Association of Exposure to Hydrocarbons Air Pollution with Incidence of Atopic Dermatitis in Children

There is growing evidence that air pollution may act as an important environmental risk factor in the development and aggravation of childhood atopic dermatitis (AD). Methods: We collected data from the Taiwan National Health Insurance research database and linked the data to the Taiwan Air Quality-Monitoring Database. Children younger than 18 years old between January 1st, 2000 and until the diagnosis of AD was made, or December 31st, 2012, were selected from the database. We measured the incidence rate and hazard ratios for AD, and stratied by quartiles (Q1-Q4) of air pollutant concentration. Multivariable Cox proportional hazards models were also applied by adjusting for age, sex, monthly income, and level of urbanization. Results: Compared with those exposed to the concentrations in the Q1 quartile, the adjusted hazard ratio (HR) for AD increased, and total hydrocarbon (THC), non-methane hydrocarbon (NMHC), and methane (CH 4 ) exposure concentrations ranged from 1.65 to 10.6, from 1.14 to 2.47, and from 1.70 to 11.9, respectively. Patients exposed to higher levels of THC, NMHC, and CH 4 had greater accumulative incidence rates of childhood AD.


Introduction
AD is a common chronic relapsing in ammatory skin disease associated with intense itching and recurrent eczematous lesions [1] . In a process referred to as the "atopic march", AD is usually an early sign of other subsequent allergic disorders [2]. Up to 80% of children with AD, particularly those with early sensitization and severe disease, will eventually develop allergic rhinitis or asthma later in childhood [3]. AD most commonly begins in early childhood. Approximately 15% -30% of children are affected worldwide, and approximately 85% of all cases begin before ve years of age [4]. It often persists into or begins in adulthood, in uencing about 10% of adults [5]. AD obviously in uences patients' quality of life and creates nancial implications. Itching and scratching are the two main symptoms that affect the quality of life (QoL) in childhood AD, impacting on the quality of sleep, and requiring a treatment regime, affecting the ability to do sporting activities, and social embarrassment [6]. The 2006 US report from the American Academy of Dermatology, the most comprehensive contemporary research on the economic impact of AD, reveals that the total annual burden of AD was $4.228 billion. AD was the fth-highest overall cost among all the skin diseases in the US, placing a tremendous nancial burden on society [7]. Hence, it is critical to identify and control risk factors in susceptible subjects for successful treatment and prevention of childhood AD.
Over the past 30 years, the prevalence of AD has increased considerably worldwide, particularly in industrialized countries [4]. Although both genetic and environmental factors are involved in the etiology of AD, the recent increase in the prevalence of AD is mainly attributed to environmental factors [8]. There is growing evidence that air pollution may act as an important environmental risk factor in the development and aggravation of childhood AD [9][10][11][12]. A variety of air pollutants, such as particulate matter (PM), nitrogen oxide compound (NO x ), environmental tobacco smoke (ETS), tra c-related air pollution (TRAP, including PM, NO, NO 2 , SO 2, CO, CO 2 , O 3 , etc.) as well as volatile organic compounds (VOCs), have been mentioned in recent cross-sectional and birth cohort studies [8] [9]. Based on the pathogenesis of AD, including skin barrier defects and immunologic dysregulation, these air pollutants probably evaporate from the surface of the skin and bind to the stratum corneum, leading to alterations in the skin microbiome, stratum corneum pH, and trans epidermal water loss (TEWL). The compounds may also tend to penetrate into the epidermis, induce oxidative stress, and activate aryl hydrocarbon receptor (AhR), which induces an in ammatory cascade in the skin [4,13,14]. For example, TRAP, especially ozone (O 3 ), has been observed to alter the resident skin ora and cause predisposition to S. aureus colonization [15]. Other dust particles and diesel exhaust particulates have also been demonstrated to exert toxicological effects on human skin [16].
Although several studies support the development or aggravation of childhood AD with air pollutants, currently available evidence on skin aspects of air pollution remains relatively scarce in contrast to airway diseases such as asthma [17]. There are still limitations in previous studies, including inaccurate study design and assessment, and the presence of confounding variables (e.g., obesity, genetics, and comorbidities). For instance, several studies have considered mixtures of substances such as environmental tobacco smoke, VOCs, and NO x , which may lead to a combined impact on human health. There was also selection bias by potential misclassi cation in some cross-sectional studies because the diagnosis of AD was based simply on reports from the patients or their parents and was not con rmed by a physician [8] .
In this study, we focused on the association between hydrocarbons and the development of childhood AD. THC, which are organic chemical compounds consisting of NMHCs and CH 4 , are responsible for approximately 85% of global energy consumption due to rapid industrialization and urbanization. Whether the air pollutants released during the combustion of hydrocarbons, particularly CH 4 , affect the body's largest organ, the skin, remains to be discussed. Hence, to evaluate the effect of exposure to these air pollutants on the risk of AD in children, we conducted this nationwide and retrospective study from real-world data in Taiwan.

