Air pollution is one of the biggest environmental threats to human health worldwide. Every year, exposure to air pollution results in the loss of millions of healthy years of life1. Numerous studies have found adverse effects of air pollution on adolescents’ health, focusing mainly on physical conditions such as respiratory symptoms2,3,4. In addition, a few studies have shown that youth who are exposed to worse air quality experience greater fatigue and emotional problems at the time of exposure and later in life5,6,7,8,9. However, prior research has focused on between-subject comparisons10, so these findings may be confounded by many individual differences between youth. It remains unknown if adolescents’ experiences of fatigue and emotional distress fluctuate on a daily level with air quality, within individuals. Such knowledge can provide greater temporal specificity for understanding air pollution as a risk-factor for youths’ emerging emotional difficulties. This within-subject, daily-level study investigated whether adolescents experienced greater fatigue and emotional distress on days that air quality was degraded in LA county, when controlling for between-subject effects. Further, we examined whether air quality was related on a daily level to fatigue and distress particularly strongly among adolescents who experienced greater physical symptoms throughout the year (e.g., headache, backpain). Finally, we considered alternative specifications such as lagged effects, and indirect pathways to shed light on potential directionality.
Air pollution and adolescents’ mood
Air pollution includes gaseous pollutants that circulate in the air—such as nitrogen dioxide (NO2), carbon monoxide (CO), and sulfur dioxide (SO2). These pollutants are emitted from transportation vehicles, coal-driven thermal power plants, industries, indoor heating, and cooking11. Ozone (O3), secondary gaseous pollutant that is formed in the lower atmosphere by chemical reactions, also contributes to air pollution. Although air pollution decreased in the United States (US) from the 1980s until 2016 due to the Clean Air Act (42 US Code 7408-9) which regulated emissions11, air pollution has since increased12.
Adolescence is a sensitive growth period characterized by rapid biological, neurological, and social changes that contribute to heightened susceptibility for the emergence of emotional challenges13. For example, adolescents undergo changes associated with puberty, physical growth, circadian rhythms, psychopathology and daily mood reactivity, all of which impact their cognitive, emotional, social and motivational processes13. Adolescents’ emotional wellbeing is related strongly to their physical surroundings and environments14. Many cross-sectional and longitudinal studies in Asia indicate that youth who are exposed to higher levels of air pollution report greater emotional disturbances such as anxiety and stress8,15. Similarly, studies in the global West have shown that adolescents who are exposed to greater air pollution on average show experience more emotional challenges5,6,9. For example, children in the UK who were lived in greater air pollution at age 10 -12 were more likely to develop depressive symptoms at age 186. This work converges with meta-analytical findings and suggests that exposure to air pollution is a risk factor for adolescents’ emotional health problems10.
The majority of studies linking air quality to fatigue and emotional health have used between-subject analyses10. Although informative, between-subjects analyses offer limited insight because they do not account for many individual differences between youth, even when covariates are included16. Relevant individual differences for the study of air quality and wellbeing include features of the youth’s environment, family life, school life, underlying physiological response patterns, and individual risk for physical and emotional challenges10. For example, families with low socio-economic status tend to live in the more polluted areas10, so adolescents’ emotional challenges could be associated with the underlying financial and social difficulties, instead of air pollution uniquely. To mitigate the confounding from between-subject designs, within-subjects analyses can clarify the link between air pollution and adolescents’ mood by treating individual youth as their own controls, and comparing their experiences and exposures on 1 day to other days. Within-subjects analyses also offer greater temporal specificity, because air pollution may be associated with adolescents’ mood on a daily level, for example within 1 or 2 days. This robust approach requires assessments collected from the same individuals repeatedly over time. Although methodologically intensive, such work can clarify whether adolescents face heightened risk for fatigue or emotional challenges on days—or the day after—they are exposed to greater pollution.
A few studies, mostly from Asia, have demonstrated significant daily-level associations between air quality and human physical and emotional health. In India and China, hospital admissions for asthma and airway obstructions increased on days that air quality was worse17 and on days after air quality was worse the previous day18, even among children19. Similarly, mortality increased on days after air quality was worse in Canada20. Two studies extended this work on physical health to examine daily links between air pollution and adults’ emotional health21,22. In China, adults’ hospital admissions for emotional health problems (e.g., schizophrenia, depression) increased on days that air quality was worse, particularly when levels of NO2 were higher21, and also 7 days later22. However, no known studies have examined daily links between air quality and adolescents’ emotional health, or have addressed daily links between air quality and emotional health in the US context. Given these within-subject, daily links between pollution and adults’ emotional health21,22, and also the previous between-subject, cross-sectional links between pollution and youths’ emotional health19, it is highly likely that pollution is also associated within-subjects on a daily level to adolescents’ emotional health. It is important to extending prior within-subjects research on this topic to the adolescent period, because adolescence is a sensitive developmental transition for the emergence emotional difficulties13. Such work can facilitate and understanding of how to support adolescents’ emotional wellbeing.
