Particle pollution – known as particulate matter (PM) – is a mixture of solid particles and liquid droplets found in the air. Some particles, like soot, are large or dark enough to be visible to the naked eye, while others are so tiny they can only be seen with an electron microscope. Particle pollution includes inhalable coarse particles, referred to as PM10 because they have a diameter greater than 2.5 micrometers and less than 10 micrometers, and fine particles, referred to as PM2.5 because they have a diameter less than 2.5 micrometers. Compare these sizes to the diameter of a single strand of human hair, which is approximately 70 micrometers – about 30 times larger than the largest fine particle. Another frame of reference is fine beach sand, one grain of which is about 90 micrograms in diameter.
PM can be composed of hundreds of different chemicals. “Primary” PM is emitted directly from a source, like a smokestack, an unpaved road, a construction site, an agricultural field or a fire. “Secondary” or “indirect” PM is formed through a complex reaction in the atmosphere of chemicals, such as sulfur dioxide (SO2) and nitrogen oxides (NOx) emitted from power plants, industrial sources and mobile sources. Most PM2.5 in the U.S. is secondary PM.
The most serious health risks associated with particle pollution comes from PM2.5, which can enter deep into the lungs and even into the bloodstream. Exposure to PM2.5 causes thousands of premature deaths each year in people with heart or lung disease. In addition, it can cause nonfatal heart attacks, irregular heart beat and decreased lung function, aggravate asthma and increase respiratory systems like coughing and breathing difficulties. Although those with health or lung disease, children and older adults are most susceptible to the adverse health impacts associated with particle pollution, even healthy adults may experience temporary symptoms from exposure to elevated PM levels.
PM also causes an array of adverse environmental effects, including visibility impairment, the acidification of lakes and streams, changes in the nutrient balance of coastal waters and large river basins, depletion of soil nutrients, damage to farm crops and sensitive forests and imbalances in the diversity of ecosystems. In addition, PM can discolor and damage stone and other materials used in buildings, monuments, statues and other objects of cultural importance.
EPA most recently revised the National Ambient Air Quality Standards (NAAQS) for PM in December 2012, at which time the agency strengthened the primary (to protect health) annual PM2.5 standard to 12 micrograms per cubic meter (μg/m3) and retained the existing 24-hour PM2.5 standard of 35 μg/m3, as well as the existing PM10 standard of 150 μg/m3. With respect to the secondary (to protect public welfare) NAAQS, EPA retained the existing standards of 15 μg/m3 for annual PM2.5, 35 μg/m3 for 24-hour PM2.5 and 150 μg/m3 for 24-hour PM10.
The phenomenon of interstate transport of air pollution across state boundaries from “upwind” sources can affect the ability of “downwind” states hundreds of miles away to attain and maintain the PM and/or ozone NAAQS. With respect to particle pollution, emissions of SO2 and NOx can react in the atmosphere to form PM2.5 pollution. The Clean Air Act includes provisions to address this transport in section 110(a)(2)(D)(i)(I) – the so-called “good neighbor” provisions. In March 2005, EPA issued the Clean Air Interstate Rule (CAIR) to help states address the problem of pollution from power plants that transports from one state to another. CAIR covers 27 eastern states and the District of Columbia and is based on a cap-and-trade system to reduce SO2 and NOx. Under CAIR, states must achieve the required emission reductions using one of two compliance options: 1) meet the state’s emissions budget by requiring power plants to participate in an EPA-administered interstate cap-and-trade system that caps emissions in two phases or 2) meet an individual state emissions budget through measures that the state chooses. After a legal challenge to CAIR, in July 2011, EPA promulgated the Cross-State Air Pollution Rule (CSAPR) to replace CAIR. CSAPR required many eastern and midwestern states to reduce power plant SO2 and NOx emissions that contribute to PM2.5 and ozone downwind states. EPA’s CSAPR was challenged in court (EME Homer City Generation, L.P. et al., v. EPA, No. 11-1302) and in August 2012, the U.S. Court of Appeals vacated CSPAR and remanded it to EPA. The court also directed that CAIR be implemented until EPA issues a replacement rule.In March 2013, EPA and the American Lung Association each appealed the D.C. Circuit Court’s decision vacating CSAPR to the U.S. Supreme Court, which heard oral arguments on December 10, 2013. On April 29, 2014, the Supreme Court reversed the lower court’s decision and remanded the cases for further proceedings consistent with the Supreme Court’s decision. In early June 2014, the D.C. Circuit initiated those proceedings, whch culminated with oral arguments on February 25, 2015. On July 28, 2015, the DC Circuit issued its opinion and judgment in EME Homer City Generation v. EPA. The court revisited this case to 1) consider, on remand from the U.S. Supreme Court, several “as-applied over-control challenges” to EPA’s 2014 emissions budgets and 2) address a number of the petitioners’ broader challenges to the Transport Rule that it did not have occasion to address the first time it heard this case. The court granted in part and denied in part the petitions for review, invalidating and remanding a number of NOx and SO2 emissions budgets and rejecting all other challenges.