Low-Level Environmental Metals Exposures, Biomarkers of Myocardial Injury and Hemodynamic Stress, and Mortality Risk

Abstract

Background

The exact pathophysiologic mechanism of environmental toxic metals (particularly lead and cadmium)–associated cardiovascular events remains poorly understood.

Objectives

The authors assessed the impacts of lead and cadmium on myocardial injury and stress and further explored their roles in the risks of all-cause and cardiovascular mortality.

Methods

We included participants with studied measurements from the National Health and Nutrition Examination Survey between 1999 and 2004, followed up to 2019. Generalized linear regression was applied to examine the associations of whole blood lead and cadmium with cardiac-specific biomarkers, inflammation, and oxidative stress. The potential pathways underpinning the associations with mortality were explored using mediation analyses.

Results

A total of 11,193 participants were included, and nearly 93.9% of individuals had lead levels below the World Health Organization guidelines for the clinical management of lead (5 μg/dL). Lead and cadmium levels were positively associated with metrics of myocardial injury (high-sensitivity troponin T and I), myocardial stress (N-terminal prohormone B-type natriuretic peptide [NT-proBNP]), inflammation and oxidative stress (C-reactive protein, total homocysteine, methylmalonic acid), and mortality risks. Significant increases of 2.23% in high-sensitivity troponin T and 7.05% in NT-proBNP were associated with per SD increase in lead levels. The higher troponin and NT-proBNP levels may mediate up to 33.10% of the associations of cardiovascular mortality with lead and cadmium, and the associations were greater in older participants and those who had diabetes and hypertension.

Conclusions

Our results suggest that the associations of low-level lead and cadmium exposures with mortality risks may be potentially mediated by worsening myocardial injury and stress.

Central Illustration

Exposure to environmental metals (particularly lead and cadmium) poses crucial threats to public health worldwide.¹ It has been identified as a major contributor to the risks of cardiovascular disease (CVD) morbidity and mortality by the American Heart Association.¹ Growing studies have shown that lead and cadmium are associated with various cardiovascular deaths, mostly related to ischemic heart disease, suggesting that an abnormal cardiac structure or function may be one of the primary biological pathways of environmental metals–driven CVDs.² The pathophysiology of ischemic heart disease includes ischemic and toxic cardiomyocyte injury, which could trigger all-cause and CVD death (eg, myocardial infarction [MI] and heart failure).³ Mechanistically, the elevations of cardiomyocyte stretch and myocyte membrane permeability posed by reactive oxygen species (ROS) and inflammatory factors are capable of prompting the release of troponin and secretion of N-terminal prohormone B-type natriuretic peptide (NT-proBNP) from the cardiac myocytes.⁴ Cardiac troponin T and I, determined by high-sensitivity (hs) assay (hs-cTnT/I), have served as the highly specific indicator for myocardial injury and diagnosis of MI.⁵ NT-proBNP is a robust indicator relevant to myocardial stretch or strain and is routinely applied to the evaluation of heart failure.⁶ Although lead and cadmium exposures have been associated with oxidative stress and inflammation in human studies, it remains poorly understood whether metal exposures might prompt the genesis of myocardial injury and stress at the population level.

Thus far, only 2 population-based studies have assessed the impact of environmental metal exposure on myocardial injury, yielding mixed findings.^7,8 Correlation analyses showed that cord blood lead and cadmium were positively related to hs-cTnI in neonatal individuals, but no significant correlations of blood lead and cadmium with troponin determined using enzyme-linked immunosorbent assay were observed in adults residing in an area with a facility recycling electronic waste.^7,8 Notably, prior investigations had relatively small sample sizes, characterized the relationships without adjusting for detailed individual factors, and focused on participants who were exposed to extremely high levels of metals or experienced unique exposure scenarios (eg, e-waste dismantling sites) that are unlikely to occur in most communities. Furthermore, no study has investigated the potential mediating role of myocardial injury and stress in heightened mortality risks attributable to metal exposures.

Here, we constructed a retrospective study of adults without diagnosed CVD by pooling available data from the National Health and Nutrition Examination Survey (NHANES) to quantitatively assess the associations and potential interlinked pathways between whole blood lead and cadmium levels and circulating biomarkers indicative of myocardial injury and hemodynamic stress, inflammation and oxidative stress, as well as all-cause and cardiovascular mortality.

Methods

Study design and study population

A racially diverse, population-based retrospective study of individuals aged 18 years or older was constructed by extracting health data from the NHANES cycles. The NHANES program is an ongoing series of complex, stratified, multistage probability cluster sampling surveys operated by the National Center for Health Statistics of the US Centers for Disease Control and Prevention.⁹ The United States noninstitutionalized civilian individuals were recruited and enrolled in this program, and each participant provided written informed consent. The Ethics Review Board of the National Center for Health Statistics has approved the study protocol of NHANES.

For this retrospective study, we initially included 18,974 participants with measurements of all 4 cardiac-specific biomarkers (troponin measured by 4 different assays, and NT-proBNP) determined in surplus specimens from NHANES between 1999 and 2004. The survival status of each participant was then followed up to December 31, 2019. We also excluded participants without measurements of inflammatory and oxidative stress biomarkers (C-reactive protein [CRP], total homocysteine [tHcy], and methylmalonic acid [MMA]), as well as those with missing available exposure data on whole blood lead and cadmium (n = 371). We further excluded 2,162 individuals missing information on covariates of interest (demographic characteristics and behavior risk factors), and the details of covariates are provided in the Supplemental Appendix. Given that the mortality data from participants under 18 years of age were not eligible for public release, 3,934 participants were excluded from the analysis. In addition, we also excluded individuals with a CVD history (self-reported physician-diagnosed coronary heart disease, congestive heart failure, angina/angina pectoris, heart attack, or stroke [n = 1,314]). Consequently, our final analytic sample consisted of 11,193 participants aged 18 years or older for further analyses.

