Basic characteristics
A total of 4,157 people were investigated in the survey and 3591 (86.48%) were included for analysis who had available data of both epidemiological data and pulmonary function tests. The participants included 1603 (44.6%) males, the average age was 55.25 ± 9.46 years, the average body mass index (BMI) was 24.44 ± 3.20 kg/m2, and the average annual exposure to PM2.5 was 24.24 ± 2.08 µg/m3. Low proportions of participants reported ever being diagnosed with other lung diseases, including 150 (3.27%) with chronic bronchitis, 50 (0.80%) with asthma, 66 (1.29%) with tuberculosis, and 88 (2.70%) who were hospitalized for lung diseases during childhood. Regarding exposure to smoking, a total of 556 (20.19%) people were current-smokers, of which the vast majority (531, 95.50%) were men and only 25 (4.50%) were women; 102 (2.13%) participants reported that their mother smoked during pregnancy. In regard to the exposure to other risk factors, 669 (18.75%) people were exposed to dust or chemicals in workplaces, 9 (0.23%) people to biofuels at home, and 12 (0.41%) people to coal at home. The basic demographic characteristics and family history of the study population were listed in Table 1.
Prevalence of COPD
Measured by spirometry, 280 people were diagnosed with COPD, and the crude prevalence rate of COPD among residents over 40 years old was 7.79% (95% CI 6.94-8.72) in Shenzhen city. Adjusted by the age and gender, the standardized prevalence of COPD was estimated to be 5.92% (4.05-8.34) (Table 2), and about 172,100 (117,800-242,300) residents over 40 years old suffered from COPD in Shenzhen (Figure 1). The age-standardized prevalence rate among men over 40 years old was 6.51% (4.48-9.09), which was higher than that among women of 5.39% (3.67-7.62), and the difference was statistically significant (p<0.001). The prevalence showed an upward trend as the age increased (trend test, p<0.001). The gender-standardized prevalence of COPD among residents aged 40-49, 50-59, 60-69 and over 70 years old were 4.30% (2.77-6.20), 7.28% (5.20-9.75), 12.72% (9.31-15.16), and 14.95% (7.28-19.19), respectively (Table 1).
The prevalence of COPD also showed difference in different areas of Shenzhen. The crude prevalence in the central area was the highest, reaching 9.34% (7.89-10.95), and lowest in the eastern region (5.44%, 3.14-8.69), of which the prevalence in Pingshan District was the lowest of only 3.51% (Figure 2A). Residents in central area were elder than in northern or eastern areas, which probably lead to the higher crude prevalence of COPD. However, standardized estimates still indicated the discrepancy among COPD prevalence in different areas. The standardized prevalence in central, northern and eastern areas were, respectively, 7.59% (5.02-10.93), 5.60% (3.80-7.93), and 3.86% (0.85-11.61) (Figure 2B).
Risk factors for COPD
To evaluate the impact on COPD of the air conditions including AQI and the ambient concentrations of six major air pollutants, we performed univariable logistic regression to include the annual average as well as the minimum and maximum values of each indicator (Table 3). The results showed that the maximum value of PM10, the average and minimum value of SO2, and the average value of CO were significantly correlated with COPD. After standardization by gender and age, the effect of the maximum value of PM10 and the minimum value of SO2 remained statistically significant. The risk of COPD increased by 3.8% (0.5-7.1) as the maximum concentration of PM10 increased every 10 µg/m3. For every 1 µg/m3 increase in the minimum concentration of SO2, the risk of COPD increased by 11.8% (2.4-22.1).
The results of univariable logistic regression on other risk factors showed that smoking, education level, medical insurance type, lung disease history, and environmental exposure were significantly correlated with COPD (Table 4). In the final multivariable logistic regression model, the risk of COPD increased by 1.206 (1.120-1.299) times as every 10 years of age increased, by 1.156 (1.053-1.270) times per every 1 µg/m3 increase of the annual minimum air SO2 concentration; people who smoked greater than 20 pack-years (OR 1.968, 95% CI 1.367-2.832), whose mother smoked during pregnancy (OR 1.881, 95%CI 1.039-3.405), or who had been diagnosed with chronic bronchitis (OR 1.733, 95%CI 1.036-2.900) or asthma (OR 4.920, 95%CI 2.425-9.982) significantly had higher risk of COPD (Table 4).
Clinical features of COPD patients
Among the 280 patients diagnosed with COPD, most (221, 78.93%) patients were classified as mild COPD (GOLD stage I), 46 (16.43%) patients were moderate COPD (GOLD stage II), and only 13 cases (4.64%) were severe COPD (GOLD stage III/IV). Among 235 patients who underwent COPD assessment test (CAT), 15 patients (6.4%) reported no related symptoms, and the other 220 patients (93.6%) reported either the respiratory symptoms (cough, sputum, sputum, chest tightness, wheezing, restricted activity) or non-respiratory symptoms (restricted going-out, poor sleep, and fatigue). The above eight symptoms were evaluated on a scale of 0-5, and the results showed that the median CAT score of these patients was five points (interquartile range 2-9 points). Among the investigated patients, 24.7% (58/235) had a total CAT score of ≥ 10 points and were regarded to have severe symptoms, as defined by GOLD [13]. Lowering the thresholds to detect symptomatic patients, we found that 55.7% (131/235) patients had a total score of over five points, and 82.6% (194/235) had a total score of over two points, where patients were clinically considered to be symptomatic (Figure 3A).
Among patients with COPD of GOLD stage I, II, III or above, the proportions of patients with severe symptoms (CAT score ≥ 10) were 18.6%, 38.5%, and 69.2%, respectively. About half of mild (52.5%) and moderate (59.0%) COPD patients gained a CAT score of ≥ five points, while most (92.3%) severe patients could be detected under this threshold. The proportions of patients who were clinically considered to be symptomatic (CAT score ≥ 2) were 79.8%, 89.7%, and 100.0%, respectively (Figure 3B). Nearly half of the patients (44.3%, 104/235) reported at least four symptoms. The proportions of patients reporting ≥ four symptoms among mild, moderate and severe COPD were 42.6%, 43.6%, and 69.2%, respectively (Figure 3C/D). The most frequently reported symptoms are cough (138, 58.7%), expectoration (134, 57.0%), poor sleep (131, 55.7%), and fatigue (123, 52.3%) (Figure 3E). Among the above four most common symptoms, patients with any of them accounted for 88.5% (208/235).