Environmental impacts are changes in the natural or built environment, resulting directly from an activity, that can have adverse effects on the air, land, water, fish, and wildlife or the inhabitants of the ecosystem.

Environmental impacts are changes in the natural or built environment, resulting directly from an activity, that can have adverse effects on the air, land, water, fish, and wildlife or the inhabitants of the ecosystem. Pollution, contamination, or destruction that occurs as a consequence of an action, that can have short-term or long-term ramifications is considered an environmental impact. Most adverse environmental impacts also have a direct link to public health and quality of life issues. Several successful reductions in pollution levels have been attributed to stricter regulations, including levels of carbon monoxide and more recent reduction in fine particulate matter.

Mass transit, as a result of the operations and maintenance of infrastructure, facilities, and vehicles, has numerous potential environmental impacts to manage, including air pollution and greenhouse gas from energy use, noise, and vibrations, water discharges, waste removal of passenger trash, harmful materials such as lead-based paint, mercury, PCBs, asbestos, contaminated soil, and groundwater. Both train and bus operations have significant environmental issues to manage on an on-going continuous basis. Besides regulatory compliance, it befits an agency to reduce environmental impacts in order to pollute less, protect natural resources, and reduce liability and save costly impacts to budgets.