Outcome measurements

Serum cardiac-specific biomarkers for all participants were analyzed using different assays on the surplus specimens stored at the University of Maryland School of Medicine. The majority (nearly 99.7%) of stored blood specimens were pristine samples (never previously thawed and refrozen) in this analytic sample. Concentrations of hs-cTnT were measured using Roche Cobas e601 autoanalyzer (the fifth-generation Elecsys assay reagents). The lower detection limit (LoD) for hs-cTnT (Roche) is 3.0 ng/L. Concentrations of hs-cTnI were measured using the Abbott ARCHITECT i2000SR platform, and the LoD for hs-cTnI (Abbott) is 1.7 ng/L. Concentrations of hs-cTnI were also measured using the Siemens Centaur XP (Siemens Healthcare Diagnostics) and the Ortho Vitros 3600 (Ortho Clinical Diagnostics). The LoD is 1.6 ng/L for hs-cTnI (Siemens) and 0.39 ng/L for hs-cTnI (Ortho). Serum concentrations of NT-proBNP were measured using the Roche Cobas e601 autoanalyzer (Roche Diagnostics Corp.), with 5 pg/mL LoD for this assay. For biomarkers indicative of systemic inflammation and oxidative stress, plasma concentrations of tHcy were measured using the Abbott IMX analyzer (Abbott Diagnostics), and serum CRP concentrations were determined by latex-enhanced nephelometry with the LoD of 0.22 mg/dL. Concentrations of circulating MMA, a surrogate indicator of mitochondrial dysfunction and oxidative stress, were determined using a gas chromatography-mass spectrometry approach.¹⁰

Data on the survival status of each participant were retrospectively ascertained by linking NHANES personal identifiers to the National Death Index with death certificates. The all-cause and cardiovascular mortality data available in the present analysis were compiled from the baseline health assessment (1999-2004) until December 31, 2019, or the date of death. CVD-related mortality was ascertained based on the recorded cause of death from the International Classification of Diseases-10 codes (I00-I09, I11, I13, I20-I51, and I60-I69).

Blood lead and cadmium measurement

The whole blood sample used for measurements of lead and cadmium was obtained from each participant using an ordinary tube after the identification of no background contamination in the materials for collection and storage. All samples were diluted before being stored at −20 °C, and whole blood lead and cadmium were analyzed by the Division of Laboratory Sciences, National Center for Environmental Health, and Centers for Disease Control and Prevention using multielement analytical techniques.

Statistical analyses

Levels of blood measurements lower than the LoD were replaced by a constant value equal to the LoD divided by $\surd 2$ based on the recommendation by NHANES.¹¹ Data for biomarkers indicative of myocardial injury and stress, inflammation, and oxidative stress were log10-transformed due to skewed distributions before analyses. Associations between environmental metal exposures and all biomarkers were assessed using generalized linear models, and the associations of metals with mortality data were examined using Cox regressions. The following covariates were considered and selected by using the least absolute shrinkage and selection operator (Lasso) regression approach for each study outcome separately (as shown in Supplemental Table 1): age, sex, race/ethnicity, body mass index (BMI), education level, hypertension, type 1 and 2 diabetes, hypercholesterolemia, chronic kidney disease, moderate physical activities over past 30 days, vigorous physical activities over past 30 days, and smoking status. Variables for age and BMI were modeled with linear and squared terms to capture potential influences of nonlinear impacts. Single-pollutant generalized linear models or Cox models were then conducted to estimate the associations of studied outcomes with environmental metal exposures.

In exploratory analyses, 2-pollutant models were first implemented to examine whether the single-pollutant effects of lead or cadmium exposure could be confounded. Furthermore, the joint effects of coexposure to lead and cadmium were also assessed. Second, receiver operating characteristic (ROC) curves were applied to assess the most responsive biomarkers associated with low-level lead and cadmium,¹² with the 25th percentile as cutoff points (1.10 μg/dL for lead, 0.30 μg/L for cadmium). Based on these cutoff points, we also depicted Kaplan-Meier survival curves for mortality events responsive to metal exposures. Third, mediation analyses were used to assess the potential effects of mediators (troponin and NT-proBNP) on the associations between exposures and mortality risks. We also explored the mediating role of inflammation and oxidative stress in metals-associated myocardial injury and stress. Fourth, subgroup analyses were conducted according to a variety of participant characteristics. Given that the majority (nearly 80.6%) of diabetic cases was type 2 diabetes, we also conducted subgroup analyses by excluding type 1 diabetic patients. In addition, subgroup analyses were performed using the median level (1.40 μg/L) of total blood mercury. Statistically significant differences (P < 0.05) in estimated associations between strata of participant characteristics were determined by 2-sample Z tests. Fifth, we examined whether the associations could still be detected by restricting analyses to exposure levels below the World Health Organization (WHO) guidelines for the clinical management of lead exposure (5 μg/dL) and the safety standard of cadmium (5 μg/L) recommended by the Occupational Safety and Health Administration (OSHA).^13,14 Sixth, to assess the potential clinical implications of the findings, we repeated regression analyses using the prevalence of elevated biomarker concentrations as outcome variables. Detailed definitions of biomarker concentrations above the thresholds used in this study were presented in the Supplemental Materials. Lastly, several sensitivity analyses were conducted to assess the robustness of our main models, including only adjustments for basic characteristics of study participants (age and sex) or additional adjustments for other environmental factors in the home. We also additionally adjusted for levels of annual family income, dietary exposure to metals, blood essential metals, and circulating albumin.¹⁵

