At the end of their useful life, building materials that aren't recycled are either burned in incinerators or deposited in landfills, where their contents leach into groundwater and air or decompose or burn in uncontrolled landfill fires - which can be the most polluting. Workers and nearby communities are exposed to toxic substances from this disposal process. Many of the substances that leach, off gas, or decompose from building materials or are released by combustion are known or suspected of being hazardous to human and animal health and have been associated with cancer, genetic mutation, reproductive or developmental harm, endocrine disruption and a large range of other chronic illnesses and acute hazards. Many more chemicals have not yet been adequately tested for toxicity. Of particular concern are persistent and bioaccumulative toxicants (PBTs) that remain in the environment without breaking down to safer compounds, travel long distances from their origin, and concentrate as they move up the food chain leading to high concentrations of concern in humans.