Literature search
The present umbrella review of systematic reviews and meta-analyses was based on a predefined research protocol. It was conducted in accordance with the preferred reporting items for overviews of systematic reviews (PRIO; Supplementary Table 1) .
We searched PubMed and Scopus databases until April 23 2020 for systematic reviews and meta-analyses on the association of any environmental risk factor and any type of childhood cancer employing a predefined search algorithm (Supplementary Material). No language or other restriction criteria were applied. Two independent investigators (MK and GM) examined the titles, abstracts and full texts of the shortlisted meta-analyses and discrepancies were resolved by consensus. We further hand searched the references of the retrieved systematic reviews and meta-analyses for systematic reviews/meta-analyses potentially missed by the initial electronic search and unpublished data.