This narrative review summarizes evidence on how exposure to toxic metals (arsenic, cadmium, lead, mercury, nickel, hexavalent chromium) initiates and sustains pulmonary fibrosis. It synthesizes mechanistic evidence from in vitro and animal studies, organized around key signaling networks: oxidative stress/ROS, TGF-beta/Smad pathway, MAPK/PI3K/Akt, Janus kinase/STAT, and mTORC1. The review addresses cross-talk among these pathways and discusses preclinical therapeutic strategies.
Key numbers
- Arsenic: induces oxidative stress, TGF-beta/Smad activation, inflammatory cytokine release, and alveolar epithelial injury. At the population level, the review cites studies finding arsenic exposure significantly increases risk of COPD, with 30% (OR) higher odds of elevated FEV1/FVC ratio at baseline; a cohort study noted 36% and 30% increased risk of COPD at 5-year and 10-year follow-up. Arsenic exposure in Pabna, Bangladesh was correlated negatively with FEV1/FVC ratio.
- Cadmium: reviewed studies found cadmium urinary concentration negatively correlated with FEV1/FVC (r = -0.199, p = 0.013), FEV1 (r = -0.221, p = 0.007), and FVC (r = -0.201, p = 0.011).
- Lead: reviewed studies showed blood lead level negatively correlated with FEV1/FVC and increased risk of respiratory symptoms.
- TGF-beta/Smad pathway is the central fibrosis axis; persistent TLR4/NF-kappaB/NLRP3 inflammation and autophagy/lysosomal dysfunction contribute to alveolar epithelial injury, fibroblast-to-myofibroblast transition, and excessive extracellular matrix deposition.
Methods (brief)
Narrative review; searched PubMed and other major databases. Covers experimental (in vitro, rodent) and human epidemiological literature. No PRISMA flow diagram or systematic search strategy reported; scope is mechanistic synthesis.
Evidence fitness
Supports: mechanistic context for arsenic-induced pulmonary fibrosis via oxidative stress, TGF-beta/Smad, and inflammatory pathways. Provides a framework for interpreting epidemiological associations between arsenic exposure and lung function decline. Limitation: narrative review (not systematic); some epidemiological figures cited are drawn from prior reviews rather than primary sources cited here directly.
Limitations
Narrative (not systematic) review; heterogeneity in cited studies; limited quantification of exposure levels associated with specific mechanistic effects; most mechanistic evidence from in vitro or animal models; translational gap to real-world human inhalation exposures is a stated limitation of the review itself.
Related evidence
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