PFAS (per- and polyfluoroalkyl substances) and asthma in young children: NHANES 2013–2014

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a class of persistent chemicals used as industrial surfactants, fire-fighting foams, and textile treatments. Early childhood exposure to perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA), and perfluorohexane sulfonic acid (PFHxS) may affect the immune system to increase the risk of allergic and respiratory diseases. However, there are substantial gaps in our knowledge about the relationship between PFAS and immune-mediated outcomes such as asthma in children. Thus, we examined the cross-sectional associations of serum PFOA, PFOS, PFNA, and PFHxS concentrations with childhood asthma. We used data from children aged 3–11 years who participated in the National Health and Nutrition Examination Survey (2013–2014). Serum PFAS concentrations were measured in serum using analytical chemistry methods. Asthma was assessed by parent-reported, doctor-diagnosed, asthma using a standardized questionnaire. Controlling for covariates, we estimated odds ratios for asthma per standard deviation increase in ln-transformed serum PFAS concentrations (n=607). We also examined effect measure modification by child age, sex, and race/ethnicity. PFOA (1.1; 95% Cl: 0.8, 1.4), PFOS (1.2; 95% Cl: 0.8, 1.7), PFNA (1.1; 95% Cl: 0.8, 1.6), and PFHxS (1.1; 95% Cl: 0.9, 1.6) were weakly associated with an increased odds of asthma. Age modified associations between serum PFOS, but not other serum PFAS, concentrations and odds of asthma (age x PFOS interaction term p-value = 0.03). Sex and race/ethnicity did not modify these associations. We observed some evidence that serum PFAS concentrations were weakly associated with increased asthma prevalence in US children.

1. Introduction

Childhood asthma is prevalent and places a significant burden on children’s health (Centers for Disease Control and Prevention, 2017). According to the United States Centers for Disease Control and Prevention, in 2015 the prevalence of asthma among 5–14 year-olds was 9.4% and medical expenses associated with asthma increased from $48.6 billion in 2002 to $50.1 billion in 2007 (Centers for Disease Control and Prevention, 2017). Identifying modifiable risk factors for asthma could prevent this disease and reduce associated morbidities. Environmental chemical exposures during early childhood are one potential modifiable risk factor that may affect the immune system to increase the risk of asthma.

Per- and polyfluoroalkyl substances (PFAS) are a class of environmentally persistent chemicals used as industrial surfactants, fire-fighting foams, and textile treatments. The strong C-F chemical bond makes PFAS extremely resistant to thermal, chemical, and biological degradation, which results in bioaccumulation up the food chain and persistence in human tissues for years (Buck et al., 2011). Consequently, PFAS have long biological half-lives in humans ranging from 3.8 to 7.3 years (Olsen et al., 2007; Ye et al., 2018). Since PFAS exposure is ubiquitous among pregnant women and children (CDC, 2019; Rappazzo et al., 2017; Romano et al., 2016; Zhang et al., 2018) and results from a recent longitudinal study indicate that serum PFAS concentrations could be higher in children compared to adults (Kingsley et al., 2018), it is possible for PFAS to affect health during multiple developmental periods.

A report by the National Toxicology Program indicated that PFOA (Perfluorooctanoic acid) and PFOS (perfluorooctane sulfonate) are PFAS chemicals which may cause an overreaction of the immune system to induce a hypersensitive state (NTP, 2016). Type 1 hypersensitivity reactions are exaggerated immune responses (i.e. allergic responses and inflammation) to foreign substances and chemicals and may exacerbate or promote allergy-related outcomes such as eczema and asthma (Eder et al., 2006; Ker and Hartert, 2009; O’Connell, 2004). There is some evidence of a relationship between early-life PFAS exposure and asthma in children but the findings have been inconsistent (Rappazzo et al., 2017). Many of these studies relied on prenatal exposures and there are still very few studies examining this exposure-outcome relationship in children under the age of 10 years (Manzano-Salgado et al., 2019; Zeng et al., 2019). As such, there are substantial gaps in our knowledge about the early-life effect PFAS exposure has on asthma in children.

To better understand the potential effects of early-life PFAS exposure on asthma in children, we used data from the 2013–2014 National Health and Nutrition Examination Study (NHANES) to estimate the associations of PFAS concentrations with asthma diagnosis among children ages 3–11 years.

2. Methods

NHANES is a cross-sectional survey designed to measure the health and nutritional status of the US population administered by the U.S. Centers for Disease Control and Prevention, National Center for Health Statistics (NCHS); details of methods and sampling methodology are described elsewhere (Centers for Disease Control and Prevention, 2019). Briefly, surveys are conducted in 2 tandem 2-year cycles which includes interviews, physical examinations, and collection of biological specimens (blood/serum and urine) which can be used for environmental exposure assessment (Centers for Disease Control and Prevention, 2019).

