From goods movement to building construction to public transportation, diesel engines are the modern-day workhorse of the American economy. Diesel engines are extremely efficient, and they power nearly every major piece of machinery and equipment on farms, on construction sites, in ports, and on highways. However, not all diesel engines are as clean as those manufactured after 2006 and later, when EPA's stringent heavy-duty highway and non-road engine standards began coming into effect. EPA estimates that approximately 11 million older diesel engines remain in use, and will continue to emit significant amounts of nitrogen oxides (NOX) and particulate matter (PM) until they wear out and are replaced. To reduce the public's exposure to pollution from these older, dirtier engines, Congress in 2005 authorized funding for the Diesel Emissions Reduction Act, a grant program designed to selectively retrofit or replace the older diesel engines most likely to impact human health. The U.S. Environmental Protection Agency (EPA) administers all Diesel Emissions Reduction Act (DERA) funding under the umbrella of the National Clean Diesel Campaign (NCDC), which promotes clean air strategies by working with manufacturers, fleet operators, air quality professionals, environmental and community organizations, and state and local officials to reduce diesel emissions.