To facilitate interpreting the results, we reported the mean percentage changes in each biomarker with 95% CIs associated with per SD increase in lead and cadmium levels. For mortality data and elevated biomarkers prevalence, the corresponding estimates were reported as HRs and ORs, respectively, with 95% CIs per SD increase in lead and cadmium levels. Statistical significance was defined at P < 0.05, and a Bonferroni corrected P < 0.005 (0.05/10) was further used to explain the effects of metal exposures on studied outcomes for multiple comparisons. All statistical analyses were performed using R (version 4.3.3; R Project for Statistical Computing). More details of statistical analyses are presented in the Supplemental Materials.

Results

Characteristics of study participants

The participants' characteristics are described in Table 1. A total of 11,193 adults were included and were followed up for a median of 16.9 years. The mean (SD) age at baseline was 49.6 (18.9) years among the study population. Nearly 47.4% were male, and 40.6% were identified as having hypertension. The mean (SD) levels of lead and cadmium measured in whole blood were 2.22 (2.11) μg/dL and 0.57 (0.55) μg/L, respectively. Furthermore, 93.9% of participants had lead exposure levels below the WHO guideline for clinical management of exposure. Results from Spearman correlations performed across measurements of studied metals, total blood mercury, and blood essential metals (including calcium, iron, sodium, and potassium) are summarized in Supplemental Figures 1 to 3. We found that lead was positively and weakly correlated with cadmium, with a correlation coefficient of 0.35 (Supplemental Figure 1).

Table 1.

Characteristics of All Eligible Participants by All-Cause and Cardiovascular Mortality Over the Whole Study Follow-Up

	Entire Population (N = 11,193)	All-Cause Mortality		Cardiovascular Mortality
		No (n = 8,078)	Yes (n = 3,115)	No (n = 10,199)	Yes (n = 994)
Follow-up time, y, median (P25, P75)	16.9 (15.2, 18.8)	17.8 (16.5,19.3)	9.7 (5.4, 13.8)	17.3 (15.6,18.8)	9.5 (5.0, 13.3)
Age, years, mean (SD)	49.6 (18.9)	42.2 (15.1)	68.2 (13.6)	47.6 (18.2)	71.0 (12.5)
Age ≥65 y, n (%)	2,961 (18.0)	795 (9.8)	2,163 (69.4)	2,216 (21.7)	742 (74.6)
Sex, n (%)
Men	5,307 (47.4)	3,623 (44.9)	1,684 (54.1)	4,754 (46.6)	553 (55.6)
Women	5,886 (52.6)	4,455 (55.1)	1,431 (45.9)	5,445 (53.4)	441 (44.4)
BMI, kg/m2, mean (SD)	28.4 (6.2)	28.4 (6.3)	28.2 (5.9)	28.4 (6.2)	28.5 (6.0)
BMI ≥25 kg/m2	7,665 (68.5)	5,520 (68.3)	2,145 (68.9)	6,956 (68.2)	709 (71.3)
Race/ethnicity, n (%)
Mexican American	2,542 (22.7)	1,994 (24.7)	548 (17.6)	2,380 (23.3)	162 (16.3)
Non-Hispanic White	5,767 (51.5)	3,899 (48.3)	1,868 (60.0)	5,149 (50.5)	618 (62.2)
Non-Hispanic Black	1,981 (17.7)	1,451 (18.0)	530 (17.0)	1,814 (17.8)	167 (16.8)
Other or other Hispanic	903 (8.1)	734 (9.1)	169 (5.4)	856 (8.4)	47 (4.7)
Education, n (%)
<High school	3,564 (31.8)	2,222 (27.5)	1,342 (43.1)	3,117 (30.6)	447 (45.0)
High school	2,680 (23.9)	1,934 (23.9)	746 (23.9)	2,457 (24.1)	223 (22.4)
≥College	4,949 (44.2)	3,922 (48.6)	1,027 (33.0)	4,625 (45.3)	324 (32.6)
Moderate PA, n (%)
Yes	5,070 (45.3)	3,931 (48.7)	1,139 (36.6)	4,733 (46.4)	337 (33.9)
No	6,123 (54.7)	4,147 (51.3)	1,976 (63.4)	5,466 (53.6)	657 (66.1)
Vigorous PA, n (%)
Yes	3,099 (27.7)	2,688 (33.3)	411 (13.2)	2,984 (29.3)	115 (11.6)
No	8,094 (72.3)	5,390 (66.7)	2,704 (86.8)	7,215 (70.7)	879 (88.4)
Hypertension, n (%)
Yes	4,540 (40.6)	2,392 (29.6)	2,148 (69.0)	3,788 (37.1)	752 (75.7)
No	6,653 (59.4)	5,686 (70.4)	967 (31.0)	6,411 (62.9)	242 (24.3)
Hypercholesterolemia, n (%)
Yes	3,137 (28.0)	1,922 (23.8)	1,215 (39.0)	2,734 (26.8)	403 (40.5)
No	8,056 (72.0)	6,156 (76.2)	1,900 (61.0)	7,465 (73.2)	591 (59.5)
CKD, n (%)
Yes	918 (8.2)	182 (2.3)	736 (23.6)	643 (6.3)	275 (27.7)
No	10,275 (91.8)	97.7 (97.7)	2,379 (76.4)	9,556 (93.7)	719 (72.3)
Diabetes, n (%)
Yes	1,325 (11.8)	581 (7.2)	744 (23.9)	1,064 (10.4)	261 (26.3)
No	9,868 (88.2)	7,497 (92.8)	2,371 (76.1)	9,135 (89.6)	733 (73.7)
Smoking status, n (%)
Smoker	3,000 (26.8)	2,188 (27.1)	812 (26.1)	2,771 (27.2)	229 (23.0)
Nonsmoker	8,193 (73.2)	5,890 (72.9)	2,303 (73.9)	7,428 (72.8)	765 (77.0)
Measured biomarkers, mean (SD)
Myocardial injury and stress
hs-cTnT (Roche), ng/L	8.37 (11.76)	5.61 (5.33)	15.51 (18.78)	7.42 (9.77)	18.10 (21.81)
hs-cTnI (Abbott), ng/L	3.97 (17.47)	2.50 (5.63)	7.79 (31.55)	3.32 (12.67)	10.58 (41.78)
hs-cTnI (Siemens), ng/L	7.32 (33.28)	4.93 (25.01)	13.51 (48.01)	6.27 (28.38)	18.04 (63.91)
hs-cTnI (Ortho), ng/L	2.02 (21.88)	0.80 (2.89)	5.18 (41.04)	1.38 (10.61)	8.51 (64.75)
NT-proBNP, pg/mL	181.6 (927.2)	68.3 (383.7)	475.6 (1,608.7)	133.6 (715.7)	674.3 (2,039.9)
Inflammation and oxidative stress, mean (SD)
CRP, mg/dL	0.51 (0.89)	0.47 (0.76)	0.62 (1.17)	0.50 (0.84)	0.62 (1.31)
tHcy, μmol/L	8.88 (4.69)	7.95 (3.46)	11.30 (6.32)	8.60 (4.28)	11.77 (7.15)
MMA, μmol/L	0.16 (0.40)	0.14 (0.19)	0.22 (0.69)	0.16 (0.21)	0.26 (1.15)
Environmental metal exposures, mean (SD)
Blood lead, μg/dL	2.22 (2.11)	1.95 (1.85)	2.92 (2.54)	2.15 (2.07)	3.00 (2.30)
Blood cadmium, μg/L	0.57 (0.55)	0.52 (0.52)	0.70 (0.61)	0.56 (0.54)	0.69 (0.61)

