Poor lung function is a predictor of respiratory diseases, cancer, cardiovascular diseases, and all-cause mortalities [1
]. Studies report that lifestyle-related chronic diseases accounted for two-thirds of all global deaths between 2002 and 2030 [3
], causing significant economic losses [4
]. Lung function depends on several biological factors such as gender, height, weight, and genetic factors [5
]. However, it is significantly affected by socio-economic and environmental factors, such as ethnicity, air pollution, nutrition, physical activity, sedentary behavior, and socioeconomic status (SES) [4
], which are highly correlated with macro-economy levels.
Despite the significant effects of economic development on lung function, studies have not fully explored the relationship between children’s lung function and economic growth. Several studies have mainly focused on the association between individual-level indicator (SES) and respiratory function [7
]. Socio-economic status does not adequately explain vital capacity [7
], because SES does not cover all economic factors. Furthermore, these studies mainly used a cross-sectional design, limiting the correct inferences on causality [11
China is a rapidly industrializing nation [13
] whose economy has grown rapidly over the past 30 years, however, this growth is associated with unique environmental hazards and significant respiratory disease burden [14
]. Notably, only a few studies have explored the effects of economic growth on lung function of children and adolescents over the last 30 years. A national survey conducted from 1984 to 2005 reported continuous decline in FVC of students (7–22 years). Furthermore, a continuous decline in FVC was reported in 2010 [15
]. However, studies have not fully explored why the turning point occurs. Similar to the relationship between income inequality and development [16
], the concept of the Kuznets curve relationship has been recently applied to explore personal health issues [17
]. For instance, previous studies reported an obesity Kuznets curve [18
] and a cardiorespiratory fitness Kuznets curve [20
]. To predict lung function and provide empirical evidence for formulating public health policy, it is important to explore whether there is a Kuznets curve relationship between lung health and economic growth. Currently, no study has explored the association of economic growth with lung function in children and adolescents.
In the current study, data from 1.5 million children and adolescents aged between 7 and 22 years were collected from 27 provinces in China between 1985 and 2014. Data were used to explore lung function status in children and adolescents and its relationship with economic development, height, and weight. The relationship between economic development and lung function was explored after adjusting for anthropometric, provincial fixed effects, time-specific effects, and other factors. The current study provides empirical evidence on the relationship between the economy and children’s lung function in China. These findings are important for policy formulation and development of effective interventions. The findings for the study serve as a reference for similar studies in other developing countries.
3.1. Trends Results
Characteristics of the study population included in the seven surveys are presented in Table 1
. National trends in FVC performance were curvilinear over time, with a gradual decline from 1985 to 2005 and an improved directional shift from 2005 to 2014 (Figure 1
). Students showed the worst respiratory function in 2005. Mean height and weight increased with increase in GDP per capita (Table 1
Mean FVC in urban boys and girls decreased from 3063.6 (SD 1077.1) in 1985 to 2753.7 (SD 1022.7) in 2005 and then increased to 2982.3 (SD 1077.3) in 2014. This trend was consistent for both genders and different age groups in both urban and rural areas. FVC decline in girls was significantly higher in both urban and rural areas from 1985 to 2005 compared with that of boys.
3.2. Linear Analysis
The performance of linear specification between socio-economic indicators in urban and rural areas and lung function of children and adolescents after adjusting for height, weight, age time effect, and fixed effects are presented in Table 2
(columns labeled ‘linear’). GDPPC models showed significant negative coefficients (−0.022 and −0.039, and−0.032 and −0.056) for boys and girls from urban and rural areas, respectively. Coefficients represented the elasticity of FVC to GDP per capita based on the log-log specification. A 1% increase in the GDP per capita was correlated with a decrease in FVC by 0.022% and 0.039% for urban boys and girls, respectively. For example, a 100% (2224 US$
) increase in the mean value of PCDI (mean = 2224 US$
) caused a 2.2% (65 mL) decrease in the mean value of FVC (mean = 2951 mL) in urban boys (p
This finding shows that urbanization ratio had significantly negative linear association with lung function in boys and girls above 30 years old. These findings further show that economic growth had a significantly negative effect on the respiratory function of children and adolescents in both urban and rural areas. Economic growth was associated with significant decrease in lung function in rural children and adolescents compared with their urban peers. In both urban and rural areas, the negative effect of economy on lung capacity of female students was significantly higher compared with that of male students.
