How, What, When & Why We Provide Forecasts and Issue Alerts

Evansville EPA / Vanderburgh County Health Department Ozone Office



Our ability to monitor and forecast air quality has improved over the years due to advances in monitoring equipment and computerized information systems. Forecasts are provided by an Air Quality Forecast Group, which includes staff from the Vanderburgh County Health Department Ozone Office, the Indiana Department of Environmental Management (IDEM), the Indianapolis Office of Environmental Services, the Metro Louisville Air Pollution Control District, and the Evansville EPA.

Forecasts are based on an analysis of current pollution levels provided by ambient air quality monitors and current and predicted weather conditions from the National Weather Service. The Forecast Group does their best to predict pollution levels as accurately as possible. Everyone should remember that ozone and particulate levels are very dependent on the weather and we do not have a complete understanding of why, when, or how these pollutants are created and transported.

For Southwestern Indiana, we are concerned with two main pollutants: ozone and particulate matter.  Ozone, a hot weather pollutant is a concern from May through September.  Monitoring and forecast information for ozone is available only during these months. Particulate Matter 2.5 micrometers in diameter (PM2.5) can be elevated during any time of the year. Monitoring data and forecasts are available all year long for PM2.5.  

The Vanderburgh County Health Department Ozone Office and the Evansville EPA, routinely make air quality forecasts by 10:30a.m. Evansville time on Monday (for Tuesday through Friday) and Friday (for Saturday through Monday). When weather conditions are uncertain or pollution levels are high, we update forecasts on a daily basis.

Our air quality forecasts are available on the City/County website:; and are also available at IDEM’s website:; and the U.S.EPA’s website at:

On a national level, the U.S.EPA provides air quality forecast information directly to commercial weather service providers, so that forecast information may be readily integrated with media weather programming.

Beginning in 2007, in an effort to provide the public with readily available information on air quality in Southwest Indiana, the Evansville EPA and the Ozone Office will partner to email Air Quality Forecasts to local media at least one day in advance. We hope the media will incorporate these air quality forecasts into the weather information they provide to the public, so that individuals who may be especially sensitive to ozone or PM2.5 can adjust their activities to protect their health.


The U.S. EPA is required by law to set National Ambient Air Quality Standards (NAAQS). The NAAQS are the U.S. EPA’s judgment on how clean the air must be to protect public health and the environment. Every five years, the U.S. EPA is required by law to review new research and studies to determine if new information may warrant revising the NAAQS to be more protective of public health and the environment.

After conducting its own research and reviewing thousands of other research studies, the U.S. EPA decides what concentration of an air pollutant over a specific time period, is sufficiently protective of public health, with an adequate margin of safety. For example, the U.S. EPA has determined that for “sensitive groups,” (active children and adults, people with asthma or respiratory problems) exposure to eight hours of ozone levels averaging 85 parts per billion (ppb) to 104 ppb is unhealthy. Eight hours of exposure to ozone levels averaging 105 ppb to 124 ppb is considered unhealthy for everyone, not just the “sensitive groups’.


PARTICULATE MATTER 2.5 micrometers in diameter (PM2.5)

PM2.5 is microscopic material found in the air--including dust, dirt, soot, smoke, and liquid droplets that are 2.5 micrometers in width or less. In comparison, a human hair is about 75 micrometers in width.

PM2.5 is made up of hundreds of different chemicals, from hundreds of different sources, such as the burning of wood and coal, automobile and truck exhaust, emissions from factories, businesses and homes, and ammonia and dust from agricultural activities and natural sources. Some PM2.5 is emitted directly from the stack or tailpipe, while other PM2.5 is created by chemical reactions in the atmosphere between other pollutants.

Unlike Ozone, PM2.5 levels can be elevated during any part of the year and any time of the day. PM2.5 levels may be much higher indoors, than outdoors.


PM2.5 is a health concern because these tiny particles get trapped deep in your lungs where they can’t be easily expelled. Exposure to elevated PM2.5 levels has been associated with increased respiratory symptoms, such as irritation of the airways and difficulty breathing, decreased lung function, aggravated asthma, chronic bronchitis, irregular heartbeat, heart attacks, and premature death.