2.1. Laboratory methods

For this study, four serum PFAS (PFOA, PFOS, perfluorohexane sulfonic acid [PFHxS], and perfluorononanoic acid [PFNA]) were quantified, originally for the measurement of cotinine, from a randomly sampled subset (one-third) of children age 3–11 years participating in the NHANES 2013–2014 survey cycle (n=639). These chemicals were not measured in any other NHANES cycle in study participants under 12 years of age (Stein et al., 2016; Ye et al., 2018). PFAS concentrations were quantified in serum samples using solid phase extraction coupled to high performance liquid chromatography-turbo ion spray ionization-tandem mass spectrometry. The limits of detection (LODs) were 0.1 ng/mL for all analytes. Details of the analytical method and quality assurance/QC procedures used are available on the NHANES website (Centers for Disease Control and Prevention (CDC), 2016; Ye et al., 2018).

2.2. Outcome and covariables

Asthma diagnosis was determined by the medical conditions data file (MCQ_H) (Question: “has a doctor or other health professional ever told you that study participant has asthma?”). We considered adjusting for variables associated with childhood serum PFAS concentrations and asthma diagnosis or believed to confound this relationship. Age (in years), sex (male or female), race/ethnicity (non-Hispanic White, non-Hispanic Black, Hispanic (includes Mexican American and Other Hispanic), and Other (non-Hispanic Asian, and other race including multiracial), and income (ratio of family income to poverty (0–5)) were extracted from the demographic variables and sample weights data file (DEMO_H).

Other variables considered including glomerular filtration rate, diet (fish consumption), history of wheezing, eczema, and respiratory infections, history of breastfeeding, parity, marital status, and parental medical history were not available for all study participants in this age group.

2.3. Statistical analyses

All analyses was performed using R software, specifically, the RNHANES, jtools, and survey packages to account for NHANES survey design (R Development Core Team and R Core Team, 2017). We used the published subsample weights (WTSS2YR) designed for the one-third subset of the full survey for all effect and variance estimates to produce estimates that are representative of the U.S. population, as recommended by NCHS. PFOA and PFOS were analyzed as the sum of the respective linear and branched polymers (∑PFOS = Sm-PFOS + nPFOS, ∑PFOA = Sb-PFOA + n-PFOA), We calculated geometric means (GM) for all four PFAS and interquartile range (IQR) for the total number of participants by age groups (3–5 and 6–11 years), sex, race/ethnicity (Hispanic, non-Hispanic White, non-Hispanic Black, and other), and income (> 1, 1 to <2, and ≥ 2). We also calcualated the weighted Pearson`s correlation Coefficients for all serum PFAS log₁₀ concentrations.

Using logistic regression, we estimated odds ratios for current asthma per standard deviation (SD) increase in ln-transformed serum PFAS concentrations after adjusting for child age (continuous), gender (categorized), race/ethnicity (categorized), income (continuous), and serum cotinine concentrations (continuous). Because age, sex, and race/ethnicity may modify associations between serum PFAS concentrations and asthma, we formally tested for effect modification in regression models by adding the interaction term between PFAS and each hypothesized modifier in the model, i.e. Asthma_i = α + β₁PFOA_i + β₂PFOA_i *sex_i + ∑β₃Covariates_i. We used Wald tests to detect the difference in the model coefficient for the PFAS variable between strata of modifier. We also conducted stratified regression analyses by age group, sex, and race/ethnicity. Given that asthmatic children have PFASs measured after asthma diagnosis, we conducted additional sensitivity analyses to address reverse causality. First, we tested if the difference in current age and age at asthma diagnosis was predictive of serum PFAS concentrations among asthmatics. We then adjusted for the difference in current age and the difference in current age and age at asthma diagnosis in a regression model models examining the effect of age on serum PFAS concentrations controlling for the same covariates in our main model.

3. Results

Of the 639 children between 3–11 years old in the NHANES 2013–2014 cycle, 607 participants were included in our analyses after excluding participants with missing data. The mean age of children included in analyses was 7.1 years (Table 1). Among the samples analyzed, there were 324 (weighted 51%) males, 150 (weighted 14%) were Black, and 207 (weighted 25%) were Hispanic. There were 93 (weighted 15%) study participants with doctor-diagnosed asthma of which 14 were under age 6 years (Table 1) and 74 (80%) of all asthmatics were diagnosed under the age of 5-years.

Table 1.

