Since the early 2000s, the EPA has employed the Air Pollutant Exposure (APEX) model in exposure and risk assessments for several reviews of the national ambient air quality standards (NAAQS). These assessments are summarized below. Versions of the APEX model are available on the Apex model download page.

Carbon Monoxide (CO) Assessments

Key feature of the EPA's 2010 Risk and Exposure (REA) for review of the primary NAAQS for CO include:

• Estimated population CO exposure, including carboxyhemoglobin (COHB) levels
•  APEX version 4.3, U.S. census data (tract level) for study area(s), monitor-based estimation of ambient air concentrations
• Two simulated at-risk populations of adults: (1) with coronary heart disease (diagnosed and undiagnosed), (2) with any type of heart disease

Nitrogen Dioxide (NO2) Assessments

Key features of the EPA’s 2008 REA for review of the primary NAAQS for NO₂ include:

• Estimated population NO₂ exposure
• APEX version 4.2, U.S. census data (block level) for study area(s), model-based estimation of ambient air concentrations
• Four simulated at-risk populations: (1) children, (2) children with asthma, (3) all people, (4) all people with asthma

Ozone (O₃) Assessments

Key features of the EPA’s 2014 REA for review of the primary NAAQS for O₃ include:

• Estimated population O₃ exposure and health risk (forced expiratory volume in one second - FEV₁)
• APEX version 4.5, U.S. census data (tract level) for study area(s), combined model and monitor-based estimation of ambient air concentrations
• Four simulated at-risk populations: (1) children, (2) children with asthma, (3) adults with asthma, (4) older adults

Key features of the EPA's 2007 REA for review of the primary NAAQS for O₃ include:

• Estimated population O₃ exposure and health risk (FEV₁)
• APEX version 4, U.S. census data (tract level) for study area(s), monitor-based estimation of ambient air concentrations
• Three simulated at-risk populations: (1) children, (2) children with asthma, (3) general population

Sulfur Dioxide (SO₂);Assessments

Key features of the EPA's 2018 REA for review of the primary NAAQS for SO_X include:

• Estimated population SO₂ exposure and health risk (specific airway resistance – sRaw)
• APEX version 4.82, U.S. census data (block level) for study area(s), model-based estimation of ambient air concentrations
• Two simulated at-risk populations: (1) children with asthma, (2) adults with asthma

Key features of the EPA's 2009 REA for review of the primary NAAQS for SO_X include:

• Estimated population SO₂ exposure and associated health risk (sRaw and FEV₁)
• APEX version 4.3, U.S. census data (block level) for study area(s), model-based estimation of ambient air concentrations
• Two simulated at-risk populations: (1) children with asthma, (2) all people with asthma

Assessment-specific input data for the APEX model are not directly available for download. For information on these files, contact Stephen Graham via email at graham.stephen@epa.gov or John Langstaff at langstaff.john@epa.gov.