.0 to 17.1% (change of 10.1%). In multivariate analysis, compared to people without disabilities, those with disabilities had a higher probability of having COPD (adjusted odds ratio, 1.42; 95% confidence interval 1.42–1.43). The results of subgroup analysis by disability characteristics suggested that disabilities due to failure of an organ, such as the kidney, lung, heart, or liver, and severe disabilities were particularly vulnerable to COPD. In conclusion, people with disabilities are more likely to have COPD compared to people without disabilities. Further longitudinal studies that examine cause-and-effect relationship between disability and COPD are needed to clarify this relationship and to further investigate any potential negative effects associated with the coexistence of these conditions.

Independent variables.We collected data on factors that may influence the prevalence of COPD, such as sex, age, household income level, place of residence, and comorbidities.Household income was categorized based on the NHIS insurance contribution, which is in turn based on the monthly wage for employees (employee-insured) and metrics of household wealth (e.g., income, property, and car ownership) for the selfemployed (self-employed-insured).The categories are as follows Medical Aid recipient, and 1st (lowest 25%), 2nd, 3rd, and 4th (highest 25%) insurance contribution quartiles.Residential area was classified as metropolitan, urban, or rural, based on the ZIP code.The cumulative burden of comorbidities for each subject was captured by the Charlson comorbidity index (CCI).Subjects were classified into four groups according to the CCI (0, 1-2, 3-4, and ≥ 5 [most severe comorbidities]).

Statistical analyses.
Descriptive statistics were generated for general participant characteristics.We calculated the prevalence of COPD for each year between 2008 and 2017.Annual COPD prevalence rates were compared between people with and without disabilities, stratified by sex.The age-standardized prevalence rate of COPD was also calculated, using the direct standardization method; the mid-year Korean population of 2005 was taken as the standard population.To examine the association between disability and COPD, we constructed a multivariable logistic regression model, adjusted for age, sex, income level, residence, and CCI, based on two datasets (2008 and 2017).All analyses were performed using SAS software (ver.9.4; SAS Institute, Cary, NC, USA).Two-sided P-values of 0.05 were considered significant.In a subgroup analysis, we calculated the agestandardized prevalence rates of COPD according to disability severity and type, and assessed trends therein, based on two datasets (2008 and 2017).A multivariable logistic regression model stratified by disability type and severity was also constructed.For subgroup analysis, we excluded people with respiratory disability.We further stratified by sex and age and conducted the multivariable logistic regression analysis adjusted for other variables, based on the most recent dataset (2017).When divided into two groups by age, we set the standard for age as 60 years, the average age of people with disabilities.
The national disability registration data distinguishes 15 types of disability.Disability severity is graded from 1 (very severe) to 6 (very mild) based on functional and clinical impairments, as determined by a medical specialist.In the present study, disability severity was classified as severe (grade 1-3) or mild (grade 4-6).Additionally, the 15 disability types were reclassified into 9 categories: physical disability, brain injury, facial disability, visual Trends and differences in COPD prevalence among people with and without disabilities.In the most recent dataset (2017), 7,085,279 participants had COPD (prevalence 13.4%; Table 2).COPD was more prevalent among people with than without disabilities (30.6% vs. 12.5%, P < 0.001), regardless of sex, age, income level, place of residence, or CCI.
From 2008 to 2017, the non-standardized and age-standardized prevalence rates of COPD increased in both groups with and without disabilities (Fig. 1A,B, Supplementary Table 3).However, COPD prevalence among people with disabilities was higher than among people without disabilities in all years.Women with disabilities had a higher prevalence of COPD than men with disabilities and all people without disabilities.
The age-standardized prevalence rates of COPD also increased over the study period in all disability severity (Fig. 1C) and type subgroups (Fig. 2A,B).People with disabilities due to failure of an organ, such as the kidney, lung, heart, or liver, had a higher prevalence of COPD than those with other types of disabilities.Also, the severe disability subgroup had a higher COPD prevalence rate than the mild disability subgroup.
The age-standardized prevalence rate of COPD among people without disabilities increased from 4.2 in 2008 to 10.9% in 2017 (change of 6.7%), whereas that among those with disabilities increased from 7.0 to 17.1% (change of 10.1%) (Table 3).Among the various types of disability, the highest COPD prevalence rate and largest increase therein were observed in people with respiratory disability (from 60.2 to 79.8%; change of 19.5%), internal organ disability (from 10.9 to 27.2%; change of 16.3%), and brain injury (from 9.6 to 22.6%; change of 13.0%).The lowest COPD prevalence rate and smallest increase were observed in people with visual (from 5.9 to 14.1%; change of 8.2%), facial (from 7.1 to 15.2%; change of 8.1%), and hearing and language (from 6.2 to 15.2%; change of 9.0%) disabilities.