Geometric mean (IQR) serum PFAS concentrations (ng/mL) in children aged 3–11 years according to demographic characteristics (N=607): NHANES 2013–2014.

Characteristics	N (weighted %)	PFOA GM (IQR)	PFOS GM (IQR)	PFHxS GM (IQR)	PFNA GM (IQR)
Overall	607	1.9 (1.4, 2.7)	3.7 (2.6, 5.5)	0.8 (0.5, 1.3)	0.7 (0.5, 1.1)
Asthmatics	93 (13)	2.0 (1.4, 2.6)	4.0 (2.9, 5.6)	0.9 (0.6, 1.4)	0.8 (0.5, 1.3)
Non-asthmatics	504 (87)	1.9 (1.4, 2.7)	3.7 (2.5, 5.4)	0.8 (0.5, 1.2)	0.8 (0.5, 1.3)
Age, years (mean ± sd)	7.1 ± 0.1
3–5	172 (33)	1.9 (1.4, 2.9)	3.4 (2.4, 4.8)	0.7 (0.5, 1.0)	0.6 (0.5, 1.1)
6–11	435 (67)	1.9 (1.4, 2.5)	4.0 (2.8, 5.8)	0.8 (0.6, 1.4)	0.8 (0.5, 1.1)
Sex
Male	324 (51)	1.9 (1.4, 2.7)	4.1 (2.6, 6.3)	0.9 (0.6, 1.5)	0.7 (0.5, 1.1)
Female	283 (49)	1.9 (1.4, 2.5)	3.5 (2.6, 4.8)	0.8 (0.5, 1.0)	0.7 (0.5, 1.1)
Race/Ethnicity
White, non-Hispanic	162 (53)	2.1 (1.5, 2.9)	4.1 (2.7, 6.0)	0.9 (0.6, 1.4)	0.7 (0.5, 1.1)
Black, non-Hispanic	150 (14)	1.5 (1.1, 2.1)	3.4 (2.3, 5.0)	0.7 (0.4, 1.2)	0.7 (0.5, 1.2)
Hispanic	207 (25)	1.7 (1.3, 2.3)	3.4 (2.5, 4.5)	0.7 (0.5, 1.1)	0.7 (0.5, 1.0)
Other	88 (9)	1.8 (1.3, 2.5)	3.4 (2.5, 6.4)	0.6 (0.4, 0.9)	0.8 (0.5, 1.2)
Poverty:Incomea
< 1	239 (31)	1.7 (1.3, 2.4)	3.5 (2.5, 5.3)	0.8 (0.5, 1.4)	0.7 (0.5, 1.0)
1 to < 2	160 (22)	1.7 (1.3, 2.4)	3.5 (2.4, 4.9)	0.8 (0.5, 1.3)	0.7 (0.5, 1.2)
≥ 2	208 (47)	2.1 (1.5, 2.8)	4.1 (2.6, 6.3)	0.8 (0.5, 1.2)	0.7 (0.5, 1.1)

Abbreviations: NHANES: National Health and Nutrition Examination Survey, IQR: interquartile range, GM: geometric mean, PFAS: per- and polyfluoroalkyl substances, PFOA: perfluorooctanoic acid, PFOS: perfluorooctane sulfonic acid, PFHxS: perfluorohexane sulfonic acid, and PFNA: perfluorononanoic acid, SD: standard deviation.

^aRatio of family income to poverty.

The GM and IQR of serum PFAS concentrations (ng/mL) by demographic characteristics are in Table 1. We observed similar GM serum PFAS concentrations according to most demographic characteristics (Table 1). However, GM serum PFOA and PFOS concentrations were higher in White, non-Hispanics compared to the other races/ethnicities and in homes with higher incomes. GM serum PFOS concentrations was also higher in asthmatics (4.0, IQR: 2.95.6) compared to non-asthmatics (3.7, IQR: 2.5–5.4) and in children age 6–11 years (4.0, IQR: 2.8–5.8) compared to 3–5-year-olds (3.4, IQR: 2.4–4.8) (Table 1). All four ln-transformed serum PFAS concentrations were moderately correlated with each other (r=0.33–0.61, all p<0.01), except for correlations between PFHxS and PFNA (r=0.23, p=0.15) (Figure 1). With the strongest correlations observed between PFOA and PFOS (r=0.55) and between PFOS and PFHxS (r=0.61).

Fig.1.