Data Source
We conducted a retrospective cohort study using the Children File, a representative database including data from half of all children randomly selected from the year 2000 registry of bene ciaries of the Taiwan National Health Insurance Research Database (NHIRD). The NHIRD was established in 1995 and covers more than 99% of the total population in Taiwan (http://www.nhi.gov.tw/english/index.aspx). It contains all medical records, including de-identi ed demographic information e.g., sex, birth dates, occupation, and place of residence, and clinical information e.g., diagnostic codes based on the International Classi cation of Disease, 9th Revision, Clinical Modi cation [ICD-9-CM], health management, and treatment. Because all the research data were anonymized and encrypted to protect the individual's privacy, consent was exempted in this study. Our study was approved by the Institute Review Board of China Medical University Hospital (CRREC-103-048) in accordance with the Helsinki Declaration.

Study Population, Outcome of Interest, End-Points, and Confounding Factors
In this study, we obtained data from children younger than 18 years, between 1 January 2000 to 31 December, 2012. Candidates with missing data or who had been diagnosed with AD before the baseline, were excluded. Because of the chronic and relapsing characteristics of AD, AD was de ned as at least 3 records of ICD-9-CM codes 691 or 691.8 made by dermatologists or pediatrics in any diagnosis eld during the inpatient or ambulatory claim process, as our outcome of interest. All participants were followed from the baseline until the diagnosis of AD was made, withdrawal from the NHI, or December 31, 2012. In this study, the mean standard deviation (SD) follow-up years in patients with AD was 6.5 (3.39). The confounding factors were age, sex, residing level of urbanization, and monthly income. The level of urbanization was de ned based on population density and was graded into four levels. The highest degree of urbanization was level 1, and the lowest was

Statistical Analysis
The demographic data in our study included age, sex, monthly income, level of residential urbanization, and daily average of exposure to air pollutants. The chi-squared test was used to test the distributed difference among daily average concentrations for each air pollutant by quartile and urbanization. The incidence rate of AD (per 1000 person-years) was counted at four different air pollutant concentration levels. Cox proportional hazard regression models were applied to estimate the hazard ratios and 95% con dence intervals (CIs) for AD in Q2-Q4 levels of air pollutant concentrations compared to the Q1 level. The multivariable model was adjusted for age, sex, monthly income, and urbanization level. We also conducted the Kaplan-Meier method to estimate the cumulative incidence of AD during the follow-up, and the log-rank test was used to test the difference among air pollutant concentration levels. All the data were analyzed using SAS 9.3 (SAS Institute Inc., Cary, NC) and the Statistical Package for the Social Sciences (Version 15.1; SPSS Inc., Chicago, IL). The signi cance level was set at p < 0.05 in all statistical tests.  Table 1. The mean age (SD) of participants was 6.50 years (3.39). The proportion of boys and girls was similar (51.6% vs. 48.4%). Most participants were from the lowest monthly income family (83.4%) and resided in the most highly urbanized areas (33.2%).

