Ozone Formation

Thousands of substances contribute to air pollution. The Federal Clean Air Act requires that the U.S. Environmental Protection Agency publish a list of criteria pollutants that negatively affect air quality and public health. The EPA established the National Ambient Air Quality Standards to ensure that public health is protected from any known or anticipated negative effects from criteria air pollutants. Ozone is one pollutant that falls onto EPA's list of criteria air pollutants and as such the state of Missouri is required to regulate the emissions that contribute to its formation.

Ozone is a primary pollutant of concern in Missouri. The St. Louis community must continue to take actions to prevent the formation of ground-level ozone. Control devices, such as vehicle emissions testing, are an important part of protecting air quality. Controlling harmful emissions and making air conscious decisions are necessary in order to protect public health and prevent the formation of ground-level ozone.

When pollution from your vehicle combines in the presence of heat and sunlight, ground-level ozone – commonly known as smog – is created. Ground-level ozone is an irritant that damages lung tissue, aggravates heart and respiratory disease and can even cause problems for healthy individuals who spend a lot of time outdoors.

Typically, ozone pollution is a problem in the St. Louis area in the hot summer months (from late May to early September) when higher temperatures cause the chemical reaction to take place. Ozone levels tend to rise in mid-morning, several hours after the rush-hour and onset of emissions-generating business operations and peak in the late afternoon.

Ozone Forecasting

In St. Louis, the 4 Warn Aircast® reports the air pollution values on a scale known as the Air Quality Index (AQI). The AQI converts the measured concentration of pollutants in the air to a number on a scale of 0 to 500. The most important number on this scale is 100, since this number corresponds to the standard established under the Clean Air Act. An AQI of 100 or below is protective of public health. When the AQI exceeds 100, sensitive populations should limit their exposure to outdoor air because it can be unsafe.

The AQI scale is broken into four ranges, each of which correspond to a different color. Air quality is best on green days, the higher the concentration of air pollutants on a given day, the higher the AQI. Current air quality forecast in St. Louis.

AQI figures enable the public to determine whether air pollution levels in a particular location are good, moderate, unhealthful, or worse. The table below shows each AQI color range and its effect on public health.

Air Quality AQI Range Weather Conditions Health Information
0 - 50 AQI Cool summer temperatures, windy and/or cloudy, recent rain or cool front None
51 - 100 AQI Temperatures mid 70's or above, light winds, sunny skies Very sensitive individuals, people with respiratory disease should limit prolonged exertion outdoors.
(Unhealthy for sensitive groups)
101 - 150 AQI Temperatures 80's or above, very light winds, sunny skies, hazy, hot HEALTH NOTICE: Sensitive children and adults and people with respiratory disease, such as asthma, should limit prolonged, moderate exertion outdoors.
151- 200 AQI Hazy, hot (90's) and humid HEALTH ADVISORY: Sensitive individuals, people with respiratory disease should avoid exertion outdoors. Others should limit prolonged or vigorous outdoor.


Air pollution readings above 100 AQI will trigger preventive action by the American Lung Association of Missouri or other local officials. On days where levels are forecasted to be Orange or Red, above 100 AQI, the American Lung Association and the St. Louis Regional Clean Air Partnership issue an Air Quality Alert. This could include health advisories for citizens or susceptible individuals to limit certain activities and potential restrictions on industrial activities. During air quality alerts, both businesses and residents in St. Louis should modify their behaviors so that emissions leading to ozone air pollution are reduced.

Emissions Testing and Clean Air

Participating in the Gateway Vehicle Inspection Program cleans the air you breathe and helps catch small problems before they become bigger, more expensive repairs. Having emissions testing performed can help alert you to malfunctions in your vehicle's pollution-control devices and lets you know when fuel is being wasted. When a vehicle receives proper maintenance, it saves you money in the long run.

Did You Know?

  • If a vehicle fails its emissions test and is repaired, dirty emissions can be reduced by as much as 80 percent.
  • A poorly maintained vehicle can release as much as 100 times the pollution of a well-maintained vehicle.
  • St. Louisans make more than six million vehicle trips each day. Of these trips, five million are single-occupancy vehicle trips.
  • People in St. Louis drive over 72 million miles every day, producing over 88 tons of ozone forming emissions daily!
  • According to the U.S. Environmental Protection Agency, although air quality has greatly improved, vehicles on the road account for at least 29 percent of air pollution emissions nationwide.
  • One person riding in a Vanpool/Carpool instead of a car can save the environment 9.1 pounds of harmful emissions every year!
  • The average commuter can save about $1,500 per year by sharing the ride instead of driving alone to work.
  • A single highway lane can accommodate 2,250 people per hour in cars and 9,000 people per hour in buses!
  • Two MetroLink tracks have the same capacity as 16 lanes of highway!
  • A full MetroBus at rush hour removes 40 cars from the highway, and a full MetroLink train at rush hour removes 125 cars from the highway.
  • Commuters can contact any of the following to link up with a carpool, vanpool or try MetroBus or MetroLink.

Preventing Air Pollution

What can I do to help protect air quality?
Steps can be taken every day to help prevent air pollution. Motorists can follow the tips below to reduce ozone-forming emissions and protect public health.

Pay attention to your vehicle's warning messages.
The “Check Engine” and “Service Engine Soon” light warn you that your engine is not performing efficiently and may be polluting too much. Bring your car to an authorized repair facility as soon as you see these warning lights.

Properly maintain your vehicle.
Give your car regular tune - ups and maintenance. Getting regular tune-ups and oil changes and keeping your tires properly inflated help maintain efficient fuel consumption and reduce air pollution. Also, use the grade of motor oil recommended by the vehicle manufacturer. This increases gas mileage significantly.

Be fuel forward.
During warm weather, fill your tank in the evening to reduce air pollution. And to prevent gases from polluting the air, don’t top off your tank when you fill up. Remember ground-level ozone forms on hot stagnant days through interactions between air pollutants such as fuel emissions and sunlight.

Drive smart.
Combine errands into one sensible trip to reduce traffic congestion and air pollution. Your car’s engine cools down in about an hour, and starting it cold generates up to five times the pollution of starting it hot. Also, share rides, take mass transit, and bicycle or walk whenever possible.