Disability characteristics associated with COPD prevalence.
In multivariate logistic analyses controlling for sex, age, income level, place of residence, and CCI, the presence of a disability was associated with a higher prevalence of COPD (Table 4).People with disabilities had higher odds of COPD compared to those without disabilities (adjusted odds ratio [aOR] 1.42, 95% confidence interval [CI] 1.42-1.43).After stratifying by sex, the odds of COPD among people with disabilities were similar in both male (aOR 1.46, 95% CI 1.45-1.47)and female (aOR 1.38, 95% CI 1.37-1.38)(Table 5A).The odds of COPD among people with disabilities were also similar in both younger people < 60 years (aOR 1.50, 95% CI 1.50-1.51)and older people ≥ 60 years (aOR 1.28, 95% CI 1.28-1.29)(Table 5B).
In the disability severity subgroup analysis after excluding people with respiratory disability, men with severe (aOR 1.48, 95% CI 1.47-1.49)and mild (aOR 1.40, 95% CI 1.39-1.41)disabilities had higher odds of COPD than those without disabilities.Similar patterns were found in women with severe (aOR 1.29, 95% CI 1.28-1.30)and mild (aOR 1.41, 95% CI 1.40-1.42)disabilities.Furthermore, all disability type subgroups had higher odds (range 1.08 to 1.78) of COPD compared to the no-disability subgroup in both men and women.The results of the multivariable analysis stratified by age were also similar.Hearing and language disability had the highest odds in the men (aOR 1.78, 95% CI 1.76-1.79)and younger people (aOR 1.78, 95% CI 1.76-1.79).Physical disability had the highest odds in the women (aOR 1.46, 95% CI 1.45-1.47)and older people (aOR 1.34, 95% CI 1. 33-1.35).

Discussion
To our knowledge, this is the first study to report that people with disabilities had a higher prevalence of COPD, which were also confirmed by the further analysis stratified by sex and age.We also identified several types of disabilities that are closely associated with COPD.Physical disability and hearing and language disability were associated with much higher prevalence of COPD, which suggests that regular screening should be provided for these individuals.Because earlier intervention can prevent lung function decline, many experts have stressed the importance of early diagnosis of COPD and concerted efforts have been made to promote spirometry screening for patients with COPD risk factors and symptoms 16 .Our findings provide important evidence of the need for public health interventions, such as screening programs for COPD, for people with disabilities.
Of the many risk factors for COPD, long-term cigarette smoking is the most important.Previous studies have provided inconsistent findings on smoking among people with disabilities.According to recent large nationwide surveys conducted in the United States 17 and United Kingdom 18 , adults with disabilities are more likely to smoke compared to those without disabilities.A study using data from the 2013 Korea National Health and Nutrition  19 .However, this Korean study did not evaluate whether people with disabilities were more likely to be heavy smokers than those without disabilities, including the smoking duration, number of cigarettes smoked daily, and pack-years.These findings suggest that differences in smoking behavior between people with and without disabilities might not be the only explanation for the high COPD prevalence seen in our study.For example, people with a disability are prone to diminished lung function 20 and arrested lung development 21 , which might make them more vulnerable to lung damage due to cigarette smoking.In addition, people with disabilities might be more exposed to secondhand smoke throughout the life course 22,23 .Further studies of the effects of cigarette smoke exposure on lung function and chronic lung disease among people with disabilities are required.Another important risk factor for COPD is exposure to indoor and outdoor air pollution, which is often problematic among people with disabilities.Poor housing ventilation is also associated with the development of COPD.Vulnerable populations, such as the elderly and those with cognitive and physical disabilities, are more likely to be affected by common hazards in the home, because they spend more time indoors 24 .A secondary analysis of a longitudinal birth cohort study in the United Kingdom found that levels of exposure to outdoor air pollution among children with an intellectual disability were significantly higher compared to those without an intellectual disability 25 , which might be attributable to residence in a socioeconomically deprived area with high levels of air pollution.According to a cross-sectional survey conducted in Canada, workers with disabilities are more exposed to occupational hazards than those without disabilities 26 , which might increase the risk of COPD.Improvements in working and housing environments can improve lung health and reduce COPD incidence 27,28 .
People with disabilities are more likely to be physically inactive 29 .This sedentary behavior is an important and highly prevalent risk factor for chronic diseases, including COPD 30 .Patients with COPD, who often complain of severe breathlessness, often limit their daily physical activity to minimize dyspnea; this inactivity can lead to muscle atrophy and deconditioning, consequently lowering activity tolerance.This vicious cycle may be exacerbated among people with disabilities.In addition, lower levels of physical activity are associated with a higher risk of exacerbation and exacerbation-related hospitalization, and also increase the risk of all-cause mortality in patients with COPD 31 .People with disabilities suffering from COPD may have a poorer prognosis than the general population.In people with disabilities, one or more physical attributes might be affected, which limits access to sport, fitness, and job-and household-related physical activity.It is necessary to promote an inclusive approach with respect to community programs and recreational, leisure, and sports activities 32 .For example, physical education in schools should be improved for all children by considering programs that are appropriate for children with disabilities.
Respiratory infection is an environmental risk factor for COPD and plays an important role in its pathogenesis and progression 33 .The nature of some disabilities may increase the risk of respiratory infection, which is one of the most common causes of mortality and healthcare utilization among people with intellectual disabilities 34 .www.nature.com/scientificreports/ Children with neurocognitive impairment often present with chronic or recurrent respiratory problems 35 , which might lead to reduced lung function and more severe respiratory symptoms in adulthood.Lower lung function in early adulthood with subsequent functional decline is as import as rapid lung function decline in normal-sized lungs with respect to the development of COPD 3 .A past history of tuberculosis is another important COPD risk factor in low-and middle-income countries 36 .Because of the overlap between tuberculosis and disability, people with disabilities are at greater risk of developing tuberculosis 37 , which in turn increases the risk of developing COPD.
One of the strengths of this study was the use of large-scale, real-world data based on a national health insurance claims database in Korea.No previous study has attempted to evaluate long-term trends in COPD prevalence among people with disabilities, or provided a detailed analysis according to the grade and type of disability.In this sense, our study has added new evidence to contribute to public health policy and practice, including the detection of health inequalities and the identification of priorities for early intervention.However, there were also some study limitations.First, our results cannot be extrapolated to other healthcare systems.Second, some clinical and demographic variables that may influence COPD development were not available from the NHID.www.nature.com/scientificreports/ In particular, because information on smoking status could only be obtained from individuals who participated in the national health checkup in the current or preceding year, it was not possible to confirm the current smoking status of more than half of the subjects based on the 2017 data.Finally, due to the cross-sectional nature of the study design, the causal relationships among variables could not be determined.Thus, further longitudinal studies are needed to verify our findings and gain insight into the mechanisms underlying these relationships.

