Understanding AQI: What Those Numbers Really Mean

You've probably seen air quality numbers on your weather app or news broadcasts, but do you really understand what they mean? The Air Quality Index (AQI) is more than just a number—it's a vital tool for protecting your health.

The Basics: What is AQI?

The Air Quality Index is a standardized system for reporting daily air quality. It tells you how clean or polluted your air is, and what associated health effects might be a concern for you. The EPA developed the AQI to provide a simple, uniform way to report air quality across the United States, and similar systems exist worldwide.

Think of AQI like a speedometer for air pollution. Just as a speedometer tells you how fast you're going, the AQI tells you how much pollution is in the air around you.

How AQI is Calculated

The AQI is calculated for five major air pollutants regulated by the Clean Air Act:

Ground-level ozone
Particle pollution (PM2.5 and PM10)
Carbon monoxide
Sulfur dioxide
Nitrogen dioxide

For each of these pollutants, EPA has established national air quality standards to protect public health. The AQI for a specific pollutant is calculated using a formula that converts pollutant concentration to a number on a scale of 0 to 500.

When you see a single AQI number reported for your area, it typically represents the highest AQI value among all measured pollutants—reflecting the worst air quality condition.

Breaking Down the AQI Scale

Good (0-50): Green Light

When the AQI is in this range, air quality is considered satisfactory. You can:

Exercise freely outdoors
Keep windows open for ventilation
Enjoy all outdoor activities

This is the ideal range, and unfortunately, it's becoming rarer in many urban areas.

Moderate (51-100): Yellow Caution

Air quality is acceptable, but there may be a concern for some people who are unusually sensitive to air pollution. You might notice:

Slight respiratory discomfort if you're sensitive
Reduced visibility due to haze

For most people, this level doesn't require any behavior changes.

Unhealthy for Sensitive Groups (101-150): Orange Warning

This is where things start getting serious. If you belong to a sensitive group—children, elderly, or those with respiratory or heart conditions—you should:

Reduce prolonged outdoor exertion
Take more breaks during outdoor activities
Watch for symptoms like coughing or difficulty breathing

The general public is less likely to be affected at this level.

Unhealthy (151-200): Red Alert

Now everyone may begin to experience health effects. At this level:

Limit outdoor exercise
Consider moving activities indoors
Keep air conditioning on recirculation mode
Sensitive groups should stay indoors

Very Unhealthy (201-300): Purple Emergency

Health alert: everyone may experience more serious health effects. This level calls for:

Avoiding all strenuous outdoor activity
Keeping all windows closed
Using air purifiers indoors
Wearing N95 masks if you must go outside

Hazardous (301-500): Maroon Crisis

Health emergency. The entire population is more likely to be affected. Take immediate protective action:

Stay indoors
Seal doors and windows
Run air purifiers continuously
Consider temporary relocation if conditions persist

Why PM2.5 Matters Most

While all pollutants contribute to the AQI, PM2.5 (fine particulate matter) deserves special attention. These microscopic particles are:

Incredibly small: About 30 times smaller than a human hair
Deeply penetrating: Can reach the deepest parts of your lungs
Bloodstream bound: Can enter your bloodstream and affect organs throughout your body

Research from Berkeley Earth found that breathing air with PM2.5 at just 22 µg/m³ for 24 hours is roughly equivalent to smoking one cigarette. This finding makes the abstract concept of air pollution suddenly very concrete.

Common Sources of Air Pollution

Understanding where pollution comes from can help you anticipate bad air quality days:

Vehicle emissions: Cars, trucks, and buses are major contributors, especially during rush hour.

Industrial activity: Factories and power plants release various pollutants.

Wildfires: Increasingly common, wildfires can send AQI soaring into hazardous levels hundreds of miles away.

Agricultural burning: Seasonal burning of crop residue creates significant pollution.

Weather conditions: Temperature inversions can trap pollutants near the ground, causing sudden spikes in AQI.

How to Stay Informed

Check daily: Make checking the AQI part of your morning routine, like checking the weather.

Use reliable sources:

AirNow.gov (US)
Your local environmental agency
Reputable weather apps with AQI features

Set alerts: Many apps allow you to set notifications when AQI exceeds certain levels.

Understand local patterns: Some areas have predictable pollution patterns—higher AQI in summer due to ozone, or worse conditions during rush hour.

Taking Action

Knowledge is only useful if you act on it. Here's how to use AQI information:

Plan outdoor activities during times when AQI is lowest (often early morning)
Adjust exercise intensity based on current conditions
Create a clean air space in your home with a HEPA air purifier
Keep N95 masks available for days when you must be outdoors during poor air quality
Support clean air policies in your community

The Bottom Line

The Air Quality Index is a powerful tool for protecting your health, but only if you use it. Make AQI awareness a daily habit, know your personal sensitivity level, and don't hesitate to modify your activities when the air quality is poor.

Your lungs will thank you.

Want to understand just how much air pollution affects you? Try our AQI to Cigarettes Calculator to visualize your exposure in terms everyone can understand.