Weighted Pearson correlation coefficients for the (standardized) logarithms of serum PFAS concentrations (ng/mL) in children age 3–11 years (n=607): NHANES (2013–2014). Abbreviations: PFAS: per- and polyfluoroalkyl substances, NHANES: National Health and Nutrition Examination Survey, PFOA: perfluorooctanoic acid, PFOS: perfluorooctane sulfonic acid, PFNA: perfluorononanoic acid, and PFHxS: perfluorohexane sulfonic acid. All p-values are less than 0.001 except between PFHxS and PFNA (p=0.15).

After covariate adjustment, PFOA (OR: 1.1; 95% Cl: 0.9, 1.4), PFOS (OR: 1.2; 95% Cl: 0.8, 1.7), PFHxS (OR: 1.1; 95% Cl: 0.9, 1.3), and PFNA (OR: 1.1; 95% Cl: 0.8, 1.6) were not significantly associated with an increased odds of asthma (Table 2). Sex and race/ethnicity did not modify the association between serum PFAS concentrations (interaction term p-values=0.170.92) (Table 2). However, age did modify associations between serum PFOS concentrations and odds of asthma (age-by-PFOS interaction p-value=0.03). Specifically, for every SD increase in serum PFOS concentrations, children aged 3–5 years had a 70% increased odds of asthma (OR: 1.7; 95% Cl: 1.0, 3.0) compared to a 10% increased odds of asthma in children aged 6–11 years (OR: 1.1; 95% Cl: 0.7, 1.6) (Table 2). Children aged 3–5 years also had a 60% greater odds of asthma compared to older children with increasing serum PFOA concentrations (OR: 1.6; 95% Cl: 1.0, 2.7 vs. 1.0; 95% Cl: 0.7, 1.3) as well; however, the age-by-PFOA interaction p-value was non-significant (p=0.47). The difference in current age and age of asthma diagnoses was not correlated with serum PFAS concentrations (Tables S1). Additionally, there were no appreciable differences in serum PFAS concentrations according to age or the difference between current age and age of diagnosis among asthmatics after controlling for covariates (Table S2).

Table 2.

Unadjusted and adjusted ORs for parent-reported asthma diagnosis among children aged 3–11 years per SD increase in ln-transformed serum PFAS concentrations (n=607): NHANES (2013–2014)

	PFOA	p	PFOS	p	PFHxS	p	PFNA	p
Unadjusted	1.0 (0.8, 1.3)		1.2 (0.9, 1.8)		1.2 (0.9, 1.5)		1.1 (0.8, 1.6)
Multipollutanta	0.8 (0.5, 1.2)		1.2 (0.7, 2.1)		1.1 (0.9, 1.4)		1.1 (0.8, 1.6)
Adjustedb	1.1 (0.9, 1.4)		1.2 (0.8, 1.7)		1.1 (0.9, 1.3)		1.2 (0.8, 1.7)
Multipollutant	0.9 (0.6, 1.4)		1.1 (0.6, 2.1)		1.0 (0.7, 1.4)		1.1 (0.7, 1.7)
Age		0.47		0.03		0.9		0.36
Age 3–5	1.6 (1.0, 2.7)		1.7 (1.0, 3.0)		0.8 (0.5, 1.2)		1.3 (0.9, 2.0)
Age 6–11	1.0 (0.7, 1.3)		1.1 (0.7, 1.6)		1.2 (0.9, 1.5)		1.1 (0.7, 1.6)
Sex		0.65		0.82		0.84		0.47
Females	1.1 (0.6, 1.7)		1.1 (0.7, 1.7)		1.2 (0.8, 1.7)		1.3 (0.7, 2.4)
Males	1.1 (0.9, 1.4)		1.2 (0.8, 2.0)		1.1 (0.8, 1.4)		1.1 (0.7, 1.5)
Race/Ethnicity		0.41		0.35		0.17		0.92
White, non-Hispanic	1.3 (0.9, 2.0)		1.4 (0.8, 2.6)		1.3 (0.9, 1.9)		1.1 (0.6, 2.1)
Black, non-Hispanic	0.9 (0.7, 1.3)		1.3 (0.8, 2.2)		1.2 (0.8, 1.7)		1.0 (0.5, 2.1)
Hispànic	1.3 (0.9, 1.9)		1.3 (0.8, 2.0)		1.3 (0.8, 2.0)		1.1 (0.6, 2.3)
Other	1.1 (0.6, 1.7)		1.1 (0.7, 1.7)		1.2 (0.8, 1.7)		1.3 (0.7, 2.4)

Abbreviations: OR: odds ratio, PFAS: per- and polyfluoroalkyl substances, NHANES: National Health and Nutrition Examination Survey, SD: standard deviation, PFOA: perfluorooctanoic acid, PFOS: perfluorooctane sulfonic acid, PFNA: perfluorononanoic acid, and PFHxS: perfluorohexane sulfonic acid.

p: age, sex, and race/ethnicity-PFAS interaction p-value.