BMI = body mass index; CKD = chronic kidney disease; CRP = C-reactive protein; hs-cTnI = hs-troponin I; hs-cTnT = high-sensitivity (hs)-troponin T; MMA = methylmalonic acid; NT-proBNP = N-terminal prohormone B-type natriuretic peptide; P₂₅ = 25th percentile; P₇₅ = 75th percentile; PA = physical activity; tHcy = total homocysteine.

Associations with biomarker responses

In single-pollutant model analyses (Figures 1A and 1C), blood lead and cadmium levels were positively associated with serum troponin determined by using one hs-cTnT (Roche) and 3 different hs-cTnI assays (hs-cTnI [Abbot], hs-cTnI [Siemens], and hs-cTnI [Ortho]). Per SD increase in whole blood lead levels, we observed significant increases of 2.23% (95% CI: 1.33-3.14) in hs-cTnT (Roche), 3.57% (95% CI: 2.30-4.85) in hs-cTnI (Abbot), 3.15% (95% CI: 1.41-4.92) in hs-cTnI (Siemens), and 4.04% (95% CI: 2.39-5.72) in hs-cTnI (Ortho). We also observed significant increases in NT-proBNP of 7.05% (95% CI: 5.02-9.13) and 6.18% (95% CI: 3.96-8.45) associated with per SD increase in blood levels of lead and cadmium, respectively. As expected, lead and cadmium exposures may prompt inflammatory and oxidative stress responses. For instance, significant increases in tHcy levels of 3.03% (95% CI: 2.41-3.65) were observed in association with per SD increase in lead levels.

Figure 1.

Association of Metal Exposures With Outcome Measures

Significant associations (P < 0.05) are shown in red; Bonferroni corrections with significance (P < 0.005) are indicated by asterisks. Single-pollutant model: adjustments for covariates; 2-pollutant model: variables in single-pollutant model plus cadmium for lead or lead for cadmium. (A) Changes in biomarker responses associated with lead. (B) Changes in mortality risks associated with lead. (C) Changes in biomarker responses associated with cadmium. (D) Changes in mortality risks associated with cadmium. CRP = C-reactive protein; hs-cTnI = hs-troponin I; hs-cTnT = high-sensitivity (hs)-troponin T; MMA = methylmalonic acid; NT-proBNP = N-terminal prohormone B-type natriuretic peptide; tHcy = total homocysteine.

