Association Between Environmental Air Pollution and Thyroid Cancer and Nodules: A Systematic Review

Background: The global incidence of thyroid cancer has increased over the past several decades. While this increase is partially due to increased detection, environmental pollutants have also emerged as a possible contributing factor. Our goal was to perform a systematic review to assess the relationship between environmental air pollution and thyroid cancer. Methods: Systematic literature search was performed using PubMed, EMBASE, Cochrane Library, Web of Science, and Scopus databases for original articles published prior to March 2024, investigating outdoor air pollution and thyroid cancer/nodules (PROSPERO CRD42024517624). Inclusion criteria included quantitative reporting of pollutant levels and effect size. Specific pollutants included ozone (O₃), particulate matter less than 2.5 (PM_2.5) or 10 microns in diameter (PM₁₀), sulfur dioxide (SO₂), nitric oxides (NO_x), carbon monoxide (CO), and polyaromatic hydrocarbons (PAHs). Study design, sample size, pollution assessment method, covariates, and strength/direction of associations between pollutants and thyroid cancer/nodule detection were extracted, and descriptive synthesis was utilized to summarize pertinent findings. Risk of bias was assessed using the National Heart, Lung, and Blood Institute quality assessment tool. Results: Of 1294 identified studies, 11 met inclusion criteria. Over 6 million patients from diverse regions were represented across studies. Pollutants studied included O₃ in 5 studies; PM_2.5, PM₁₀, SO₂, and NO_x in 3 studies; unspecified PM and CO in 2 studies; and PAHs in 1 study. Primary outcome was thyroid cancer diagnosis among 9 studies and thyroid nodule detection in 2. All studies examining NO_x and O₃ reported increased risks ranging from 1.03 to 1.5-fold and 1.1 to 1.3-fold, respectively. Both studies assessing PM_2.5 reported 1.18 to 1.23-fold increased odds of thyroid cancer diagnosis, and the magnitude of association increased with increasing duration or concentration of PM_2.5 Inconsistent results were observed for levels of CO, PM₁₀, and SO₂. Conclusion: While an emerging body of literature suggests a potential association between air pollution and thyroid cancer, the quality of evidence is limited by study design constraints, variability in exposure assessment, and inconsistent adjustment for potential confounding factors. The heterogeneity in study designs and methodologies present challenges in interpreting results, underscoring the need for standardized approaches in future research.