Association Between Lung Function of Children and Their Socioeconomic Conditions: A Systematic Review

Background

Rationale

The importance of socioeconomic status (SES) in determining lung function highlights broader social and economic factors that can affect lung health. Social class or economic status is linked to lung health in most existing studies, and studies suggest that individuals with lower socioeconomic status have underdeveloped lungs, are more likely to develop lung diseases later in life, with poor lung functions.¹

A number of factors may contribute to this association, such as limited access to healthcare, exposure to environmental pollutants, unhealthy living conditions, and lifestyle factors associated with lower SES.² Considering that lung volume increases from birth until early adulthood, studying this developmental period allows assessment of inequalities in lung function attained.³ It is important to address these factors during these formative years because the respiratory system is vulnerable to adverse influences, such as pollution, tobacco smoke, and poor nutrition. These influences may impact lung development, modulate lung function, and contribute to the development of airway diseases during this critical period.⁴

The association between disadvantaged socioeconomic conditions and poorer lung function attainment among adults has been explored in various studies^5–8 and presents a varied picture of the magnitude and direction of this association. There is an observable lack in this research topic with particular focus on children. The goal of systematically reviewing the impact of early life socioeconomic circumstances on lung function in children, stratified by various socioeconomic (SE) factors was to highlight its important implications for public health. Interventions targeted at reducing exposure to environmental pollutants, promoting healthy lifestyles, and addressing socioeconomic disparities during childhood and adolescence can have long-lasting effects on respiratory health into adulthood. Understanding these associations is essential for public health initiatives aimed at improving lung outcomes and preventing lung diseases.

Objectives

Thus this study aims to evaluate this association through a PRISMA⁹ guided systematic review, by providing a comprehensive overview of the existing evidence. The purpose of this study was to review the most recent medical literature addressing the relationship between SES and lung function in children below 18 years, to provide updated evidence of the association by synthesizing data from multiple studies, potentially revealing patterns, trends, and the overall strength of the association between socioeconomic circumstances and lung function. Do lower SES factors associate with poor lung function for children below 18 years was the research question for this systematic review. P (Population), I (Intervention), C (Comparison), and O (Outcome) (PICO) of this aspect is covered in the Table 1.

Table 1 The PICO Matched Inclusion and Exclusion Criteria for the Study Selection.

Methods

Protocol and Registration

This review adheres to the PRISMA guidelines and is registered with the International Prospective Register of Systematic Reviews (PROSPERO) (registration number CRD 42020197658). The institutional review board ruled out the need for an ethical approval for a systematic review.

Search Strategy

A well-structured search strategy tailored to address the objectives of this study was adopted to browse the relevant electronic databases from 2014 to 2023. A broad spectrum of literature on the topic was captured by incorporating a range of relevant PICO format and Medical subject heading (MeSH) terms and keywords related to lung function, spirometry, socioeconomic factors, type of study, time frame, and demographic descriptors. Table 2 is descriptive of the databases accessed and the search strategy followed in this study.

Table 2 Keyword Strategy Used in the Database Search.

Inclusion and Exclusion Criteria

Inclusion criteria for the studies were: (1) Studies comparing higher and lower socioeconomic class, rural and urban population with any methodologies of clinical trials, cross-sectional, longitudinal retrospective, observational studies and case series. (2) Studies that report lung function with at least one spirometry value (eg forced expiratory volume in the first second (FEV₁); forced vital capacity (FVC); ratio between FEV₁ and FVC, FEV₁/FVC; forced expiratory flow (FEF)).

Studies reporting outcomes of lung function correlated to at least one socioeconomic indicator (eg parent’s education and occupation, family income, nutritional status, etc). Studies on adults, studies on general population, other health and genetic factors, animal studies, narrative review letters to editor, editorials, commentaries, and abstracts only available were excluded. Table 1 is descriptive of the eligibility criteria designed for this study.

Data Extraction

The data extraction was planned to cover all the key information essential for the thorough understanding of the studies included, ensuring a comprehensive and transparent synthesis of the included studies. This study adopted a robust screening process to systematically review and select relevant articles that align with the research objectives of the study. The assimilated articles were screened for all titles, abstracts and keywords and those articles clearly failing to meet the inclusion criteria were removed. The reference lists of the reviewed articles were also further screened for potentially relevant articles that the electronic search failed to identify. An in-depth evaluation of the full texts was done for a closer examination of the content and methodology of each article against the pre-defined criteria for eligibility. A further reassessment of the selected articles was done to ensure their adequacy for data extraction by confirming that the necessary information and data are present. The data extraction design and plan is described in Table 3.