Associations with mortality risks

As shown in Figures 1B and 1D, higher whole blood levels of lead and cadmium were associated with higher risks of all-cause and cardiovascular mortality. Per SD increase in exposure levels, the HR for all-cause mortality was 1.088 (95% CI: 1.060-1.117) for lead and 1.217 (95% CI: 1.177-1.257) for cadmium. Also, per SD increase in exposure levels, the HR for cardiovascular mortality was 1.101 (95% CI: 1.055-1.150) for lead and 1.273 (95% CI: 1.202-1.349) for cadmium.

Exploratory analyses

Two-pollutant models showed that the observed associations were generally robust, except for the lead effect on CRP and cadmium effect on MMA (Figures 1A and 1C). The joint effect analyses revealed that coexposure to lead and cadmium at higher levels displayed strong impacts on studied outcomes (Supplemental Table 2). Mediation models indicated that the associations of lead and cadmium with all-cause and cardiovascular mortality could be possibly mediated through increased biomarkers relevant to myocardial injury and stress as well as systemic inflammation and oxidative stress (Table 2). We estimated that hs-cTnT mediated 18.31%, hs-cTnI (Abbott) mediated 16.08%, and NT-proBNP mediated 33.10% of the estimated association between lead exposure and cardiovascular mortality, respectively. Furthermore, the significant increases in tHcy mediated 37.01% of the lead effects on hs-cTnT.

Table 2.

Estimates of the Mediation Effects in Associations of Metal Exposures With Outcome Measures

Mediators	Outcomes	Mediation Proportion (%) with 95% CIs
		Lead Exposure	Cadmium Exposure
The associations with myocardial injury and stress mediated by inflammation and oxidative stress
CRP
	hs-cTnT (Roche)	1.38 (−0.03 to 4.96)	3.46 (1.07-8.70)∗
	hs-cTnI (Abbott)	2.42 (0.02-6.41)	11.49 (4.76-32.86)
	hs-cTnI (Siemens)	1.14 (−0.08 to 4.80)	4.29 (0.10-18.96)
	hs-cTnI (Ortho)	2.74 (0.20-7.44)	6.77 (2.87-14.64)∗
	NT-proBNP	3.77 (0.32-8.39)	9.66 (4.79-18.08)∗
tHcy
	hs-cTnT (Roche)	37.01 (20.03-87.84)∗	34.32 (19.92-73.59)∗
	hs-cTnI (Abbott)	11.99 (6.98-21.77)∗	20.05 (9.08-62.95)
	hs-cTnI (Siemens)	16.60 (8.72-50.16)∗	19.29 (8.75-75.94)
	hs-cTnI (Ortho)	11.58 (6.23-23.11)∗	9.37 (5.00-20.97)∗
	NT-proBNP	7.35 (3.78-13.36)∗	6.60 (3.54-12.00)∗
MMA
	hs-cTnT (Roche)	13.35 (7.20-28.94)∗	5.45 (0.78-12.08)
	hs-cTnI (Abbott)	7.61 (3.95-14.44)∗	5.42 (0.41-17.88)
	hs-cTnI (Siemens)	8.39 (3.89-21.77)∗	4.25 (0.24-16.33)
	hs-cTnI (Ortho)	7.81 (3.85-15.96)∗	3.10 (0.19-8.55)
	NT-proBNP	7.75 (4.47-13.75)∗	3.27 (0.17-7.17)
The associations with mortality mediated by myocardial injury and stress, inflammation, and oxidative stress
All-cause mortality	hs-cTnT (Roche)	14.99 (8.46-25.62)∗	5.75 (3.38-8.63)∗
	hs-cTnI (Abbott)	11.48 (6.90-18.20)∗	2.80 (0.91-4.77)∗
	hs-cTnI (Siemens)	6.05 (2.72-10.92)∗	1.89 (0.42-3.48)∗
	hs-cTnI (Ortho)	11.81 (6.52-19.30)∗	4.81 (2.55-7.46)∗
	NT-proBNP	23.68 (15.15-38.55)∗	9.01 (5.90-12.92)∗
	CRP	3.82 (0.09-8.28)	3.61 (1.78-5.82)∗
	tHcy	16.99 (11.60-24.73)∗	5.80 (3.70-8.36)∗
	MMA	7.99 (4.73-13.20)∗	1.33 (0.19-2.69)
Cardiovascular mortality
	hs-cTnT (Roche)	18.31 (9.12-45.65)∗	6.01 (3.46-9.91)∗
	hs-cTnI (Abbott)	16.08 (9.16-32.42)∗	3.66 (1.30-6.59)∗
	hs-cTnI (Siemens)	9.07 (3.87-19.27)∗	2.73 (0.80-5.29)
	hs-cTnI (Ortho)	16.76 (8.89-32.59)∗	6.73 (3.40-11.15)∗
	NT-proBNP	33.10 (18.61-87.80)∗	11.29 (7.08-17.47)∗
	CRP	2.87 (0.28-7.04)∗	2.61 (1.21-4.77)∗
	tHcy	16.68 (9.65-30.20)∗	5.63 (3.27-8.73)∗
	MMA	7.57 (3.82-15.37)∗	1.17 (0.08-2.53)

Mediated effects with 95% CIs. We assessed the statistical significance (P < 0.05) by evaluating whether the 95% CI contains 0; Bonferroni corrections with significance (P < 0.005) are indicated by asterisks.

Abbreviations as in Table 1.

