Asbestos
Used widely in floor tile, pipe and boiler insulation, ceiling and wall plaster, and window glazing until the late 1970s and 1980s. Intact, well-managed material is low risk. Once it is damaged, worn or disturbed — by renovation, a leak, even hail — fibers can be released and breathed in.
Inhaled fibers can cause mesothelioma, lung cancer and asbestosis, often 20 to 50 years after exposure. Teachers, who spend whole careers in the same buildings, carry a measurable share of that risk.