Criteria Pollutant Monitoring

Criteria pollutants are six commonly-occurring air pollutants that have been deemed harmful to public health and the environment through the Clean Air Act.

The Louisville Metro Air Pollution Control District (APCD) monitors for these pollutants at six monitoring stations throughout Louisville Metro to determine the Air Quality Index (AQI) and the area’s attainment with National Ambient Air Quality Standards. The methods used to monitor for criteria pollutants are strictly regulated by EPA and standardized across the country.

The pollutants are: ground-level ozone (O3), Particle Pollution (PM10 and PM2.5), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), and lead (Pb).

Air Quality Index

The Air Quality Index (AQI) is tool to used to communicate the impact of air quality on health. It calculated for five criteria pollutants: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Live AQI values measured at through the APCD's monitoring network are shared through Louisville Air Watch. Historical AQI Values can be accessed using the EPA's Air Data website and through tools on APCD's Air Quality Data webpage.

Dots around Louisville map signaling the live AQI
Louisville Air Watch provides live AQI values from APCD's monitoring sites.

National Ambient Air Quality Standards

The monitored levels of pollutants also determine Louisville's attainment of the National Ambient Air Quality Standards (NAAQS), which are set through the Clean Air Act. Louisville's current status for each standard is in the table below. 


Standard (Averaging Time)


Carbon Monoxide

9 ppm (8 hours)


Carbon Monoxide

35 ppm (1 hour)



0.15 μg/m3 (Rolling 3-month average)


Nitrogen Dioxide

100 ppb (1 hour)


Nitrogen Dioxide

53 ppb (1 year)



0.070 ppm (8 hours)


Particle Pollution (PM10)

150 μg/m3 (24 hours)


Particle Pollution (PM2.5)

9.0 μg/m3 (1 year)


Particle Pollution (PM2.5)

35 μg/m3 (24 hours)


Sulfur Dioxide

75 ppb (1 hour)


Sulfur Dioxide

0.5 ppm (3 hours)


1In 2016, the EPA authorized the APCD to discontinue active monitoring of airborne lead (Pb) because levels are far below the NAAQS and are not expected to increase at the present time.

2On February 7, 2024, US EPA strengthened the NAAQS for fine particle pollution from 12 µg/m³ to 9 µg/m³. Louisville’s attainment status is based off the previous standard. Louisville will receive an initial designation status for the new standard two years after this rule is finalized.


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