Results from ROC analyses, as shown in Figure 2, further revealed that several biomarkers could display as sensitive predictors to discriminate health responses from wide ranges of blood lead levels at cutoff points of the 25th percentile (low vs high), which contributed to the area under the ROC curve values as follows: 0.7879 (95% CI: 0.7773-0.7985) for hs-cTnT, 0.7882 (95% CI: 0.7777-0.7987) for hs-cTnI (Abbott), 0.7964 (95% CI: 0.7862-0.8067) for hs-cTnI (Siemens), 0.7802 (95% CI: 0.7695-0.7908) for hs-cTnI (Ortho), and 0.8649 (95% CI: 0.8564-0.8734) for tHcy. The associations with mortality remained significant when lead and cadmium levels were dichotomized at cutoff points of the 25th percentile (log-rank P < 0.001) (Supplemental Figure 4).

Figure 2.

Exploring the Most Responsive Biomarkers With Metal Exposures

We used whole blood metal metrics with cutoff points at the first quartile (1.10 μg/dL for lead, 0.30 μg/L for cadmium) to model receiver operating characteristic curves. (A) Receiver operating characteristic curves for lead exposure. (B) Receiver operating characteristic curves for cadmium exposure. Abbreviations as in Figure 1.

Results from the subgroup analyses by demographic characteristics and disease susceptibility factors on cardiac effects of metals are presented in Figure 3 and Supplemental Figure 5. Specifically, the effects of NT-proBNP attributed to lead were greater in those aged ≥65 years (11.28%; 95% CI: 7.17-15.56), those who had diabetes (23.54%; 95% CI: 14.22-33.62), hypertension (11.57%; 95% CI: 7.84-15.43) and chronic kidney disease (32.98%; 95% CI: 22.55-44.29), in comparison to their respective reference groups. The associations were also generally greater in patients with diabetic status when excluding those with type 1 diabetes (Supplemental Table 3). There were no consistent results of effect modification by participants' race/ethnicity (Figure 3, Supplemental Figure 5) and levels of total blood mercury (Supplemental Table 4).

Figure 3.

Associations with Selected Biomarkers Stratified by Participant Characteristics

Significant associations (P < 0.05) are shown in red; Bonferroni corrections with significance (P < 0.005) are indicated by asterisks. The difference represents the P value of the differences in association estimates relative to the reference subgroups. BMI = body mass index; CKD = chronic kidney disease; PA = physical activity; other abbreviations as in Figure 1.

In addition, as shown in Supplemental Table 5, we found significant associations with lead and cadmium in subsets of individual exposure levels below specific values, such as the WHO guidelines of lead exposure (5 μg/dL) and the OSHA safety standard of cadmium exposure (5 μg/L). We further found positive and significant associations with the prevalence of elevated biomarker outcome measures, such as hs-cTnT and NT-proBNP (Supplemental Table 6). For instance, lead and cadmium levels were associated with elevated hs-cTnT (Roche) based on the definition of sex-specific 99th percentile upper reference limit for the assay (hs-cTnT concentrations above 14.0 and 22.0 ng/L for females and males, respectively). In addition, the results from sensitivity analyses by only controlling for age and sex in regression models as well as additional adjustments for other environmental factors in the home assessed by questionnaires such as the source of tap water, water treatment devices used, or pest control in the past month, did not substantially change the results derived from the main models (Supplemental Table 7). The overall results were also robust by additional adjustments for participants' annual family income, dietary exposure to metals, blood essential metals, or circulating albumin (Supplemental Table 8).

Discussion

As shown in Central Illustration, we have shown here that environmental metal exposures, including lead and cadmium, are associated with increased cardiac-specific biomarkers of injury and hemodynamic stress, as well as increased inflammatory and oxidative stress responses. We further revealed prospective associations of higher lead and cadmium levels with higher risks of all-cause and cardiovascular mortality. Several biomarkers (eg, troponin and tHcy) may serve as sensitive indicators capturing the health effect of lead exposure. Importantly, the mortality risks attributed to these metals have been shown to be potentially mediated through myocardial injury and stress. Our findings provide novel mechanistic evidence supporting the associations of environmental metals (particularly lead and cadmium) with developing or exacerbating acute and chronic CVD events observed in previous epidemiologic studies.

Central Illustration.

Cardiovascular Effects of Metal Exposures in General Population

Lead exposure frequently occurs in our daily-life environments, especially in regions near emission sources via contact with soils and dust contaminated by lead.¹ For cadmium, the general population can also be exposed to various sources, such as inhaled ambient air, cigarette smoke, waste incinerators, coal-fired industries, or power plants.¹ Accumulating population-based studies have shown that lead and cadmium exposures are associated with increased risks of MI, left ventricular dysfunction, and autonomic imbalance.^1,16,17 Despite the well-established relationships, the available evidence of heavy metals-induced myocardial injury is limited and comes mainly from experimental studies. Troponin, as a reliable biomarker of myocardial injury, has been shown to be positively associated with clinical manifestations of CVD events and ischemic burden.³ Elevations in the expression of troponin were observed in the heart tissues of cockerel chicks following lead exposure.¹⁸ In a murine model of lead exposure, serum levels of troponin increased significantly in the lead-exposed groups.¹⁹ Likewise, oral administration of cadmium chloride in the rabbits showed that cadmium accumulation was higher in the heart tissues, along with elevated serum troponin levels.²⁰ To date, few studies have assessed the impacts of lead and cadmium on circulating troponin in humans.^7,8 A study of neonatal patients reported cord blood lead and cadmium levels associated with elevations of hs-cTnI (Siemens), whereas null associations with troponin measured by enzyme-linked immunosorbent assay approach were found among adults from an area of recycling electronic waste with relatively higher lead exposure levels.^7,8 The disparate results across human investigations might originate from varied research contexts, including sample size, study population characteristics, and analytic approach. Our analysis leveraged geographically and racially diverse individuals from the NHANES platform together with blood samples to estimate exposure levels of lead and cadmium, which provided improved statistical power and spatial variations for the measured pollutants compared with previous investigations. In this study, lead and cadmium exposures showed consistent effects on troponin determined by 4 sensitive assays in single- and 2-pollutant models, suggesting that the troponin-raising effects posed by lead and cadmium could be independent of each other. Collectively, our findings provide novel evidence that myocardial injury could be significantly worsened by exposure to environmental metals among this study population.