Table 3 Data Extraction Plan Adopted in the Study

Quality Assessment and Risk of Bias

The Newcastle–Ottawa scale (NOS)¹⁰ is a widely used quality assessment tool ideal for non-randomized studies, providing a standardized method to evaluate the quality and potential biases of the included studies. The NOS scoring involves assigning a star rating or a grade based on the quality of each study. The scale ranges from zero to eight for cohort studies and from zero to six for cross-sectional studies. Any potential bias for selection of participants, comparability and measurements are covered by this comprehensive assessment. The results of this quality assessment of the included studies are expressed in the results with ratings assigned to each study, providing a clear summary of the overall quality assessment and risk of bias among the included literature.

Data Analysis

The approach to summarize the extracted information was focused on both the direction and magnitude of the association between socioeconomic indicators and lung function indices, aiming to capture a nuanced understanding of the relationship. A positive association was understood when the advantaged socioeconomic circumstances were associated with an increase in lung function and the disadvantaged socioeconomic circumstances were associated with a decrease in lung function. The negative association was acknowledged when advantaged socioeconomic circumstances were associated with a decrease in lung function and the disadvantaged socioeconomic circumstances were associated with an increase in lung function. A summarized presentation of all the stay data assimilated in this systematic review is shown in a tabular form in the Results section.

Main Outcome

Most available evidence suggests that FEV₁ is by far the most reported index in medical literature as it provides information on airflow based on airway caliber and elasticity.¹¹ The main outcome was to evaluate the association between the most reported and common SES factors across the included studies (like parent's education, parent's occupation, family income, nutritional status of participants, participant’s neighborhood) and lung function of children below 18 years of age. This was evaluated by undertaking logistic regressions to evaluate the association between the SES indicators and lung function from the reported FEV₁ adjusted for the following SES indices: parent's education, parent’s occupation, family income, nutritional status of participants, housing conditions, and participant’s neighborhood. The Results section has a tabular description of this analysis. Owing to the heterogeneity in the studies analyses and reported outcomes, a full-fledged meta-analysis was not considered.

Results

Study Selection

This systematic review search ended with 20 studies^12–31 included from an initial hit of 513 studies. Following the removal of duplicates, 290 studies were screened (titles and abstracts) and 187 articles were excluded and only 103 were assessed for full eligibility, leaving 20 studies to be included in this systematic review. The PRISMA flow diagram of study selection procedure is shown in Figure 1. A total of 89,619 participants were included from these included studies and Table 4 provides the summary of the attributes included in the studies. The review was summarized narratively after the explicit quality assessment process. Statistical pooling of all the lung function results matching to each SES factor in the study from the included studies was not practical due to the myriad of difference in their methodologies and reporting measurements. This highlights a key research gap in the standardized SES indices measurements and unified reporting which inhibits a better comprehension of the key mechanism of lung function over time related to SES factors.

Table 4 Characteristics of the Included Studies and the Data Analyzed

Figure 1 PRISMA flow diagram of the literature search. Adapted from Page M J, McKenzie JE, Bossuyt PM et al . The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ. 2021; 372. Creative Commons.9

Quality Assessment and Risk of Bias

The methodological quality of the included articles are shown in Table 5. Among the included studies, two studies were scored less due to the inadequate follow up of cohorts. The publication bias was not feasible as the studies included had a high level of heterogeneity from the different methodologies and reported outcomes.

Table 5 Scores Based on the Newcastle–Ottawa Scale for Assessing Methodology Quality

Participant Details

Samples sizes ranged from 90²³ to 54,904³¹ participants with ages ranging from 5 to 18 years, and the demographic of the samples are briefly described in Table 5.

Synthesis of Results

Table 5 gives the characteristics of the included articles, seven^{14–18,23,28} longitudinal and tthirteen^{12,13,19–22,24–27,29–31} cross-sectional studies. Due to the highly heterogeneous nature of the included studies a meta-analysis was excluded and the study therefore progressed with a tabular synthesis. From the 20 articles incorporated in this review, most studies considered parent’s education and family income as the socioeconomic indicators. Different strata of SES measurement were identified: individual, family, neighborhood, and school. At the individual level, SES was collected by self-report and was represented by proxies such as maternal and paternal education level and nutritional status. The family level included household economic resources, sibling size, taking into consideration multiple or all family members. Neighborhood-level measures encompassed a particular geographical area and were usually derived from census data, including measures of health, income and education.

