This rule sets forth the initial air quality designations and classifications for all areas in the United States, including Indian country, for the fine particles (PM2.5) National Ambient Air Quality Standards (NAAQS). The EPA is issuing this rule so that citizens will know whether the air quality where they live and work is healthful or unhealthful. Health studies have shown significant associations between exposure to PM2.5 and premature death from heart or lung disease. Fine particles can also aggravate heart and lung diseases and have been linked to effects such as cardiovascular symptoms, cardiac arrhythmias, heart attacks, respiratory symptoms, asthma attacks, and bronchitis. These effects can result in increased hospital emissions, emergency room visits, absences from school or work, and restricted activity days. Individuals that may be particularly sensitive to PM2.5 exposure include people with heart or lung disease, older adults, and children. This rule establishes the boundaries for areas designated as nonattainment, unclassifiable, or attainment/unclassifiable. This rule does not establish or address State and Tribal obligations for planning and control requirements that apply to nonattainment areas for the PM2.5 standards. The EPA will publish a separate rule which will set forth the planning and control requirements that apply to nonattainment areas for the PM2.5 standards.