People with heart or lung diseases—such as coronary artery disease, congestive heart failure, asthma or chronic obstructive pulmonary disease (COPD)—are at increased risk, because particles can aggravate these diseases. People with diabetes also may be at increased risk, possibly because they are more likely to have underlying cardiovascular disease.

Older adults are at increased risk, perhaps because they may have undiagnosed heart or lung disease, or diabetes. Many studies show that when particle levels are high, older adults are more likely to be hospitalized, and some may die of aggravated heart or lung disease. Children are at increased risk for several reasons: their lungs are still developing; they spend more time at high activity levels; and they are more likely to have asthma or acute respiratory diseases, which can be aggravated when particle levels are high.


The more strenuous your activity and the longer you are active, the more likely you are to be affected by particle pollution. If your activity involves prolonged or heavy exertion, reduce your activity time—or substitute another activity that involves less exertion. Go for a walk instead of a jog, for example. Postpone strenuous activities until particle levels are lower. Don't exercise near busy roads, for particle levels are generally higher in these areas.


The Air Quality Index (AQI) is the system which provides health related information on current and forecasted air quality. For each pollutant, the AQI sets pollution ranges which correlate to colors:


Corresponding ranges for PM2.5

Color Air Quality averaged over 24 hours (from midnight to midnight):






0 to 12.0 micrograms per cubic meter (µg/m3)






12.1 to 35.4 µg/m3 averaged over 24 hours





35.5 to 55.4 µg/m3 averaged over 24 hours






55.5 to 150.4 µg/m3 averaged over 24 hours


The Evansville EPA and Vanderburgg County Health Department Ozone Office will call Particulate Alerts whenever PM2.5 is predicted to reach the new NAAQS of 35µg/m3 averaged over 24 hours.

This is not the same as calling a Particulate Alert if the PM2.5 on-line monitor registers one or two hours at or over 35µg/m3. There is a well-known and widely accepted concept that the duration of exposure, the nature of the pollutant, the concentration of pollutant, and the sensitivity of the individual, are factors which determine the level of harm or lack of harm to an individual. After years of research and thousands of studies, the U.S.EPA has determined that a PM2.5 NAAQS of 35µg/m3 24-hour average is sufficiently protective of public health with an adequate margin of safety, so that is the level at which we will call Particulate Alerts.


In Southwest Indiana, there are PM2.5 monitors located in Knox County (south of Vincennes), Evansville, Jasper and Dale. Information on monitor locations throughout the state and on-line monitoring data can be accessed at Most of the monitors take in samples of the air, which must then be shipped to a laboratory for analysis.  The results may not be available for weeks or months. There are a few monitors which provide more timely information, which you can view at the internet address provided above. Please remember, there is a delay of about two hours between the time the on-line monitors sample the ambient air, and the time the data shows up on the website. The data from the on-line monitors is less accurate, than the data from the monitors which must be shipped to laboratories for analysis. The on-line monitor is useful as a forecast tool, but the final PM2.5 levels determined by laboratory analysis may differ from the on-line monitor’s reading for the same day.


Ozone is a “hot weather” pollutant. For Southern Indiana, May through September is considered “Ozone Season.” Ozone forecasts and monitoring data will be available only during these months. Ozone is formed when strong sunlight triggers chemical reactions between Nitrogen Oxides (NOx) from sources such as boilers and vehicle exhaust, and Volatile Organic Compounds (VOCs) which include gasoline, parts cleaners, solvents, paints, etc.

Ozone levels rise and fall with the sun, with the highest levels typically monitored around 3 p.m. Ozone levels are lower in the morning and the evening, so people should consider changing their work or exercise schedules to avoid the peak levels in the afternoon. Ozone levels are lower indoors, than outdoors.


Breathing higher levels of ozone can trigger a variety of health problems--including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. Ground-level ozone also can reduce lung function and inflame the linings of the lungs, possibly making allergy symptoms worse.


Active adults and children, the elderly, and people with respiratory conditions such as asthma or emphysema, may experience adverse health effects from ozone at lower concentrations than do other people and are considered “Sensitive Groups” by the U.S. EPA.


