How Is Air Quality Measured? AQI Explained

June 08, 2026 / Diing Shenq Yiauw

You've probably noticed an AQI number pop up on a weather app or local news report and wondered what it actually means. It's more relevant than most people realize.

The Air Quality Index is the standard system governments use to communicate how polluted the air is, both outside and, increasingly, inside.

Here's what those numbers mean, how air quality is measured, and what you can do to improve the air where you live.

What Is the Air Quality Index (AQI)?

The Air Quality Index (AQI) is a numerical scale developed by the U.S. Environmental Protection Agency to translate complex air pollution data into a single, readable number that runs from 0–500. The higher the number, the greater the pollution level and the greater the potential health concern.

It tracks 5 major air pollutants regulated under the Clean Air Act. Here's a quick breakdown of what each one is and where it typically comes from:

Ground-level ozone: Forms when vehicle and industrial emissions react with sunlight. Levels tend to be highest on warm, sunny days in urban areas.
Particle pollution (PM2.5 and PM10): Tiny particles from vehicle exhaust, wildfires, industrial activity, and windblown dust. PM2.5 is the most concerning because particles this small can travel deep into the lungs.
Carbon monoxide: Produced by burning fossil fuels. Common sources include vehicle exhaust, gas appliances, and wood-burning stoves.
Nitrogen dioxide: Generated mainly by vehicle exhaust and power plants. Prolonged exposure can irritate the airways.
Sulfur dioxide: Released primarily through industrial processes and burning coal or oil. Less common in residential areas but relevant near power plants or refineries.

Each pollutant receives its own sub-index score, and the daily AQI value reported is the highest of those 5.

AQI Categories Explained: What the Numbers & Colors Mean

AQI Value	Color	Category	What It Means
0–50	Green	Good	Air quality is satisfactory; little to no risk
51–100	Yellow	Moderate	Acceptable overall; some risk for unusually sensitive people
101–150	Orange	Unhealthy for Sensitive Groups	Children, older adults, and those with respiratory conditions may be affected
151–200	Red	Unhealthy	Everyone may begin to experience health effects
201–300	Purple	Very Unhealthy	Health alert; serious effects for the general population
301–500	Maroon	Hazardous	Health emergency; everyone is at risk

Source: U.S. EPA AirNow

Once the AQI climbs above 100, sensitive groups, including people with asthma, heart conditions, or chronic respiratory disease, should reduce prolonged outdoor activity.

Above 150, that guidance extends to everyone.

At 200 and beyond, staying indoors with windows closed and running an air purifier is the most practical protective step available to most households.

How Is Air Quality Monitored?

Outdoor Air Quality

Government air quality networks rely on stationary monitoring stations positioned across cities and regions.

The EPA's AirNow network, for example, draws real-time data from thousands of stations operated by state, local, and tribal agencies. The readings are converted into AQI scores and published hourly at AirNow.gov.

Because stations are spread across large areas, the reported AQI reflects general conditions in a region rather than the exact air quality in your backyard or living room.

Indoor Air Quality

In practice, you have two main options for monitoring the air in your home: a standalone air quality monitor that displays continuous readings, or a smart air purifier with built-in sensors that monitors and responds to conditions automatically.

Standalone monitors are a good fit if you want detailed data across multiple pollutants. Smart purifiers suit most households looking for a lower-effort setup, since they handle monitoring and filtration in one device.

Learning how to test indoor air quality at home is a great step towards taking control over the air in your home and leading to a healthier indoor space.

Why AQI Matters for Your Health

On days with poor outdoor air quality, pollutants don't stay outside.

The CDC notes that air pollution can worsen respiratory conditions, trigger symptoms in people with asthma, and disproportionately affect children, older adults, and anyone with a pre-existing heart or lung condition.

The AQI gives you the information to make smarter decisions and, on bad air days, that often starts with what you do at home.

What Affects Indoor Air Quality?

Common Sources of Indoor Pollution

Most indoor pollution comes from everyday activities:

Cooking, particularly frying or using a gas range, can cause sharp PM2.5 spikes.
Cleaning sprays and air fresheners release VOCs.
Candles and incense generate fine particles and carbon monoxide.
Pet dander and hair add biological allergens to the mix.