Results
In our study, we collected the data of participants under conditions of THC, NMHC, and CH 4 exposure based on the location of the Taiwan Air Quality Monitoring Station. Four concentrations of each air pollutant were categorized by quartiles, ranging from Q1 (the lowest concentration) to Q4 (the highest concentration). Tables 2-4 show the baseline characteristics of candidates exposed to 4 levels of concentrations of THC, NMHC, and CH 4 . Children with the highest exposure concentrations of THC, NMHC, and CH 4 lived in areas with higher urbanization.
The incidence rate for AD increased with THC, NMHC, and CH 4 exposure concentration, increasing from 0.69 to 6.45, from 1.72 to 4.37, and from 0.73 to 7.74 per 1000 person-years, respectively. In the multivariable Cox proportional hazard regression, the adjusted HR for AD increased with the THC, NMHC, and CH 4 exposure concentrations from 1.65 to 10.6, 1.14. 2.47, and 1.70. 11.9, respectively, compared with those exposed to the corresponding concentrations in Q1 level (1.00) ( Table 5).
The Kaplan-Meier plots in Fig 1 demonstrate the accumulative incidence of AD in participants exposed to 4 levels of THC, NMHC, and CH 4 concentrations, respectively. Patients exposed to higher levels of THC, NMHC, and CH 4 had a greater accumulative incidence rate of AD.