Another finding relevant to the impacts of environmental metals on the myocardium is the associations of blood lead and cadmium with NT-proBNP, a biomarker of myocardial stretch or strain. NT-proBNP, which is well known to be a potent trigger of heart failure risk, has also served as one of the most crucial predictors of future atherosclerotic CVD events.²¹ The family of natriuretic peptides primarily includes atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), and they are swiftly upregulated in response to cardiac wall stress.²² On stress-induced production of BNP, pro-BNP in cardiomyocytes (particularly myocytes in the left ventricle) can be further cleaved into the active types of hormone BNP and inactive type of NT-proBNP.²² Although the association between environmental metals and NT-proBNP has not been previously investigated in humans, the effects on ANP and BNP are increasingly corroborated by experimental studies. Mice with low-dose exposure to cadmium revealed a significant upregulation in ANP levels in heart tissues,²³ and effects on serum levels and protein expressions of ANP and BNP were also shown recently with lead exposure in rats.²⁴ In people occupationally exposed to high levels of lead, altered genetic variability of the BNP system was observed, and certain polymorphisms of the BNP gene have been linked to higher circulating levels of BNP and NT-proBNP.²⁵ Recent and limited epidemiologic studies have been conducted to characterize the linkages between environmental metals (eg, blood lead) and systolic left ventricle dysfunction,¹⁶ a pathophysiologic condition often occurring before the onset of heart failure. Also, electrocardiographic abnormalities, such as the cardiac autonomic nervous system, were linearly associated with lead exposure,¹⁷ which is largely in line with our findings of the exposure-response analyses. Here, we showed for the first time that exposure to lead and cadmium may prompt alterations in NT-proBNP levels. Although the magnitude of the effects attributable to these environmental metals was relatively modest, the burden of cardiac health worsened in the overall population would have enormous ramifications given that lead and cadmium have been listed as the top 10 environmental substances of major health concern needing action at a global scale by WHO.²⁶

We systematically investigated the potential mediating role of inflammation and oxidative stress in metals-associated myocardial injury and stress. Furthermore, we assessed these alterations of health responses in the associations with all-cause and cardiovascular mortality. Existing evidence shows that the pathophysiological processes of cardiomyocyte injury can be driven by multiple pathways, such as inflammation, the disorder of iron homeostasis posed by the overproduction of ROS, and prompting mitochondrial permeability transition pore formation.²⁷ It has been shown that environmental metals, such as lead and cadmium, may trigger the generation of inflammatory cytokines and overgeneration of ROS, and troponin levels can be enhanced by lead-induced activation of immune cells.^19,28 CRP, as an acute-phase plasma protein, is generated from the liver, and elevated CRP could be involved in the progressions of myocardial ischemia and reperfusion, possibly due to its property of acute inflammation.²⁹ Interestingly, the lead effects on CRP were small and appeared to be insignificant alter adjustment for cadmium, suggesting that the acute inflammatory response posed by lead might be confounded by cadmium. Vascular inflammation and oxidative stress from the imbalance between ROS production and the antioxidant system have also been proposed as a potential mechanism of homocysteine-related cardiovascular morbidity and mortality.^30,31 Recent population-based evidence indicates that homocysteine levels are positively associated with elevated hs-cTnT/I and NT-proBNP, and these myocardial injury biomarkers can substantially mediate homocysteine-associated CVD risks.³² As crucial risk factors of cardiac-related diseases, high BMI levels are closely correlated with the degree of chronic inflammation.³³ We found that BMI ≥25 kg/m² slightly modified the environmental metals attributed to the detrimental cardiac effects in the study participants. Our analyses also showed positive associations of lead and cadmium exposures with MMA, a biomarker of mitochondrial dysfunction and oxidative stress. MMA serves as a mitochondrial toxin that can disturb mitochondrial energy metabolism by triggering ROS generation.^34,35 Mechanistic evidence has shown that the cardiomyocyte injury (eg, increased troponin) provoked by lead and cadmium could be mitigated by inhibition of the ROS production,^18,36 and excessive ROS may prompt mitochondrial membrane depolarization, thereby leading to cardiomyocyte apoptosis.³⁷ Taken together, our findings are in line with previous experimental studies by supporting the study hypothesis that inflammatory and oxidative stress responses originating from lead and cadmium could potentially worsen myocardial injury and stress, and these pathophysiologic responses may also possibly mediate the associations between mortality and metal exposures.