3.3. Quadratic Analysis
Analysis using linear models showed a negative correlation between economic growth and lung function, which was inconsistent with the descriptive results. A quadratic model was established to obtain a specification that best represents the data. The quadratic specification results are presented in Table 2
(columns labeled ‘quadratic’). The coefficient on the log of GDP per capita was significantly negative, whereas that on the squared log of GDP per capita was significantly positive for urban full sample (β1
= −0.866 (p
< 0.01); β2
= 0.057 (p
< 0.01)) and rural full sample (β1
= −0.575 (p
< 0.01); β2
= 0.036 (p
< 0.01)). The turning points (exp(−β1
)) were at 1978 US$
per capita for the urban full sample and 2921 US$
for the rural full sample. The turning points τ, that is, the minimum point of the U-curve for urban and rural samples, occurred between 2005 and 2010, as shown in Table 1
Analysis of urbanization ratio indicated that β1
> 0 and β2
< 0, were statistically significant, implying that urbanization ratio exhibited a nonlinear relationship with lung function in all samples. In addition, turning points τ in the U- curve of urbanization ratio with lung function occurred between 2005 and 2010. In urban and rural areas, the estimates based on male and female student samples exhibited a “U” curve (Figure 2
and Figure 3
). The turning point in all models for rural students occurred in later years, compared with models for urban peers.
3.4. Periodical Analysis
To further observe the trajectory of associations, associations for the total sample and separately by sex, region within 10 years (1985–1995, 1995–2005, 2005–2014) were explored (Figure 4
and Figure 5
). For the whole sample, each 1-standard-deviation increase in GDPPC was associated with a 1.27-standard-deviation decrease in lung function of students from 1985–1995. Moreover each 1-standard-deviation increase in GDPPC was correlated with a 0.84-standard-deviation in decrease of in lung function of students from 1995–2005. The negative impact in 1995–2005 was significantly less compared with that from 1985–1995. On the contrary, economic growth was significantly positively correlated with lung function for all sample from 2005–2014. The findings on analyses of urban, rural, boys’ and girls’ samples were similar to those of the analyses of the whole sample. Findings on standardized association of urbanization ratio with lung function at each period were similar with the GDPPC findings. A significantly negative effect of urbanization level was observed between 1985–1995 and 1995–2005, whereas a positive effect of urbanization level was observed between 2005–2014.
Analysis showed a U-shaped relationship between economic growth and lung function by precisely representing the data of each sample. Lung function of students initially decreased with GDP per capita or urbanization ratio until it reached a minimum, after which lung function increased with increase in GDP per capita or urbanization ratio. These findings indicated that the relationship between economic growth and lung function varied in different development stages. Notably, in less-developed provinces, economic growth was negatively correlated with lung function whereas in developed provinces economic growth was positively correlated with lung function.
All regressions shown are adjusted for BMI, age, and provincial and time-specific effects and any time-varying differences common to all provinces. t statistics is shown in parentheses. GDP = gross domestic product.
All regressions shown are adjusted for BMI, age, and provincial and time-specific effects and any time-varying differences common to all provinces. t statistics is shown in parentheses.
