On November 21, 2024, the Board adopted amendments to keep Illinois’ ambient air quality standards “identical in substance” to the National Ambient Air Quality Standards (NAAQS). The amendments reflect two actions taken by the United States Environmental Protection Agency (USEPA) during the first half of 2024. First, USEPA revised the primary annual NAAQS for fine particulate matter (PM2.5) from 12.0 micrograms per cubic meter of air (12.0 µg/m3 ) to 9.0 µg/m3 . USEPA set the more stringent standard due to human health effects associated with long- and short-term exposures to PM2.5 in ambient air. Second, USEPA updated its List of Designated Reference and Equivalent Methods. This update modified existing method designations and designated a new Federal Equivalent Method (FEM) for measuring concentrations of ozone (O3) in ambient air.
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