https://19january2021snapshot.epa.gov/co-pollution/setting-and-reviewing-standards-control-carbon-monoxide-co-pollution-outdoor-air_.html monthly 1.00 https://19january2021snapshot.epa.gov/co-pollution/basic-information-about-carbon-monoxide-co-outdoor-air-pollution_.html monthly 0.80 https://19january2021snapshot.epa.gov/co-pollution/technical-data-and-reports-carbon-monoxide-measurements-and-sip-status_.html monthly 0.80 https://19january2021snapshot.epa.gov/co-pollution/applying-or-implementing-outdoor-air-carbon-monoxide-co-standards_.html monthly 0.80 https://19january2021snapshot.epa.gov/co-pollution/national-ambient-air-quality-standards-naaqs-carbon-monoxide-co_.html monthly 0.80 https://19january2021snapshot.epa.gov/co-pollution/fact-sheets-and-additional-information-regarding-national-ambient-air-quality-standards_.html monthly 0.80 https://19january2021snapshot.epa.gov/co-pollution/forms/contact-us-about-carbon-monoxide-co-pollution_.html monthly 0.80