Trend and heterogeneity in forced vital capacity among Chinese students during 1985–2019: results from Chinese National Survey on Students’ Constitution and Health

Our study utilized the latest national and provincial data to explore long-term trends and heterogeneity in FVC among Chinese students aged 7–22 years from 1985 to 2019. We found that in Chinese students, APFV in 2019 was earlier than that in 1985, with female experiencing further advancement than male. During the 35 years, FVC exhibited a “V-shaped” trend, sex differences in FVC increased, and urban students exhibited greater FVC than rural students. In 2019, the age trajectory of FVC growth in 30 provinces could be divided into four scenarios, and most Chinese ethnic minority students had smaller FVC than Han. FVC level was dominantly influenced by age and height, followed by socioeconomic and environmental factors. A comprehensive understanding of growth pattern and influencing factors of FVC is essential to formulate effective strategies to improve respiratory health of Chinese students.

APFV has not been extensively studied in children. We found that compared to 1985, APFV was earlier in 2019, especially for female. This shift may be due to an earlier onset of puberty, which has been a long-term trend and along with earlier height spurt. Chinese Han girls have shown an advanced trend in age at menarche, decreasing from 13.03-year-old in 1995 to 12.00-year-old in 2019 [16]. Spermarche in Han boy has also begun earlier (14.6-year-old in 1995 vs. 13.9-year-old in 2019), especially for rural boy, which may be related to enhanced prevalence of childhood obesity [17]. Because adequate store of fat and energy would allow earlier onset of puberty [18]. We also found that female had larger decrease in APFV than male, which would be a result of sex difference in the advancement of puberty onset. Male has higher height than female in almost all the time of childhood, later puberty onset and higher velocity of height growth led to greater attainment of adulthood lung function in male [8, 19]. Our results also showed that male had later APFV, larger PFV, and higher FVC than female. More studies are needed to elucidate secular trends of FVC in children and adolescents.

We observed that age-specific FVC declined from 1985 to 2005, improved during 2005–2019 for both sexes and regions, but most age groups did not rebound to 1985 level. This finding was similar to previous results [14], and FVC levels in children and adolescents in Xinjiang, China, were also observed to reach a minimum in 2005 [20]. We updated the data to 2019, included eight successive CNSSCH, and expanded it to be nationwide. FVC decline of Chinese Han children may be related to insufficient physical activity, obesity and air pollution [21, 22]. In 2005, a survey in China revealed that over 60% of students aged 13–18 did not have enough time for physical activities [20]. According to a 2016 survey, only 29.9% of students followed the recommended daily physical activity standards [23]. To address this issue, the Chinese government introduced policies like the National Program for Child Development (2001–2010) and the Suggestions on Strengthening Adolescents’ Sports (2007). Besides, the air pollution in China has undergone significant amelioration since 2005, as evidenced by an appreciable 8.1% reduction in sulfur dioxide emission and a concomitant 1.6% decline in PM_2.5 emissions from 2006 to 2010 [24]. China has been working towards improving urban landscape, which could create more green spaces and enhance physical activity, helping to improve lung function [25, 26]. All these may help to explain the rebound in FVC among Chinese Han students after 2005. Interestingly, male aged 13–15 years old and female aged 10–12 years old showed largest FVC increment during 1985–2019, possibly due to difference in age-specific increment in height [27]. Secular trend in FVC resulted from the complex interaction between physical activity, nutrition, air pollution, and greenness.

Our study found that sex difference in FVC expanded in 13–15 years old, and urban students exhibited greater FVC than rural students. Growth rate of trunk during prepubertal and pubertal periods was different by sex. Although age of puberty onset decreased in both boys and girls, but more obvious in females than males [16, 17], so the development potential of FVC in female was less than male, and the gap increased year by year. For Chinese urban children and adolescents, the largest sex difference in height increment was also observed at around 14-year-old during 1975–2005 [27]. Persistent height advantage of urban students over the past 30 years is part of the explanation of urban-rural heterogeneity in FVC [28]. Studies of urban-rural differences in physical fitness of children and adolescents have been conducted in many countries. For instance, cardiopulmonary fitness among Mexican urban children and adolescents was found to surpass that of their rural counterparts [29]. Conversely, physical fitness levels were lower among children and adolescents in urban areas of Austria compared to those living in rural regions [30]. Interestingly, no disparity in cardiopulmonary fitness was observed between rural and urban adolescents in Kosovo [31]. These inconsistent findings may be attributed to differences in culture and urban-rural economic gap among these countries. Construction of livable and healthy cities and villages requires appropriate urban planning and infrastructure to promote physical activity. This is especially important in the prevention and control of chronic respiratory diseases.