The findings of the study show a U-curve relationship between economic growth and lung function of children and adolescents. Lung function of students initially decreased with decrease in socio-economic indicators and then increased with increase in socio-economic indicators. These findings are consistent with the FVC report from 1984 to 2014, which shows a U-shaped trajectory [15
] (Figure 1
). In addition, the findings were consistent with longitudinal analyses between SES and lung function [24
]. Analysis showed a decrease in the lung function of participants in the lowest childhood SES group. The highest group shows increased growth, whereas the middle group shows intermediate growth [24
]. Although several studies report that the socio-economic circumstances are positively correlated with lung function [12
], most of these studies used cross-sectional designs and only explored the effect at the individual level. A previous study reported an inverse correlation between SES and lung function. The study reported that the association between SES and lung function is correlated with the area’s economic level [28
The current study presents several plausible explanations for the FVC–Kuznets curve pattern between economic growth and lung function in Chinese children and adolescents. China has experienced rapid economic development and urbanization since the implementation of the “Reform and Opening-up” policy in 1978. Studies report significant commuter exposures, air pollution, and construction pollutants in the environment [29
]. Several studies report that air pollution is associated with a decline in respiratory function and a higher risk of respiratory illnesses [30
]. Physical inactivity and sedentary lifestyles due to rapid development of the social economy and urbanization are also associated with decrease in lung function [8
]. In addition, a sedentary lifestyle is highly associated with low respiratory muscle strength and breathing difficulties [33
The government and society have taken measures to protect the environment even with socio-economic development [29
]. Environmental policies have accelerated the arrival of the turning point for the whole country despite the low likelihood of the Environment–Kuznets curve turning points being reached soon [34
]. China is now entering an important stage of increased urbanization by optimizing urban landscapes and forms [29
], despite the air quality getting worse in cities with higher urbanization levels [35
]. It is postulated that residing in greener areas is positively associated with better lung function [30
Studies report a positive association between physical activity, physical fitness, and lung capacity [10
]. Physical exercise controls several crucial elements of aerobic conditions. Forceful inhalation and deflation of the lungs strengthens respiratory muscles [36
]. The endurance running level of Chinese urban students between 1984 and 2014 showed a U-shaped trajectory [20
]. In addition, a U-shaped relationship was reported between economic growth and endurance running of children and adolescents between 1985 and 2014 [20
]. Furthermore, dietary factors are potential mediators of the association between economic growth and lung function [37
]. The nutritional status of an ethnically similar population is associated with significant differences in lung function during childhood [38
]. Improving nutrition during growth and development can increase maximal lung function and decrease the risk of adult lung diseases [39
To the best of our knowledge, this is the first study to explore the longitudinal relationship between economic growth and respiratory function of children and adolescents using a large sample size. In the current study, lung function trends were predicted after adjusting for a series of factors, including height, weight, and time trends. The study period coincides with the 30 years of China’s reform and opening up, during which significant unbalanced changes of the socio-economic status occurred. The unbalanced development has led to several health concerns and inequalities in social welfare and income. Health characteristics of children and adolescents have also changed with the increase in socio-economic development. This study provides a basis for governments to formulate policies for distributing welfare payments and providing health education and health services. Moreover, it provides a basis for developing specific interventions that target priority populations, such as children and adolescents from areas with middle and low economies. The study further provides a basis for formulation of policies in other developing countries, including the value of using multiple indicators for tracking social and economic determinants of the respiratory function in children and adolescents.
Although the study has several strengths, such as using tracking data across 30 years spanning major economic change, use of large sample size across multiple provinces in China, and inclusion of several robustness checks, it had a few limitations. First, FVC was determined using different instruments across the 30 years. However, this did not significantly affect the assessment of respiratory function trends over time, mainly over the seven survey cycles. Second, the lung function indicator was specific. However, the main purpose of the study was to track lung function trends. Third, socioeconomic indicators were calculated based on provincial-level data, rather than average individual-level data. However, province-level socioeconomic inequalities reflect the effects of macroeconomic change on individual health outcomes of children and adolescents [40
]. Fourth, data on other factors such as air pollution, physical activity, and socioeconomic status were not collected, which might substantially affect changing trends of children’s lung function. However, these factors might be mediators of macroeconomic effects on lung function and, therefore, do not require moderation [22
]. Finally, the study did not explore the effect of environmental protection laws on the trends of lung function. However, this has little effect on the robustness of the results since several legal regulations were controlled using the two-way fixed effects estimation.