1. Introduction
Scholars addressing child and adolescent developmental issues globally have paid substantial attention to the related concepts, antecedents, intervention programs, and consequences [
1,
2,
3,
4]. Considering their pervasiveness and negative consequences for personal growth and societal processes, it is vital for various actors, including governments, academia, school educators, and parents, to put more effort into preventing such developmental issues. In particular, researchers need to understand the underlying factors and mechanisms in order to explain different developmental issues among such age groups.
In line with the predictions of ecological system theory [
5], the environment can have a paramount influence on the development of adolescents. While the notion of the environment is largely treated as a social condition in the field of psychology, a handful of studies have investigated the physical environment or physical context [
6,
7], i.e., the “physical properties of the settings and objects people interact with” [
8], and argued for its association with individual human development. Air quality, for example, has emerged as a critical physical element, although sometimes as a distal factor of the physical surroundings, that influences the mental health, capacity development, and behavior patterns of adolescents.
Air pollution, a byproduct of the process of industrialization and urbanization in most countries, turns out to be a major cause of the deterioration of human beings’ quality of life. Various statistics show that exposure to toxic air may lead to acute and long-term health problems, such as heart disease, stroke, lung cancer, asthma, and other respiratory issues, for both adults and children [
9,
10]. It also affects human beings in the form of lowered cognitive performance, defensive or unethical behavior, or other spill-over behaviors [
11]. In addition, extant studies in other disciplines, such as economics and business management, focus on the impact of air pollution on individuals’ productivity and investment decision-making [
12,
13,
14,
15]. Overall, air pollution has been shown to be a major risk factor for human beings. More recently, some scholars have focused on whether people exposed to high air pollution, including nitrogen dioxide (NO
2), sulfur dioxide (SO
2), particulate matter 2.5 (PM
2.5), ozone (O
3), etc., suffer from psychological distress [
16], greater unhappiness [
17,
18], anxiety or depression symptoms [
17,
19,
20,
21], or other negative mental health issues [
22,
23]. For example, life dissatisfaction, hedonic unhappiness, and depression are found to be statistically associated with air pollution in multiple cities in China [
18]. Therefore, such emotional states are indispensable factors for understanding the role of air pollution on human beings. Nevertheless, most of these studies investigate older adults as their subjects, while studies with a close-up focus on adolescents are still emerging across different institutional contexts.
The studies on the impact of air pollution on adolescent development are also highly focused on physical health outcomes and cognitive development, including children’s hospitalization, learning abilities, school absence, or campus violence [
24,
25,
26,
27,
28,
29]. Similar research focuses exist in studies using a sample of Chinese students [
30,
31]. Nevertheless, these related studies examined children’s school attainment or test scores as the outcome variables, while paying insufficient attention to how air pollution affects their emotional state and, eventually, their development (i.e., behavioral or capacity outcomes) in a full picture.
By applying existing knowledge about environmental psychology [
32], we aim to fill this research gap, and argue that emotional state is the underlying mechanism that explains the impact of air quality on adolescent development. We investigate this association in a longitudinal sample group comprising adolescents in grades seven to nine in Shenzhen, China, from the years 2016 to 2018. In particular, our research question investigates the impact of air pollution, a reliably recorded environmental stressor, on the emotional status and development of adolescents. To further delimitate the specific mechanisms to explain such associations, we use several key dimensions of emotional disorders (e.g., anxiety, neuroticism, withdrawal motivations, etc.) that are particularly related to adolescent development [
33,
34], and find them related to behavioral changes as the main mediation mechanism. These effects hold after we control for major confounding factors at different levels.
By exploring the effects of air pollution on the same student cohort in terms of their emotional development, as well as behavioral and cognitive outcomes, we aim to contribute to the theory and practice in several aspects. First, we contribute to the extant research in the field of environmental psychology, as we can precisely identify both the relationship between air quality and adolescent’s development and the triggering mechanisms of emotional disorders. Indeed, an in-depth understanding of adolescent development should encompass elements including emotions, internal attributes, behavior patterns, and capacity-building. Second, unlike the major studies in this area [
17], this paper uses a unique longitudinal design that tracks changes and associations between air quality, emotional status, and development of the same group of adolescents. Without a longitudinal study to identify the continuous relationship among the variables, our suspicions of a relationship between air quality and child development would remain inconclusive and underdeveloped, and incapable of providing any normative suggestions to practitioners. Third, in practice, our research question is both relevant and timely, as adolescent development is key for society’s human capital attainment, and this is particularly true for developing countries in Asia, where sophisticated prevention and intervention systems are still lacking [
1]. By identifying the relationship between air quality, emotional status, and adolescent development, this longitudinal study hopes to move forward the related policy making process.
