MIDSTATE, Pa. (WHTM) — Over the last few months, you may have noticed many Air Quality Alerts or Air Quality Action Days being issued by the Pennsylvania Department of Environmental Protection (DEP).

But, why has this been happening? The weather has a lot to do with it. To understand when and why these action days are issued, you have to understand the Air Quality Index (AQI)

The AQI is used to monitor air quality throughout the United States. There are five levels to the AQI, as quoted by the Pennsylvania Department of Environmental Protection:

  •  An AQI value of 0 to 50 is considered “good” and pollution at this level is considered to pose little to no risk to human health. 
  • A value of 51 to 100 is considered “moderate” and pollution at this level poses little risk for most but may be a risk for those unusually sensitive to air pollution. 
  • A value of 101 to 150 is considered “unhealthy for sensitive groups” and those sensitive to air pollution may experience negative health effects from pollution at this level. 
  • A value of 151 to 200 is considered “unhealthy” and pollution at this level becomes a concern for everyone. 
  • A value of 201 to 300 the greater risk of air pollution becomes to human health.
  • Anything above a value of 301 can cause a health warning of emergency conditions, and everyone is more likely to be affected.

Most of these air quality action days are due to something called fine particulate matter. So, what exactly is that?

Courtesy of Airnow.gov

The DEP says that this matter in the atmosphere can come from a few sources. Some come from wildfires, chimneys, and unpaved roads. It can also come in the form of chemical reactions from emissions of other sources such as cars, trucks, power plants, and industries.

Certain weather conditions can cause these particulates to be trapped near the ground, causing elevated concentrations. When the weather forecast calls for elevated concentrations of an AQI over 100, that is when an Air Quality Action Day is issued.

An example of one of these weather conditions is a temperature inversion. This is when the temperature of the air increases with altitude, rather than it decreasing. The greater the increase, the stronger the inversion.

This tends to happen most in the spring and fall months, due to fluctuating temperatures and more active weather patterns. However, this can happen in the winter months when the air mass aloft is warmer than usual.

The action day that was declared for Wednesday, Jan. 11 was because of a temperature inversion. This inversion was caused by an air mass of high pressure. This high pressure combined with very light winds caused the action day to be issued.

The DEP states that the last few occurrences of action days have been fairly short-lived, before conditions improve either due to a change in air mass or a storm system bringing precipitation and a cold front, essentially “cleaning” the air out.

For example, on Thursday, Jan 12, showers from a cold front are moving into the region and will cause the temperature inversion to stop. Better air quality is expected into Thursday and Friday, due to an air mass change, according to the DEP.

Current air quality information for the Midstate can be found by typing your zip code into https://www.airnow.gov.