Discussion
In this population-based longitudinal study, we demonstrated that Taiwanese children exposed to higher concentrations of THC, NMHC, and CH 4 were at increased risk of developing AD, regardless of adjustment for potential confounding factors such as age, sex, monthly income, and urbanization level. Our cohort study also revealed a clear dose-response relationship between air pollution and AD. Overall, the current study is distinctive in several respects. First, we assessed the real-world data from the Children' s le. Children, one of the most susceptible subgroups of the population due to their immature systems, are undoubtedly more vulnerable to the health effects of air pollution than adults [18]. Second, our AD diagnosis was con rmed precisely by the physician, so the potential for selection bias was minimized. Third, in order to identify the dermatologic effect of a single component, our study could be one of the rst to investigate the relationship between AD and an active greenhouse gas, CH 4 .
Taiwan is located in east Asia, the most polluted region of the world, and is now facing severe air pollution, especially in major urban areas, owing to the rapid increase in population and industrial development, as well as transportation demands [3]. While the number of children with AD continues to increase in both developed and developing countries, the prevalence of AD in Taiwan appears to have grown dramatically over recent decades [19]. According to the Taiwan National Study 2000 to 2007, the overall eight-year prevalence of AD is approximately 6.7%, and has roughly doubled since then [20]. Due to such rapid growth in the number of AD cases with increased urbanization and industrialization, the role of environmental factors, especially airborne pollution, has drawn increasing attention. Over the past ten years, a number of studies have shown that air pollutants, such as PM, TRAP, VOCs, and ETS, are associated with the development and exacerbation of AD. Multiple comprehensive studies have been conducted in the pediatric age group with a large data set. For example, in a French study enrolling 4,907 children who had resided at their current addresses for 3 years or longer, lifetime AD was signi cantly associated with 3-year averaged concentrations of PM 10 , NO 2 , NO x , and CO (adjusted ORs 1.13, 1.23, 1.06, and 1.08, respectively,) [21]. In a Munich prospective birth cohort study including 2,860 children at four years of age demonstrated that NO 2 exposure (per 6.4 mg/m 3 ) was associated with both physician-diagnosed AD and parental reports of symptoms for AD (OR 1.18 and 1.11, respectively,) [22]. In a cross-sectional study during 2011-2012 in Shanghai enrolling 3,358 preschool children indicated that positive correlation between increased gestational and lifetime exposures to a mixture of SO 2 , NO 2 and PM 10 during total lifetime and childhood AD (ORs 1.78, and 1.87, respectively) [23]. In a US National Survey of Children's Health, 91,642 children found that moderate to severe eczema was associated with elevated levels of NO 3 and PM2.5 (OR 1.249 and 1.070, respectively,) [24]. A few studies also revealed that prenatal exposure to VOCs and ETS are likely to induce a TH2-dominant immune status or the development of AD after birth [25][26][27]. In our study, we found that the adjusted HRs for AD increased with the THC (from 1.65 to 10.6), NMHC (from 1.14 to 2.47), and CH 4 (from 1.70 to 11.9) exposure concentrations compared with those exposed to the corresponding concentrations in Q1 level.
Rapid industrialization coupled with urbanization has led to accumulated global waste production because of the continuously increasing demand for energy. Hydrocarbons, organic chemical compounds consisting of hydrogen and carbon, form the basis of the majority of global energy production by fossil fuel combustion and evaporation of gasoline. Both NMHC and CH 4 are composed of THCs. Most hydrocarbons on earth naturally occur from the decomposition of organic matter in petroleum and are generated by human activity. NMHCs, often referred to as VOCs, are unstable forms of substances, such as benzene and their derivatives.
A great number of animal and epidemiological studies have disclosed negative effects of VOCs on skin barrier function. A prospective study in Korea revealed that a 1-ppb increase in outdoor benzene and total VOC concentration were associated with, respectively, 27.38% and 25.86%, respectively, in AD symptoms [11]. Kim et al. also found that exposure to airborne formaldehyde leads to an increase in TEWL and stratum corneum pH both in healthy and AD groups [12]. A rat model of AD conducted by Han et al. showed that formaldehyde exposure aggravated pruritus and skin in ammation. These results suggest that formaldehyde penetrated the injured skin barrier and exacerbated Th1 responses and serum IgE levels in the AD rats [28]. Certain VOCs and polycyclic aromatic hydrocarbons (PAHs) in several previous studies have been proposed to activate the ligandactivated transcription factor AhR, leading to downstream activation of in ammation and itch mediators such as artemin [29,30]. CH 4 , a nontoxic greenhouse gas, is scarcely reported for other adverse health effects of direct exposure, except for high concentrations leading to suffocation. Our study rst identi ed that CH 4 exposure contributes to an increased risk of AD development. One possible reason for this might be that the extra production of CH 4 from rapid industrialization and urbanization contributes to the higher potential for pathogen transmission [31]. Microbial superinfection leads to an exacerbation of AD [1].
Although our study was a large-scale and population-based cohort, there were still several limitations. First, although AD is a complex and multifactorial disorder, we did not consider other environmental factors, including temperature, humidity, and ultraviolet light that might interact with airborne pollutants. Besides, other potential risk factors for AD, such as atopic family history, dietary factors, pet and prenatal exposure, and even severity of AD could not be estimated in this study due to the lack of information in the children's les. Thirdly, according to our results, a total of 7304 children (2.96%) were diagnosed with AD during the study period. The relatively low prevalence might be attributable to the medical records we chose as our database. Patients with mild AD may neither seek medical services nor be coded for the clinical diagnosis of AD by a physician. In other words, our study population was considerably representative of moderate-to-severe AD presentation of intense itching and relapsing eczematous skin lesions for more than six months. Finally, we did not investigate indoor air pollution in our study because children have greater participation in the home environment [32].

Conclusion
In conclusion, our ndings indicated that exposure to higher concentrations of THC, NMHC, and CH 4 might The data were analyzed anonymously and informed consent is not applicable. This study has been approved by the Institute Review Board of China Medical University Hospital (CRREC-103-048) and complies with the principles outlined in the Helsinki Declaration.

Consent for publication:
This manuscript is an original article that has not been previously published and will not be submitted to any other journal. All the authors have read this manuscript and agree that the work is ready for submission, and accept responsibility for the manuscript's contents.
Availability of data and materials: Data available on request due to privacy/ethical restrictions.