4. Discussion
Our research findings are largely consistent with previous findings of the negative impact of air pollution on children’s development. By adding to the literature on whether air pollution affects adolescents’ physical health status, cognitive performance, and school attendance [
17,
29,
30], with this study we have made our first theoretical contribution by focusing on the impact of the physical environment, and specifically air quality, on a much broader concept of youth development, the PYD. We found that exposure to more severe air pollution was associated with lower PYD. The results were robust with the analyses conducted for both the aggregated PYD measure and individual ones. Therefore, it provides substantial support for the notion that air pollution is a substantial risk factor for the development of adolescents. In particular, adolescent development should be perceived as a total concept, incorporating elements regarding adolescents’ behavioral competence, identity, prosocial attributes, and other general qualities. In addition, our robustness check shows that the impact of air quality on adolescent development holds for the whole time period of three years, and for each individual year. Therefore, toxic air is obviously a fundamental threat to the growth of adolescents.
Our second theoretical contribution shows the specific mechanisms of how air pollution eventually makes adolescents vulnerable to such an adverse problem; therefore, it provides a complete framework to understand the exact paths that relate air pollution to adolescent development. Furthermore, the longitudinal design of this study with lagged effects of air quality on emotions and development reminds us of the extended influence of air quality. Overall, the empirical findings verify that school children are more likely to suffer emotional disorders due to exposure to air pollution, which further leads to deteriorated development. For example, we demonstrate that air quality may trigger emotional disorders in students, such as symptoms of anxiety, neuroticism, and withdrawal motivations, which further leads to the low PYD. This is largely consistent with the current literature [
19,
20,
21]. We posit that anxiety is a natural reaction to toxic levels of pollution, which may make adolescents unsatisfied with the current situation, lower their subjective well-being, and make it difficult for them to sustain their personal development potential.
Interestingly, the current empirical findings do not show a positive relationship between air pollution and depression in our participants. This may be because they are adolescents. Usually, depressive symptoms as a result of adverse air quality are more prevalent in the elderly, as shown in previous studies [
23,
38]. In contrast, the emotional disorders of adolescents who are at the early stage of their lives are more likely to be reflected as anxiety toward the future, social isolation, or withdrawal intentions from others, as well as neuroticism with unstable moods. Indeed, it is particularly valuable for academia and policymakers to investigate the negative effects of air pollution on an array of populations, and develop corresponding prevention mechanisms effectively.
It should be noted that China is a less aged country, with one fifth of the population consisting of children who are less than 18 years old. Undoubtedly, such a negative impact of low air quality may lead to major social costs for adolescent development resulting from high expenditures in the public health service and undermined human capital development for the country. For example, it was estimated that air pollution leads to annual economic losses that amount to 1% to 3.8% of GDP, considering the two factors of direct medical costs and willingness to pay [
39]. It will eventually cause more personal suffering and economic problems that are a substantial burden to the society on a global level. Essentially, the burgeoning industrialization process over the past forty years in China is as double-edged sword; that is, both economic development and ecological degradation are intertwined, which requires urgent solutions.
We recognize that our future research can be improved in several aspects. First, the findings may be constrained by the single research site of Shenzhen, which is often perceived as one of the cleanest cities in China. Nevertheless, given the uneven distribution of environmental conditions in China, the overall interpretation should be cautious as the adverse association between air quality and students’ emotional health and development may vary in other areas. A comparative study including adolescents from other places or countries is warranted [
40]. Second, while our current focus is the composite measure of air pollution, AQI, additional analyses of other indicators are required to show if there are any individual effects on adolescents that are hidden by our combined effects. In addition, we also need to control for potential environmental confounders, such as daily temperature, relative humidity, and duration of sunlight. Future research needs to assess such variance across different pollutants and health outcomes. Fourth, our results must be interpreted with caution, as there are always confounding factors related to potential residual and unmeasured errors.
This study has several theoretical and practical implications. First, our research question is highly relevant, as ambient air pollution has become one of the major environmental hazards to health in the last decades, including damage to the brain and respiratory system, cognitive performance, and with cardiovascular effects [
38,
41,
42,
43]. Therefore, we aim to bring the attention of academic researchers and related parties in the adolescent development setting (i.e., children, parents, and schools) in order to prevent and minimize such damage. Secondly, government, as policymakers, needs to play an important role in advancing air quality governance and taking effective actions to combat it. They need to fully understand whether there is a causal impact of air pollution on human health issues, as found in the current study. In addition, as air quality may further affect individuals’ participation in economic activities, productivity, and the future economic development of nations, policy makers need to formulate numerous policies and incentive schemas to curb different sources of air pollution. Third, players in the private sector and non-profits can also take collaborative efforts and focus on how to leverage resources from multiple sources to prevent the occurrence of more air pollution, which will benefit adolescent development in the long run.