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Air Quality Action Day (AQAD) Advisories

  • Air Quality
  • Current: Air Quality Action Day (AQAD) Advisories

Air Quality Action Days (AQADs) are days when ground level ozone pollution or fine particulate matter could build to unhealthy levels in the outdoor air. Fine particulate matter is known as PM2.5 because it refers to microscopic dust, soot, liquid droplets, and smoke particles that are 2.5 micrometers wide or smaller.

IDEM issues AQAD advisories for ozone (O3) and PM2.5 based on air quality forecasts, air quality standards, and Air Quality Index (AQI) categories. When AQADs are predicted, Hoosiers can take action to protect their health and protect air quality.

Air Quality Forecasts

Ground level ozone and PM2.5 are two pollutants of particular concern across the nation. Indiana collects ground-level ozone data from March 1 through October 31 each year, when the pollutant is most likely to form, and PM2.5 data year-round. IDEM meteorologists and air quality professionals from local metropolitan areas and neighboring states examine concentrations of the pollutants and weather conditions to issue air quality forecasts.


Ozone is not emitted directly into the air but can form from a chemical reaction between volatile organic compounds (VOCs) and nitrogen oxides (NOx)—known as ozone precursors—in the presence of heat and sunlight. PM2.5 is emitted directly in smoke or can form from chemical reactions of nitrogen oxides and sulfur dioxides in the atmosphere. PM2.5 also comes from activities that stir up tiny airborne dust, as well as natural events such as volcanos and wildfires.

Ozone, ozone precursors, and PM2.5 can be carried to Indiana by the wind from hundreds of miles away or generated locally. In Indiana, motor vehicles are the primary source of the ozone precursors.

Ground-level ozone can cause respiratory problems for anyone; however, sensitive groups such as young children, the elderly, and people with respiratory conditions can be particularly vulnerable to exposure. Individuals who suffer from asthma, chronic bronchitis, emphysema, or other lung disease will generally experience more serious health effects.

Because PM2.5 is extremely small, the particles can deposit deep in lungs and are difficult to exhale. For comparison, a human hair is 75 micrometers wide. Being exposed to PM2.5 may cause coughing and difficulty in breathing. Exposure over several days may increase the chance of these symptoms. Health risks are greater for individuals with heart or lung diseases such as coronary artery disease, congestive heart failure, asthma, chronic obstructive pulmonary disease (COPD), and diabetes.

Health professionals advise limiting physical exertion outdoors to reduce the chance for ill health effects from poor air quality.

Common Weather Conditions for AQAD Advisories

Typical conditions for ozone AQADs in Indiana are high temperatures approaching 80° Fahrenheit or above, clear skies, dry atmosphere, calm to light southerly winds, very little air mixing, high NOx values the previous night, and/or persistent high pressure over the eastern Midwest states and East Coast. Ground-level ozone pollution is known as a summertime pollutant because long, still days and warm temperatures contribute to its formation.

Typical conditions for PM2.5 AQADs in Indiana are temperature inversions, light winds, clear skies, persistent high pressure, high humidity values, transport from high PM2.5 locations (such as wildfires), and/or warm and humid air over snow cover during the winter.

Air Quality Standards, AQI Categories, and Predictions That Trigger AQAD Advisories

Indiana follows requirements in the National Ambient Air Quality Standards (NAAQS) to monitor ozone and PM2.5 in the outdoor air and alerts the public when either pollutant could reach or exceed certain limits. For ozone, Indiana takes action when the average concentration over any 8-hour timeframe could reach or go above 70 parts of ozone per billion parts of air. For PM2.5, Indiana takes action when the average concentration for 24-hours could reach or exceed 35 micrograms per cubic meter (µg/m3). Air quality forecasters also refer to U.S. EPA’s Air Quality Index (AQI), which is a tool used to communicate outdoor air quality. The higher the AQI value, the greater the level of air pollution and associated health concerns. IDEM will issue an AQAD for the following AQI and pollutant predictions:

AQAD Advisories

ppb = parts per billion
µg/m3 = micrograms per cubic meter
Ozone AQI is based on an 8-hour average
PM2.5 AQI is based on a 24-hour average

Health impacts associated with these AQI categories are:

  • High Moderate: Occasionally, an action day is declared when the AQI is in the high Moderate range, or Code Yellow, if the levels are expected to approach Code Orange levels. Health alerts issued under these conditions are intended to inform the general public that there may be a moderate health concern for individuals who are unusually sensitive.
  • Unhealthy for Sensitive Groups (USG): This triggers a health alert to inform the general public about potential impacts to sensitive groups, including people with lung disease, older adults, and children who are more vulnerable to ozone and people with heart and lung disease, older adults and children who are more vulnerable to PM2.5.
  • Unhealthy: This triggers a health alert to inform the general public that everyone may begin to experience some adverse health effects and members of sensitive groups may experience more serious effects.
  • Very Unhealthy: This triggers a health alert to inform the general public that everyone may experience more serious health effects.
  • Hazardous: This triggers a health warning of emergency conditions that are more likely to affect the entire population.

Take Action

The IDEM SmogWatch website provides the latest forecasts as well as the ability for users sign up and receive AQAD advisories by email.

When Air Quality Action Day (AQAD) advisories are issued for your area, take precautions such as limiting physical exertion outdoors. Follow these simple pollution prevention tips to reduce emissions from daily activities:

  • Carpool, walk, bike, or use public transportation when possible.
  • Refuel vehicles after dusk.
  • Avoid excess idling and drive-through windows.
  • Consolidate trips and avoid fast-starts.
  • Postpone using gasoline-powered garden equipment or mowing the lawn until late evening, when temperatures are cooler.
  • Work from home to reduce vehicle emissions, if your employer provides the option.
  • Use energy efficient lighting and appliances recommended by the Energy Star Program.
  • Turn off appliances and lights when not in use to reduce emissions from energy production.
  • Adjust your thermostat by turning it up in the summer and down in the winter to reduce emissions from energy production.
  • Recycle to reduce emissions related to producing paper, plastic, glass bottles, aluminum cans, and cardboard.
  • Use “low VOC” or “zero VOC” paint and cleaning products.
  • Consider burning gas logs instead of wood to reduce smoke.
  • Avoid burning clean wood waste such as leaves and brush. If possible, recycle yard waste by shredding or chipping it at home or use a registered collection site. Never burn trash.

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