​FVC difference between Han Chinese students and the national average showed four patterns, and most Chinese ethnic minority students had smaller FVC than Han. In 2019, the spatial distribution of FVC across 30 Chinese provinces presented a trend of “high in the North and low in the South, high in the East and low in the West”. A study revealed that students in the Chinese eastern region generally had higher heights than their western peers during six CNSSCH surveys (1985–2010), both in rural and urban settings [32]. In addition, numerous studies have provided evidence that air pollutants the adversely affected lung function of children, both in short-term and long-term exposures [22]. An examination of air pollutants in 336 Chinese cities from 2014 to 2019 revealed that air quality index was highest in northwestern and northern China, as well as in Henan province [33]. Several studies have shown that environmental pollutants have a lag effect on lung function, which continues to affect childhood growth and development [34]. This may explain the widening age-related gap in FVC between northwest China and the national average. What’s more, Northwestern China is also located at a higher altitude, and some studies have shown a negative correlation between vital capacity and altitude in people aged 6–18 years, as the lung compensatory mechanism was not fully developed [35]. We found that vast majority of Chinese minority students had lower FVC than Han students in 2019. A study by Pan et al. found that boys from North Korea and Kazakhstan had higher FVC levels than the national average, while girls from Korea, Tujia, Kazakh and Yi ethnic groups also exceeded the national average [36]. In contrast, we found only Kazak students consistently surpassed the Han average in all age groups, while other ethnic groups exceeded the Han average only in some specific age groups. The 26 ethnic groups covered by CNSSCH 2019 were mainly located in the Yunnan-Guizhou Plateau, Sichuan Basin, and Wuling Mountains, which are underdeveloped regions of China. Lung function varies significantly among ethnic groups due to body size, socioeconomic status, and environmental factors. Sitting height and chest width accounted for 16% of ethnic variation of lung function, and growth rate and timing of pubertal maturation also varied among ethnic groups [37]. Yan et al. discovered that among five nationalities in China aged 35 to 70, only Mongolians exhibited greater FVC than Han, and Dai had the lowest FVC [11]. Although the age of the study population differed, our findings also suggested that Mongolian boys and girls aged 7–15 years had larger FVC than Chinese Han counterparts, but Dai and Yi had relatively lower FVC throughout childhood. ​Ethnic differences in lung function arise from a complex interplay of genetic, socioeconomic and environmental factors. To address the developmental disparities in China, China implemented strategies like the “West China Development” policy. Due to the large population, small shifts in disease prevalence have a large impact on the number of patients. Developing ethnicity-specific reference equations is critical for accurate lung function prediction and reducing misdiagnosis of respiratory diseases.

We confirmed the strong association between height and FVC, genetic factors determine growth potential. Relationship between socioeconomic status and students’ FVC was complex. Findings of meta-analysis of 33 papers showed that children with disadvantage socioeconomic circumstances had lower lung function, and respiratory health inequalities were higher in boys than girls [38]. Another study included seven successive Chinese National Survey on Students’ Constitution and Health, and they found the “U-shaped” relationships of GDP per capita and urbanization rate with FVC in both boys and girls by using quadratic analysis [39]. In our study, GAM was performed, and ranges of GDP per capita and urbanization rate were larger than previous study [39], which would enable us to capture more complex patterns of FVC change with improvement of socioeconomic circumstance, like “W-shaped” and inverse “N-shaped” curves, not merely “U-shaped” curve. Sex-specific relationship of urbanization rate with FVC were also identified in our study, further study conducted in the Asian population would help to confirm our findings. Latitude and longitude determine geographical location, which affects students’ socioeconomic status and exposure to different environmental factors. Research has confirmed that the detrimental effects of air pollution on lung function and respiratory health [22]. These effects were attributed to various mechanisms, such as the induction of airway inflammation and oxidative stress [40]. Therefore, it is crucial to reduce air pollution levels and implement measures to improve air quality in order to safeguard lung function. The impact of greenness on children’s lung function remains debated. Evidence from the Avon Longitudinal Study of Parents and Children birth cohort suggested that residential greenspace has a positive effect on lung function levels at 8, 15, and 24 years old [41], but a study of 3,428 Norwegian and Swedish participants showed that increased normalized difference vegetation index was associated with lower FVC [42]. Boeyen et al. observed no significant association between surrounding greenness and lung function among 360 school children from Western Australia [43]. We found an inverse “V-shaped” relationship of park green space area with FVC in the whole population and males, but nearly “N-shaped” relationship in the females. Different measurement methods of greenness may produce different results. Increasing in park green space led to improvement in air quality, and increased levels of biodiversity and physical activity [44]. On the other hand, allergic risk increased with green space area increase due to pollen plants. As known, females have smaller lung size, narrower airway diameter, and shorter expiratory time constant than males. Sex difference in the relationship of park green space with FVC may result from sex difference in physiological characteristics of lung. Interaction of these factors has contributed to the inter-provincial FVC variation in China. Thus, implementation of regional policies aimed at reducing unfavorable environmental and economic disparities is a necessary condition for improving lung function.

Present study had a number of notable strengths. Firstly, it stood out as the first study to examine trend and heterogeneity in FVC among Chinese students from 1985 to 2019, a 35-year period marked by rapid urbanization and industrialization in China. Notably, nationally representative survey with large sample size improved the credibility of our findings. Secondly, we applied SITAR model to investigate the age and velocity of FVC spurt, making it the first to undertake such an investigation among children and adolescents. Lastly, this study considered not only the latest age-specific FVC level of Chinese Han students but also 26 minorities, thereby enriching known evidence. Our study also had following limitations. Firstly, CNSSCH was a cross-sectional study rather than a large, high-density cohort study. To some extent, our findings reflect long-term trends that cannot be accurately measured [17], but it can help to study FVC growth and development in Chinese children and adolescents. Secondly, types of spirometers used for the eight surveys of FVC were not the same, but strict quality control was performed, which would not hinder the evaluation of long-term trend in FVC. Thirdly, although FVC is an objective measure, it may be influenced by subjective factors, like degree of forceful exhalation and posture. However, CNSSCH technicians were well trained and followed a uniform standard, and acceptable range of instrument errors ≤ 3%. Finally, use of regional-level exposure data in our study may introduce ecological bias, and effects of environmental factors on children’s FVC need further study based on individual exposure data.