Eligibility criteria
Eligible articles included systematic reviews and meta-analyses of observational epidemiological studies that examined the association of potential risk factors with childhood cancer types. Exposure variables included environmental exposures of either parent (mother and/or father) or child during preconception, pregnancy, delivery or childhood. Outcome variables included any type of cancer diagnosed in children aged 0-14 years. Studies on broader age groups which included sub-analyses on children 0-14 years were also eligible. Meta-analyses that solely examined the association of genetic or epigenetic factors with childhood cancer risk were excluded. Likewise, meta-analyses that investigated cancer survival or other outcomes among patients with cancer, as well as meta-analyses not reporting comprehensive study-specific information, such as effect sizes, 95% confidence intervals (CI) or sample sizes were also excluded. When one or more eligible systematic reviews and/or meta-analyses on the same research question were identified, the one with the largest number of component studies, usually the most recently published study, was included. Each identified study was cross-checked for each quality and issues of overlap before including in the final set of eligible studies.