Air Quality Explained: AQI Levels, Health Effects, and What to Do

Home Published 28/10/2023 Updated Air quality reflects how clean or polluted the air is and how healthy it is for you. To help you understand the condition, the Air Quality Index (AQI) in the US converts measurements of common outdoor pollutants into a simple-to-read scale.

Of course, the AQI number is just where things start. However, to help you make informed decisions, check the AQI category and the specific pollutant driving it to a certain level, along with your health and planned activity, plus air quality forecasts that advise how long this situation might last. This guide explains AQI, PM2. Outdoor Exercise, Indoor Filtration and Ventilation, and What’s your plan when conditions worsen today

Question	Quick answer
What is AQI?	An index used to communicate outdoor pollution and health concerns
Good	0–50
Moderate	51–100
Unhealthy for Sensitive Groups	101–150
Unhealthy	151–200
Very Unhealthy	201–300
Hazardous	301+
Where can U.S. readers check it?	AirNow and state/local air agencies
Is AQI the same as PM2.5?	No
Can indoor air also be polluted?	Yes

The six AQI categories above are the current U.S. categories published by AirNow.

What Is Air Quality?

Air conditions depend on the type and concentration of pollutants present. Pollution can come from traffic, power generation, industry, wildfires, combustion, dust, and indoor activities.

Outdoor conditions can also affect indoor spaces because outside particles and gases can enter through doors, windows, ventilation systems, and leaks in a building.

What Does Good Air Quality Mean?

Under the U.S. AQI system, a reading from 0 to 50 is categorized as Good. AirNow describes this range as satisfactory, with little or no expected risk from pollution.

“Good,” however, does not mean the atmosphere contains absolutely no pollutants. It means measured concentrations correspond to the lowest AQI health-concern category.

• Read more: Air Quality

Indoor vs. Outdoor Air Quality

The AQI you see in a weather app or on AirNow primarily describes outdoor conditions. Your indoor environment can be different.

Indoor pollutants can come from cooking, tobacco smoke, fireplaces, cleaning and consumer products, building materials, mold, dust, and particles entering from outdoors. EPA notes that indoor air can contain both particles and gases, including PM2.5, PM10, volatile organic compounds, formaldehyde, mold, and pollen.

This means staying inside does not automatically guarantee cleaner air. What matters is what is happening both outdoors and inside the building.

What Is the Air Quality Index (AQI)?

The Air Quality Index is a health-communication tool. Instead of expecting people to interpret individual pollutant concentrations, the AQI converts measurements into categories associated with different levels of health concern.

How Does the U.S. AQI Work?

EPA establishes an AQI for five major pollutants regulated under the Clean Air Act:

• Ground-level ozone
• Particle pollution, including PM2.5 and PM10
• Carbon monoxide (CO)
• Sulfur dioxide (SO₂)
• Nitrogen dioxide (NO₂)

The pollutant producing the highest AQI generally determines the reported AQI for that place and period.

That is why knowing the dominant pollutant matters. An AQI driven by wildfire-related PM2.5 is a different pollution situation from an elevated summertime ozone reading.

What Do the AQI Colors Mean?

The U.S. system uses six colors to make conditions easier to recognize:

Green → Yellow → Orange → Red → Purple → Maroon

Green corresponds to Good, yellow to Moderate, orange to Unhealthy for Sensitive Groups, red to Unhealthy, purple to Very Unhealthy, and maroon to Hazardous.

Don’t rely on color alone. Check the actual number and category, particularly if you use multiple websites or apps.

AQI vs. PM2.5: What’s the Difference?

AQI and PM2.5 are related but not interchangeable.

AQI	PM2.5
A health-communication index	A type of particulate pollutant
Can be driven by different pollutants	Specifically particles 2.5 micrometers or smaller
Displayed as an index value/category	Commonly measured in µg/m³
Helps communicate health concern	Describes a physical concentration when measured

For example, AQI 100 does not mean PM2.5 is 100 µg/m³. The AQI is calculated by translating pollutant concentrations through defined breakpoints.

Think of PM2.5 as a measurement that may go into the calculation, and AQI as the health-oriented number readers see after that conversion.

What Do Air Quality Levels Mean?

The most useful way to read AQI is to connect the number with the action it may require.

