In December 2012, the U.S. Environmental Protection Agency (EPA) finalized a new national clean air standard for fine particulate matter (PM 2.5), commonly referred to as soot. These microscopic particles are often emitted from diesel engines and power plants. When inhaled, the particles lodge deep inside the lungs and can cause asthma, acute bronchitis, heart attack, stroke, and even premature death, especially in vulnerable populations such as children and the elderly. EPA moved forward to strengthen the standard after new data confirmed that the standard set in 1997 did not adequately protect the public.