Establishing environmental causes of childhood cancer: a view from the IARC Monographs.

In 2025, the IARC Monographs programme identified two new modifiable potential causes of childhood acute lymphoblastic leukaemia, with limited evidence: automotive gasoline and human cytomegalovirus. Previously, for childhood acute lymphoblastic leukaemia only one potential cause was identified (parental tobacco smoking, with limited evidence). The Monographs programme, throughout its history, has identified few other causes for specific childhood cancers. Here, we aim to: i) summarise IARC Monographs classifications relevant for childhood cancer aetiology; ii) describe the evidence that contributed to the recent classifications of automotive gasoline and human cytomegalovirus; and iii) list agents with evidence for an association with childhood cancer that are currently prioritised for an IARC Monographs evaluation in the 2025-2029 period. Several factors have been increasingly identified as potential modifiable causes of childhood cancers. Methodologically, studies have been improved by including assessment of the exposure prior to diagnosis (e.g., use of geocoding methodology or stored neonatal bloodspots) that were instrumental in the automotive gasoline and human cytomegalovirus classifications. Several agents currently prioritised for IARC Monographs evaluation, mainly individual pesticides or air pollution components, present sporadic positive findings for specific childhood cancer subtypes. IARC Monographs evaluation of cannabis smoking is planned for November 2026. Sporadic positive associations for specific childhood cancer subtypes were also reported for cannabis smoking (through parental habits) in case-control studies in the 1990s and early 2000s. Studies informative for cancer hazard identification are likely those able to well define the exposure (e.g., individual pesticides), the window of exposures (e.g., in utero) and the outcome (e.g., avoiding combining childhood cancers).