What environmental factors, if there's any, that can cause brain cancer?

None are known: The stuff about electric power lines, microwave ovens and cell phones isn't generally accepted by the scientific community and don't fit with what we know about cancer. From time to time there is talk of an epidemiologic link. If ct scans and/or pesticides are placing people at extra risk, the increased risk is not striking. There's inconsistent data for solvents & nitrites.