State of the Science on 1,4-Dioxane: Emission Sources, Global Contamination, and Regulatory Disparities

1,4-Dioxane has emerged as a critical global environmental concern because of its carcinogenic potential, persistence, and ubiquitous occurrence. This review systematically addresses key knowledge gaps in its emission trends, spatiotemporal characteristics on a global scale, and challenges for unified regulatory frameworks. By compiling emission data, we provided the first quantitative global estimates for 1,4-dioxane emissions and revealed a remarkable increase in industrial discharges from 645–1522 tonnes in ca. 2000 to 3868 tonnes in 2023. The emission profile has shifted from direct releases as a chlorinated solvent stabilizer in the 20th century to contemporary unintentional byproduct emissions associated with the production of poly(ethylene terephthalate) and ethoxylated surfactants. The previously overlooked domestic emission was found to contribute 711–832 tonnes annually (2023 estimate), ascribing to the use of household products containing ethoxylated surfactants. Along with shifts in emission patterns, affected environmental matrixes have expanded beyond groundwater to include surface water and drinking water, with rising concentrations observed in nonhistorical production regions. Meanwhile, the environmental persistence and long-range transport capacity of 1,4-dioxane continue to exacerbate legacy contamination issues in historically industrialized regions. However, divergent interpretations of its toxicological mechanisms, particularly regarding the carcinogenic mode of action, have resulted in inconsistent drinking water standards and regulatory approaches worldwide. This review highlights future efforts regarding more accurate emission inventories, enhanced spatiotemporal monitoring of environmental occurrence, and health risks assessment, alongside the need for coordinated regulatory efforts to address the growing concerns of 1,4-dioxane contamination.