In energy-efficient homes, tighter building seals reduce air exchange, which means pollutants can accumulate without adequate ventilation to clear them.

A peer-reviewed study in the International Journal of Environmental Research and Public Health found that people with asthma and COPD experience greater lung function decline from indoor PM2.5 and PM10 exposure than healthy adults, including from routine activities like frying food or burning candles.

Seasonal factors have an effect, too:

Pollen season pushes outdoor allergens through every open window.
Wildfire smoke can drive AQI values into hazardous territory and saturate homes with ultra-fine particles within hours.
During winter, tighter home seals concentrate whatever pollutants are already inside.

How Air Purifiers Help Improve AQI at Home

Air purifiers work by continuously drawing in room air, passing it through filtration layers that capture particles, gases, and other pollutants, then returning cleaner air to the space.

When run consistently, they can help reduce the buildup of PM2.5, VOCs, formaldehyde, pet dander, and other indoor contaminants that the outdoor AQI number alone doesn't account for.

The difference between a basic model and a smarter one comes down to awareness.

Purifiers with built-in air quality sensors detect when pollutant levels rise and adjust fan speed automatically, so the unit works harder when you're cooking, when smoke drifts in from outside, or when VOCs spike after cleaning. No manual switching required.

Before choosing, it helps to understand which filtration approach suits your situation. Exploring the types of air purifiers available and the differences between HEPA and True HEPA filtration can help you match the technology to your actual needs.

For larger spaces where continuous monitoring matters, the Dreame AirPursue™ PM20 tracks 7 air quality metrics in real time, covering up to 1,883 sq. ft. (175m²) in 15 minutes, with Auto Mode that adjusts airflow based on live sensor readings.

In smaller rooms, the Dreame AP10 Pet Air Purifier brings the same monitoring-and-response approach to spaces up to 592 sq. ft. (55m²).

Frequently Asked Questions

What is Considered a Good AQI Level Indoors?

A reading below 50 is the target. For PM2.5 specifically, the EPA sets an annual average standard of 9 micrograms per cubic meter as a benchmark for healthy indoor exposure. Purifiers with onboard sensors can help you track whether your home is consistently hitting that mark.

What Happens If Air Quality Is 500?

An AQI of 500 is the ceiling of the "Hazardous" category. At this level, health agencies advise everyone to stay indoors and avoid all outdoor physical activity. Readings this high are rare and typically associated with severe wildfire events or major industrial incidents. Running an air purifier indoors significantly reduces your exposure during those periods.

How Can You Tell If the Air in Your House Is Making You Sick?

Common signs include persistent headaches, fatigue, dry or irritated eyes, frequent sneezing, worsening allergy or asthma symptoms, and a cough that noticeably improves when you leave the house. These symptoms can have many causes, but testing your indoor air quality at home is a practical starting point for figuring out what's in your air.

How Do Home Air Quality Monitors Work?

Home monitors rely on two main sensor types to detect what's in your air. Laser particle counters identify particulate matter by shining a beam of light through an air sample; the way particles scatter that light reveals their size and concentration. Electrochemical sensors handle gases by detecting minute changes in electrical current when specific molecules react with the sensor.

Improve Your Indoor Air Quality With Smarter Monitoring and Filtration

The AQI is a useful starting point, but the number that matters most is the one inside your home. Understanding what drives poor indoor air quality, and having the tools to monitor and respond to it makes a real difference to what you breathe day to day.

If you're ready to take action, browse our range of intelligent air purifiers to find the right fit for your space.

References:

U.S. EPA — AirNow (2024): Air Quality Index (AQI) Basics. Available at: https://www.airnow.gov/aqi/aqi-basics/
U.S. EPA (2024): Introduction to Indoor Air Quality. Available at: https://www.epa.gov/indoor-air-quality-iaq/introduction-indoor-air-quality
CDC (2024): About Air Quality. Available at: https://www.cdc.gov/air-quality/about/index.html
Annesi-Maesano I. et al. (2022): Indoor Air Pollution and the Health of Vulnerable Groups: A Systematic Review Focused on Particulate Matter (PM), Volatile Organic Compounds (VOCs) and Their Effects on Children and People with Pre-Existing Lung Disease. Int J Environ Res Public Health. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9316830/