Main Outcome

The main outcome of interest was FEV₁ and it has been the most widely reported index of lung function in the included studies.^12–31 Moreover, it allows the determination of the FEV / FVC ratio, which is used to detect the presence of airway obstruction and to diagnose lung diseases. All the included studies reported estimates for FEV₁, either as mean values of volume,^{12–14,16,17,20,21,23,25,26,28,30} mean difference,^{15,18,19,22,24,27,31} or z-scores.²⁹ Analysis using multivariable alternating logistic regression showed that the odds of having reduced lung function from the observed FEV₁ spirometry values from the included studies was positively associated with low socioeconomic status indicators like low family income, low parental income, crowded housing, smoking habits, and poor neighborhood as described in Table 6. A positive association between the odds of lower respiratory function and a lower SES composite score, with an interquartile OR of 1.67 (95% CI 1.03–1.3) was observed. Overall, children, adolescents and young adults from disadvantaged socioeconomic circumstances presented with significantly lower FEV₁ liters when compared with those from advantaged socioeconomic circumstances.

Table 6 Adjusted Logistic Regression Model for FEV1 (L) Based on SES Indices, from All the Included Studies

Discussion

This study systematically reviewed the evidence on the association between socioeconomic circumstances and lung function in children, adolescents and young adults considering sex differences. From the 20 papers included, 16^{12,15–17,19–27,29–31} showed that lower socioeconomic circumstances were positively associated with lower lung function in early ages, while four studies^13,14,18,28 reported no association between SES and lung function in children of lower SES. Despite prior evidence suggesting that boys of different age groups experienced greater socioeconomic inequalities,^32–34 anthropometric differences did not adequately explain the gender differences caused by socioeconomic differences.

Historically, spirometry assessments have been used primarily to assess lung function in patients with respiratory symptoms, asthma, and cystic fibrosis.³⁵ Its use in pediatric and adolescent patients to monitor lung growth has been less explored to monitor asthma and chronic obstructive pulmonary disease which are the greatest burden on patients and society.³⁶ Tracking lung function in healthy children during this period may also enable the detection of early life differences in lung growth and lung function attainment, both of which may have clinical significance for future management of pediatric lung diseases.^37–39

This systematic review, concludes that lower SES was associated with higher odds of having a reduced lung function. Income, neighborhood and housing are usually underestimated variables in most study populations, a composite score including the family income and parent's education may be better proxies of SES for this review analyses. Overall, the relationship between a low respiratory function and SES is highlighted in this study to guide future investigations and implement interventions to reduce the global burden of lung and respiratory burden among the pediatric population.

Strengths and Limitations

The inclusion of a range of study designs, involving multiple databases by a robust search strategy, and a thorough search of the literature strengthened the scope of this review. However, this study does possesses some limitations inherent to a systematic review which is acknowledged here.

The use of appropriate reference values is crucial for interpreting spirometry results, which was mentioned only by a third of the included studies. Indicators of socioeconomic status were highly variable across the included studies, which contributed statistically significant differences which are hard to detect and non-feasible to analyze. Since most the included studies presented different estimates of FEV₁ (mean values, predicted values, percentages, z-scores), and the statistical analysis was heterogeneous, it was difficult to compare them quantitatively. This created a potential source of selection bias and thus a possibility of a meta-analysis was waived.

The studies included in this qualitative syntheses were mainly cross-sectional studies (n = 13) compared with longitudinal studies (n = 7). Longitudinal studies have an advantage as these studies collect data over time and are more appropriate for assessing causal relationships, thus in this context are expected to show a higher effect of disadvantaged socioeconomic circumstances on lung function. However, both cross-sectional and longitudinal studies from the included studies showed relatively similar effect sizes. Further, since both exposure and outcome are measured at an early age, it can be speculated that effects are not yet completely established, and perhaps if outcomes are measured at adulthood, the differences would be more pronounced.

Conclusions

It is noted in this systematic review that children, adolescents, and young adults from lower SES present with lower lung function. In children of all ages and in countries with markedly different living standards and environmental exposures, their SES is associated with reduced lung function in both boys and girls. In spite of the fact that it is unclear how poverty in particular and lower SES in general impact lung function, there are some commonalities between economic conditions and known lung health risks. A lower SES thus has critical implications for the health of the lungs early in life. Health policy makers should be able to utilize this evidence to tackle lung health in equalities at a young age by explaining the social patterning of lung diseases during adulthood and at older ages.

There are multiple mechanisms through which SES impacts health outcomes, as evidenced by a myriad of gradient variations.⁴⁰ The lower the SES, the lower the birth weight, the poorer the nutrition, the greater the likelihood of physical abuse, and the fewer the opportunities for quality education.^41,42 This correlation makes it imperative to measure SES robustly, identify the mechanisms by which SES affects child health outcomes, and design interventions and/or policies that target modifiable SES mechanisms.