Does exposure to lead or other environmental toxins explain most ADHD cases?

Not for most ADHD cases, no, though environmental toxin exposure is a genuinely studied risk factor worth taking seriously in specific circumstances. Prenatal exposure to lead or certain other toxins has been associated with a modestly elevated ADHD risk in research, but this explains only a small fraction of overall cases, not the majority. Genetics remains the dominant known factor, accounting for the large majority of variance in who develops ADHD according to twin study heritability estimates. References: https://www.caddra.ca/wp-content/uploads/Canadian-ADHD-Practice-Guidelines-4.1-January-6-2021.pdf