Our collective findings might have crucial public health and clinical implications. We observed the adverse effects of lead and cadmium exposure at levels much lower than the current limits recommended by international and national bodies.^13,38 Nearly 93.9% of study participants had blood lead levels below the WHO guideline for clinical management of exposure to lead (5 μg/dL). Similarly, nearly 99.8% had blood cadmium levels below 5 μg/L, the safety standard proposed by OSHA. These findings reveal an underappreciated role of low-level environmental metal exposures, especially in those with pre-existing chronic diseases (eg, cardiometabolic disorders). Indeed, we noted that the hs-cTnT/I and NT-proBNP-raising effects in association with blood lead and cadmium were stronger among participants with hypertension, diabetes, and kidney dysfunction, providing evidence of an increased risk due to pre-existing disease. Prior studies have shown that elevated troponin levels, even within the normal range, are an independent predictor of all-cause and cardiovascular mortality in patients with and without CVDs.^39,40 Apart from assessing the effects of hs-cTn as continuous variables in this study, lead and cadmium were also observed in association with the prevalence of elevated several biomarker concentrations (eg, hs-cTnT) above the thresholds that are used clinically to capture myocardial injury. These findings highlight the need to understand the health risks of low-level exposure and to consider the potential for increased risks in susceptible subpopulations.

Study limitations

Several limitations should be noted when interpreting the findings. First, hs-cTnT/I and NT-proBNP measurements were determined in stored serum samples, although the inter-assay coefficients of variability for these used assays were excellent, and they have been suggestive of accuracy and reliability in long-term stored blood specimens previously.⁴¹ Second, a suite of potential covariates was considered in regression models, but the associated estimates might not completely exclude unknown or unmeasured factors. Third, blood metals generally reflect recent exposures, whereas metals from bone represent chronic exposures (eg, years to decades), and toenail metals represent medium-term time exposures (eg, several months).42, 43, 44 Furthermore, most urinary metals reflect recent exposures due to their short half-lives, but cadmium has a long elimination half-life (approximately 10-30 years), and 24-hour urinary excretion of cadmium has been applied as a biomarker indicative of lifetime exposure in epidemiological studies.⁴³ Notably, although whole blood lead could capture recent exposures, they may also reflect the body burden from past exposures, possibly due to lead mobilization from the bone system back into the blood.⁴⁴ Previous studies have shown that nearly 45% to 75% of the blood lead may have originated from bone in individuals without excessive exposure to lead.^45,46 Nevertheless, exposure levels estimated in the present analysis based on the whole blood samples might have limited the ability to attribute the lead-associated adverse health impacts to different time windows of exposure. Therefore, assessments of whole blood lead and cadmium combined with bone lead and urine cadmium, as well as those measured in toenail samples, may provide more reliable estimates of recent, medium-term, and chronic exposure levels. Fourth, studies showed that environmental metals, such as arsenic, mercury, copper, and zinc, could heighten cardiovascular risks, and cadmium-induced iron dysregulation (eg, elevated iron levels) could worsen the function of endothelial cells through activation of the ferroptosis pathway.^2,47,48 In exploratory analyses, we found that the associations were greater at higher levels of coexposure to lead and cadmium. Therefore, it is worth exploring the potential joint effects of multiple metals on cardiovascular health responses in future studies. Lastly, although our results from mediation analyses were largely in line with existing experimental evidence, the static nature of available data on exposure metrics and studied outcomes did not allow us to assess temporal changes in the interests of the mediator and provide a causal interpretation of the mediating effects. Mediation models based on prospective designs with longitudinal measurements are required to validate our observational findings of the potential interlinked pathways. Despite these limitations, our study has some strengths, including highly standardized protocols employed in NHANES, outcome measurements by multiple state-of-the-art assays, a large sample size, and a diverse study population, all of which might facilitate comprehensive comparison and the generalizability of our results.

Conclusions

We have shown here that cadmium and lead exposures, even at low levels, are significantly associated with heightened myocardial injury and hemodynamic stress, worsened systemic inflammatory and oxidative stress, as well as increased risks of all-cause and cardiovascular mortality in the general population. Our novel findings extend the existing pathophysiologic evidence of detrimental cardiac effects in a real-world scenario with relatively low exposure levels of environmental metals. These findings also support continued efforts to reduce environmental metal exposures, particularly lead and cadmium, to effectively prevent or mitigate the detrimental effects on the myocardium and subsequent mortality risks.

Perspectives.

COMPETENCY IN MEDICAL KNOWLEDGE: In this large population-based investigation, with the majority of participants having blood lead levels below the WHO guideline for clinical management of exposure and cadmium levels below the Safety Standard proposed by OSHA, we showed that lead and cadmium exposures were significantly associated with biomarkers of myocardial injury and hemodynamic stress determined by multiple state-of-the-art assays, which might potentially mediate the associations of these metal exposures with mortality risks. Detection of the adverse effects on the myocardium supports an underappreciated public health risk from low-level metal exposures, which are commonly experienced and detected in the general population daily in the communities.

TRANSLATIONAL OUTLOOK: As in our study population, billions of individuals across numerous regions worldwide continuously face low-level environmental metal exposures. Our collective findings support that cardiac-specific biomarkers might serve as clinical relevance indicators for discerning metals-associated cardiac dysfunctions and thereby assist physicians in patient monitoring and treatment, as well as guide policymakers and public health leaders in developing protective strategies to reduce sources of metal exposure. Furthermore, the potential effect modification by demographic characteristics and disease susceptibility factors identified in our study could provide valuable information for health care providers to communicate the potential risks with vulnerable populations.

Funding support and author disclosures

The authors have reported that they have no relationships relevant to the contents of this paper to disclose.