The federal government has set long-awaited limits on airborne mercury generated by cement kilns, but they apply only to new or recently renovated plants and don’t require existing ones to make any significant changes.

Once fully in effect, the rules announced Monday will prevent between 1,300 and 3,000 pounds of mercury nationwide from escaping into the atmosphere each year, the U.S. Environmental Protection Agency said. Mercury can damage nervous systems, cause developmental problems for children and pollute waters.