Conclusion
In conclusion, our study revealed a higher prevalence of COPD among people with than without disabilities.
Respiratory system disease is a particularly important physical health issue in people with disabilities.Health problems caused by a combination of COPD and disability can impose a huge social and economic burden.The various barriers that the disabled encounter when seeking health care, and the difficulty of adhering to COPD treatment, must be overcome by developing appropriate public health interventions. https://doi.org/10.1038/s41598-023-39319-8

Figure 2 .
Figure 2. Age-standardized prevalence rate of chronic obstructive pulmonary disease between 2008 and 2017 stratified by type of disability.(A) Types of disabilities with a higher than average COPD prevalence among all disability subtypes.(B) Types of disabilities with at least an average COPD prevalence among all disability subtypes.

Table 1 .
Characteristic of people with and without disabilities in the 2017 dataset.SD standard deviation, CCI Charlson cormorbidity index.a Medical Aid beneficiaries were merged into the first quartile group.

Table 2 .
Characteristics and prevalence of chronic obstructive pulmonary disease among people with and without disabilities in the 2017 dataset.COPD chronic obstructive pulmonary disease, CCI Charlson cormorbidity index.a P values for comparison of prevalence by disability status within stratified participants according to various characteristics.b Medical aid beneficiaries were merged into the first quartile group.

Table 3 .
Prevalence of chronic obstructive pulmonary disease stratified by severity and type of disability in the 2008 and 2017 datasets.COPD chronic obstructive pulmonary disease.

Table 4 .
Association between disability and chronic obstructive pulmonary disease in the 2008 and 2017 datasets: results of multivariate logistic regression analysis.OR odds ratio, CI confidence interval, CCI Charlson cormorbidity index.a Adjusted for age (continuous), sex, income level, place of residence, and Charlson cormorbidity index.

Table 5 .
Stratified analysis to assess association between disability and chronic obstructive pulmonary disease in the 2017 dataset; (A) stratified by sex, and (B) stratified by age.OR odds ratio, CI confidence interval.a Adjusted for age (continuous), income level, place of residence, and Charlson cormorbidity index.b Adjusted for sex, income level, place of residence, and Charlson cormorbidity index.