10 Air Quality Terms You Need to Know

Air Quality Terms

When it comes to air quality, there can be a lot of technical terms and jargon you need to know. As with any science, it’s important to define these terms so we all have the same understanding and language when we communicate.

We believe in making research not just understandable—but also accessible.

As part of that goal, we’re defining the terms here so you can better understand them. That way you can stay informed about what’s happening when it comes to climate sciences.

Here are 10 essential climate terms to know and their definitions.

Air Quality Index

An Air Quality Index, or AQI, is a nationally standardized indicator for monitoring and predicting daily air quality used around the world. These are also represented on AQI scales that concentrate on potential health impacts that can occur after inhaling contaminated air.

In the United States in particular, the Clean Air Act regulates the 4 most prevalent ambient air pollutants, including ground-level ozone, particle pollution (PM10 and PM2.5), carbon monoxide (CO), and sulfur dioxide (SO2).

Air Quality Advisories

These are the alerts typically issued whenever the Air Quality Index (or AQI) reaches harmful levels. They are referred to by various air pollution control organizations, governments, and authorities at multiple levels.

For example, if you have an air quality alerts app, it would tell you of any upcoming air quality advisories.

Carbon Monoxide (CO)

Carbon Monoxide (CO) is a colorless and odorless gas. When exposed in excessive quantities, carbon monoxide can become dangerous. Any time something burns, CO emits. Motors, lorries, or machinery that burns fossil fuels are the main emitters of CO in the ambient air.

Air Pollutant

Particles, gases, or droplets suspended in the air that cause harm to Earth’s ecology and human health are referred to as air pollutants. We can categorize them into 3 main categories: biological pollutants, air toxins, and criterion air pollutants.

Air Quality Alerts

When pollutant concentrations reach high levels according to nationally established Air Quality Categories (AQC), an air quality warning may be issued. AQCs are average threshold concentrations measured hourly rather than national criteria for pollutants, which are daily averages from midnight to midnight.

The two categories of air quality warnings are:

  1. Hourly Alerts: Whenever air quality is bad or worse, any station within the standard-compliant surveillance system may issue an hourly air quality warning.
  2. Daily Air Quality Prediction Notifications: An air quality warning forecast is issued when there is a forecast that air quality will deteriorate the next day. Forecast notifications are for particles based on the guidelines (i.e., 24-hour averages).

Coarse particles or PM10

PM10, or particulate matter 10, refers to coarse particles with a diameter typically more than 2.5 micrometers and less than or equal to 10 micrometers. Some examples of PM10 are mold spores, dust, smoke, bacteria and viruses.

Generally, PM is measured by drawing in the air and measuring the concentration level on a filter. It can also be measured with other technology like light sensors, fans, and lasers.

Fine particles PM2.5

Particulate matter with a diameter of 2.5mm or less is called fine particles. These are also known as PM2.5. Also included in this category of particles are nanoparticles and ultrafine particles, often defined as having dimensions smaller than 0.1 micrometers.

Like with PM10, there are some overlapping examples of PM2.5 like mold spores, dust, and smoke. However, this also includes smaller particles like pollen, ash, and soot.

HEPA (High-Efficiency Particulate Air) Filters

This refers to mechanical air filters with pleats that come in the form of HEPA filters. When it comes to the high-end models, this kind of air filter can eliminate 99.97% or more indoor pollution like dust, pollen, mold, germs, and any other air pollutants smaller than 0.3 microns (µm).


This is the tendency for solid objects to have the ability to absorb moisture from the environment or atmosphere. As a result, organic material can change its appearance and size after adsorption.

Ground-Level Ozone

Ground-level ozone is produced through chemical interactions between nitrogen oxides (NOx) and volatile organic molecules. When sunlight is present, pollutants released from factories, power plants, industrial boilers, refineries, and other sources undergo chemical reactions.

Knowing standard climate terms is the first step to making science and academia more accessible. It’s also important if you want to better understand how air quality monitoring works and stay informed.

Worried about air quality?

Monitor the Particulate Matter levels around the world with our free, real-time PurpleAir Map. or join PurpleAir's mission to make air quality data accessible to everyone by investing in an air quality monitor for your home. 

Together, we can be informed and make changes in our daily habits and the community to improve air quality.