^aRegression models adjusted for other serum PFAS concentrations

^bRegression models adjusted for sex, age, race/ethnicity, serum cotinine, and poverty to income ratio

Total sample size n=607

4. Discussion

In this cross-sectional study, we observed some evidence that serum PFAS concentrations were weakly associated with increased asthma prevalence in US children between 3–11 years of age. Our findings indicate age significantly modified the associations between serum PFOA and PFOS concentrations and asthma diagnosis. In secondary analyses, increasing serum PFOA and PFOS concentrations were also associated with greater odds of asthma diagnosis in children aged 3–5 compared to children age 6–11 years. We did not find significant effect measure modifications of the associations between serum PFAS concentrations and asthma diagnosis based on sex or race/ethnicity.

Of the few studies that measured serum PFAS levels in young children, we observed PFAS levels in our study were relatively similar with those reported in other US studies (Harris et al., 2017; Kingsley et al., 2018; Schecter et al., 2012) and among adolescents and adults (CDC, 2019). Additionally, there have been relatively few studies examining the associations between PFAS and asthma in young children (Rappazzo et al., 2017). A case-control study of Taiwanese children reported that increasing serum PFOA and PFOS concentrations were positively associated with asthma severity scores among asthmatic children aged 9–16 years and another study found higher serum PFOA and PFOS concentrations among 10–15 year old asthmatic children compared to non-asthmatics (Dong et al., 2013; Zhu et al., 2016). Recently, one study found a significant associations between PFHxS and asthma at age 10 years in girls but not with other PFASs (Kvalem et al., 2020) and a Norwegian study also reported a significant linear associations between increasing PFAS exposure, PFOS specifically, and current asthma in high school students (Averina et al., 2019).

While the mechanisms by which PFAS increases asthma development are not completely elucidated, there is limited evidence to suggest that early-life exposure to PFOA and PFOS could induce immune dysfunction by enhancing Type 2 helper T-cell dominance. (Dong et al., 2013; NTP, 2016; Zhu et al., 2016). Type 2 helper T-cell dominance is consistent with immune response patterns found in Type 1 hypersensitivity disorders such as asthma (Chalubinski and Kowalski, 2006). Epidemiologic studies have found associations between increasing prenatal and child serum PFOA and PFOS concentrations and decreased serum concentrations of vaccine antibodies (Grandjean et al., 2012; Granum et al., 2013; Stein et al., 2016; Zeng et al., 2019). Additional studies found positive associations between serum PFOA and PFOS concentrations and immune-mediate biomarkers and allergy-related health outcomes (Dong et al., 2013; Impinen et al., 2019; Kvalem et al., 2020; Manzano-Salgado et al., 2019). However, some studies that found no associations between serum PFOA and PFOS concentrations and hypersensitivity health outcomes (Ashley-Martin et al., 2015; Impinen et al., 2018; Smit et al., 2015).

There were some limitations of this study. Although we observed evidence of an association between PFOS and PFOA and odds of asthma in children ages 3–5 years; there were relatively fewer children with asthma in this age category, which resulted in larger standard errors and less precise effect estimates compared to 6–11-year-old children. Second, this study relied on parent-reported questionnaires to identify asthma in children which may result in misclassification of the outcome, thus potentially attenuating these results. Due to limitations in the survey design of NHANES, there are many potential confounders which we did not have available for model adjustment (i.e. glomerular filtration rate, parental history of asthma, breastfeeding duration), therefore there is potential residual confounding from unmeasured or unknown factors. Finally, NHANES is a cross-sectional survey which limits the ability to establish the temporality of the exposure and outcome sequence. PFAS can persist in the body for years, we cannot discern whether exposure to PFAS occurred before or after asthma development. However, the majority of the study participants reported asthma diagnoses under age 5 years and serum PFAS concentrations were not associated with age or the difference between current age and age of asthma diagnoses which is evidence against reverse causation.

There are substantial gaps in our knowledge about the relationship between PFAS exposure and asthma risk in children. Although we did not observe an overall significant association between serum PFAS concentrations and asthma in children; we did find an association with PFOA and PFOS in children under age 6 years. Given the ubiquitous exposure observed in US children, this association warrants further investigation. Future studies could use prospective cohorts to examine links of repeated PFAS measurements from early infancy into adolescence with immune markers and allergy and asthma related health outcomes. This would help determining if there are periods of heightened susceptibility to PFAS exposure for asthma risk and enhance our knowledge of the role PFAS exposure may play in immune and asthma development and morbidity in children.