"National Primary and Secondary Ambient Air Quality Standards", 40 CFR—Protection of the Environment, Chapter I, Part 50, Section 50.3, 1998 National Ambient Air Standards.
Archivado desde el original el 14 de abril de 2006. "National Primary and Secondary Ambient Air Quality Standards", 40 CFR—Protection of the Environment, Chapter I, Part 50, Section 50.3, 1998 National Ambient Air Standards (enlace roto disponible en Internet Archive; véase el historial y la última versión).WikiMatrix WikiMatrix