An air quality alert is in effect for some parts of the Chicago as heat and humidity continue across the area for a fourth straight day.
According to the National Weather Service, Air Quality Action Day was in effect for Lake, Porter, Northern La Porte, Newton and DuBois Counties tin Northwest Indiana through midnight Wednesday. According to the NWS, the forecast was made due to "elevated ozone levels."
"Ozone levels are expected to be at unhealthy levels for sensitive groups," the alert added.
According to AirNow.gov, a federal website that tracks air quality, levels for those parts as of Wednesday afternoon registered as yellow, or "moderate," which ranks as level two on a six-level scale. However, ozone levels still pushed the air quality into the orange, or "unhealthy for sensitive" groups category, which ranks as level three.
The Action Day comes days after a similar alert was issued for multiple counties in Illinois, including Cook, DuPage, Kane, Kendall, Grundy, Lake, McHenry and Will Counties. The alert in those parts, issued Monday and Tuesday, was also due to "elevated ground-level ozone," according to the Illinois Environmental Protection Agency.
Wednesday afternoon, Chicago's Air Quality Index ranked at a PM2.5 of 58, or "moderate" on the scale. However, the primary pollutant for the area, also listed as ozone, noted Wednesday's air quality could still be unhealthy for sensitive groups at times.
What is an Air Pollution Action Day?
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According to NWS, "an Air Pollution Action Day is declared when weather conditions are such that widespread ozone and or particulate levels are expected to be at or above the Unhealthy for Sensitive Groups category of the air quality index for multiple days."
In Illinois, an Air Pollution Action Day is issued when air quality is forecasted to be at or above the Orange or “Unhealthy/Sensitive Groups” category for two or more consecutive days.
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As part of Monday's alert for Illinois, officials urged "active children and adults, especially people with pulmonary or respiratory disease such as asthma" to limit activities outdoors.
What factors can make air quality bad?
There are two major factors that can impact air quality in an area: ground-level ozone or particulate matter, also known as PM2.5.
As of Wednesday afternoon, Chicago's PM2.5 ranked at 58, which some groups could be sensitive to.
"If you are unusually sensitive to particle pollution, consider reducing your activity level or shorten the amount of time you are active outdoors," AirNow.gov said.
According to the Environmental Protection Agency, particulate matter, also called particle pollution, is the term for a "mixture of solid particles and liquid droplets found in the air."
"Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope," the EPA states.
PM2.5 and ground-level ozone are among five major air pollutants regulated by the Clean Air Act, which also includes carbon monoxide, sulfur dioxide and nitrogen dioxide.
While it may seem unrelated, the weather can dramatically impact both ozone and particulate pollution levels.
As the Chicago area continues to experience hot and humid temperatures and sunny skies, weather could play a role in the region's air quality.
That's because "sunshine can cause some pollutants to undergo chemical reactions, resulting in the development of smog" and "higher air temperatures can speed up chemical reactions in the air," according to NWS.
The EPA noted that "ground level ozone forms as a result of chemical reactions between nitrogen oxides and
volatile organic compounds," meaning "emissions from vehicles, power plants, and industrial sources chemically react with sunlight."