Potential mechanisms linking air pollution to mood
Air pollution may impact adolescents’ mood via several biological pathways. One pathway is inflammation, which taxes the body physiologically and can contribute to feelings of fatigue. Air pollution increases humans’ levels of circulating inflammation in the blood, such as C-reactive protein23. In turn, inflammation is robustly linked to heightened symptoms of exhaustion, anxiety and depression in both adolescents and adults24. Specifically, inflammation impacts both the body and brain via multiple different neuroendocrinological and immunological mechanisms that occur simultaneously. For instance, inflammation increases monoamine levels, contributes to dysregulation of the hypothalamic–pituitary–adrenal axis, and activates pathologic microglial cells23. All of these biological pathways are physiologically arduous for the body and can contribute to feelings of fatigue. Moreover, exposure to air pollution also impairs the immunological, neuroendocrine and autonomic pathways which promote healthy physiological homeostasis, rest and sleep25, which can further contribute to fatigue. Accordingly, pollution may “get under the skin” as inflammatory processes that contribute to adolescents’ fatigue by taxing the body and reducing adolescents’ ability to maintain homeostasis and rest adequately23. In turn, fatigue reduces adolescents’ capacity to effectively self-regulate difficult emotions and maintain emotional wellbeing throughout the day26. This research suggests that air pollution may impact adolescents’ mood via an indirect pathway by which youth become more fatigued, which in turn leads to greater emotional problems such as anxiety and depressive symptoms that day or even the following day.
Youth with physical symptoms may be more sensitive to air pollution
Air pollution does not impact adolescents’ moods uniformly. Adolescents who experience higher levels of ongoing physical symptoms (e.g., headaches, stomachaches, shortness of breath) may be particularly sensitive to the daily effects of air pollution on fatigue and emotional health1. Compromised physical health can make individuals more susceptible to physiological stress from the environment27. Further, ongoing physical symptoms associated with health challenges or illness are linked bidirectionally with depression, neuroendocrine dysregulation, and inflammation28. Adolescents who experience greater ongoing physical symptoms face higher risk for emotional problems, and also likely suffer underlying cardiometabolic risks that make their body more susceptible to impairments caused by air pollution (e.g., inflammation, poor sleep, neuroendocrine changes)29. In addition, adolescents who experience more physical symptoms have more compromised immune systems30, which can hinder their ability to ward off and cope with the negative physiological effects of air pollution. For example, children with asthma were more vulnerable to the additional negative physical health impacts of air pollution compared to their non-asthmatic peers31,32. Accordingly, it is likely that air pollution is associated on a daily level with feelings of fatigue and emotional distress more strongly among adolescents who experience relatively higher levels of ongoing physical symptoms compared to their peers.
The goal of this study was to understand if air pollution is related on a daily level to adolescents’ experiences of fatigue and emotional distress. We linked adolescents’ daily reports of fatigue and emotional distress to objective daily, county-level air pollution data collected by the EPA in LA County from 2009 to 201133. All participants lived in LA County at the time they completed daily diaries. We investigated the following questions: (1) When air pollution is higher, do adolescents feel more fatigue and emotional distress the same day and the following day? We operationalized emotional distress as experiences of anxiety, sadness and stress. For air quality, we focused on four major criteria-gas air pollutants regulated by the Clean Air Act: NO2, CO, O3, and SO233. We hypothesized that when air pollution was higher, adolescents would report greater fatigue and emotional distress the same day and the next day. (2) Is air quality related to adolescents’ fatigue and emotional distress the same day and the next day more strongly among those who report higher levels of ongoing physical symptoms that year? We hypothesized that air pollution would be more strongly associated on a daily level with fatigue and distress among adolescents who reported relatively higher levels of physical symptoms that year compared to their peers. (3) Finally, are the daily-level links between air pollution and distress mediated via an indirect pathway through daily levels of fatigue? We hypothesized that daily levels of fatigue would significantly mediate the daily, positive link between air pollution and emotional distress. Since these processes likely occur both within and across days, we tested both same-day and 1-day-lagged mediation models. In addition to these primary analyses, we tested alternative specifications as detailed further below.
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