Like PM2.5, the best way to protect your health, is to limit your exposure to ozone. Reschedule your strenuous activities to avoid the peak ozone times of 12:00 p.m. to 6:00 p.m. During the afternoon hours, reduce your physical activity or just stay inside, where ozone levels are lower.


The Air Quality Index (AQI) is the system which provides health related information on current and forecasted air quality. For each pollutant, the AQI sets pollution ranges which correlate to colors:

Corresponding ranges for Ozone

Color Air Quality averaged over 8 hours:






0 to 59 parts per billion (ppb) avg. over 8 hours






60 to 75 parts per billion (ppb) avg. over 8 hours





76 to 95 parts per billion (ppb) avg. over 8 hours






96 to 115 parts per billion (ppb) avg. over 8 hours


Please remember, there is a delay of about two hours between the time the ozone monitors sample the ambient air, and the time the data shows up on the website. The ozone monitors not only provide on-line data, but are also accurate enough that they are used to determine the area’s attainment status, after the data has been quality-assured. On-line monitoring data and monitor locations throughout the state can be accessed at


Since 1997, the U.S. EPA has considered an 8-hour average of 84 ppb or less to be protective of public health, with an adequate margin of safety. In 2008 the NAAQS were made more stringent, and the “Unhealthy for Sensitive Groups” level was set at 76 ppb over an 8-hour average. The Evansville EPA and the Vanderburgh County Health Department Ozone Office have issued Ozone Alerts when ozone levels, over an 8-hour time frame, were predicted to reach 76 ppb or greater.


Although not required by federal or state law, or the Municipal Code of Evansville, the Evansville EPA and the Vanderburgh County Health Department Ozone Office sincerely believe issuing Ozone and Particulate Alerts to be one of our most important responsibilities.

The decision to issue an Ozone or Particulate Alert is made jointly with the other agencies in the Forecast Group. We’ll do our best to issue the Ozone Alerts or Particulate Alerts by 10:30 a.m. the day before the Alert is needed; although there may be times the weather conditions are so uncertain or change so suddenly that we won’t be able to meet that deadline.

The Evansville EPA notifies the media and many other groups throughout Southwestern Indiana, including city / county governments, school systems, hospitals, emergency medical providers, Chambers of Commerce, businesses, industries, day care centers, and nursing homes.

Email is the preferred method of providing notification, but we also send faxes and in the case of some smaller day care centers, we will telephone them. If you want to receive our Alert notifications, please contact the Evansville EPA at 812/435-6145 or send a message to

The main goal of an Ozone or Particulate Alert is to provide warning (preferably in advance) of degraded air quality, to protect the health of “Sensitive Groups”--those with respiratory conditions, the elderly and children. This advance warning also provides notice to caregivers so that they can plan to protect their students, clients, and patients.

A secondary, but very important goal of an Ozone or Particulate Alert, is to ask everyone to voluntarily reduce their contributions to air pollution.


Ozone and PM2.5 may be different pollutants, but many reduction strategies work for both:

CONSERVE ENERGY – reduce the use of all types of energy: electricity, natural gas, gasoline and diesel fuel.

LIMIT DRIVING – This reduces Nitrogen Oxide emissions and the need to refuel.

POSTPONE REFUELING – Gasoline is a Volatile Organic Compound.  Small amounts are emitted whenever you fill your gas tank.

AVOID TOPPING OFF your tank – even more gasoline fumes are emitted when you top-off.

POSTPONE YARD WORK – It’s not a good idea for even healthy people to exert themselves when ozone or PM2.5 levels are high.  Gasoline-powered lawn equipment emit Nitrogen Oxides, Volatile Organic Compounds and particulates, all of which add to the ozone/particulate problem.

NO OPEN BURNING is allowed in the City of Evansville or four miles outside the City limits during Ozone or Particulate Alerts. Smoke is a source of PM2.5 and during Ozone Alerts, we need to protect our remaining air quality.

POSTPONE GRILLING - Use of gas or charcoal cooking grills is allowed, but does add to the pollution, so we ask that you postpone grilling until air quality improves.


If you have any questions, comments, or suggestions, please contact:

Dr. Joanne Alexandrovich, Vanderburgh Co. Health Dept., 812/435-5764


Jacob Keating, Evansville EPA, 812/435-6145



Last updated: 8/28/2014 3:54:29 PM