AQI	Category	Who should pay attention?	Practical consideration
0–50	Good	Everyone	Usual activities are generally appropriate
51–100	Moderate	Especially unusually sensitive people	Watch for individual sensitivity
101–150	Unhealthy for Sensitive Groups	Sensitive groups	Reduce exposure as applicable to the pollutant
151–200	Unhealthy	Everyone, especially sensitive groups	Reduce prolonged or heavy outdoor exposure
201–300	Very Unhealthy	Everyone	Significantly reduce exposure
301+	Hazardous	Everyone	Follow health and emergency guidance

AirNow’s exact recommendations vary by pollutant and population, so check the activity guidance rather than treating this table as a medical rule.

AQI 0–50: Good

Pollution poses little or no expected risk under the AQI framework. Most people can continue normal outdoor activities.

AQI 51–100: Moderate

Conditions are generally acceptable, although some people who are unusually sensitive to pollution may experience effects.

AQI 101–150: Unhealthy for Sensitive Groups

At Code Orange, sensitive groups should pay closer attention.

Who counts as sensitive depends partly on the pollutant. For particle pollution, AirNow identifies people with heart or lung disease, older adults, and children among sensitive groups. For ozone, sensitive groups include people with lung disease and children and adults who are active outdoors.

AQI 151–200: Unhealthy

At this level, effects become a concern for some members of the general public, while sensitive groups may experience more serious effects. Exposure reduction becomes relevant for everyone.

AQI 201–300: Very Unhealthy

This category represents a health alert with increased risk for everyone. Moving strenuous activities indoors may help when indoor conditions are cleaner.

AQI 301+: Hazardous

This is the highest AQI category. AirNow characterizes it as a health warning of emergency conditions in which everyone is more likely to be affected. Follow local public-health and emergency instructions.

What Pollutants Affect Air Quality?

Different pollutants behave differently, come from different sources, and may peak under different conditions.

PM2.5 and PM10

Particulate matter is a mixture of small airborne particles.

PM10 includes particles with diameters of 10 micrometers or smaller. PM2.5 refers to much finer particles—2.5 micrometers or smaller.

PM2.5 can come from combustion and smoke, including wildfire smoke. Its small size allows it to penetrate deeply into the respiratory system, which is one reason fine-particle pollution receives substantial health attention.

Suggested visual: Human hair → PM10 → PM2.5 size comparison.

Ground-Level Ozone

Ground-level ozone is not the protective ozone layer high in the atmosphere.

It forms through chemical reactions involving other pollutants in sunlight, so ozone problems often become important during sunny conditions. This is especially relevant when planning prolonged or strenuous outdoor activity.

• Read more: Air Quality

Nitrogen Dioxide, Sulfur Dioxide, and Carbon Monoxide

Nitrogen dioxide (NO₂) is associated with combustion sources including vehicles and power generation.

Sulfur dioxide (SO₂) can result from burning sulfur-containing fuels and some industrial activities.

Carbon monoxide (CO) is produced by incomplete combustion. Unlike particle pollution, CO requires specific source-control and safety measures; a normal particle air cleaner is not a substitute for a working CO alarm or eliminating a dangerous combustion source.

Pollutant	Typical sources/examples	Key consideration
PM2.5	Smoke, combustion, fine particles	Can travel long distances
PM10	Dust and larger airborne particles	Coarser than PM2.5
Ozone	Forms through atmospheric reactions	Often relevant on sunny days
NO₂	Traffic and combustion	Often associated with urban combustion
SO₂	Fuel combustion and industry	Source patterns vary by region
CO	Incomplete combustion	Requires direct safety/source control

What Causes Poor Air Quality?

Pollution levels are shaped by both emissions and weather. A city can produce pollutants every day but experience much worse conditions when weather prevents those pollutants from dispersing.

Traffic and Vehicle Emissions

Cars, trucks, buses, and other combustion sources contribute gases and particles. Exposure can vary considerably by location, traffic volume, vehicle mix, weather, and distance from busy roads.

Industry and Power Generation

Industrial processes and power generation can release particulate matter or gases that contribute directly or indirectly to pollution.

The dominant source is not the same everywhere, so avoid assuming traffic, industry, or any other single source explains every local AQI problem.

Wildfires and Smoke

Wildfire smoke is particularly important in the United States because smoke can travel far beyond the fire itself.

Fine particles are a major concern during smoke events. A region hundreds of miles from a fire may experience elevated PM2.5, and the sky does not always need to look dramatically smoky for measurements to rise.

Weather and Temperature Inversions

Weather determines how easily pollutants disperse.

During a temperature inversion, warmer air above cooler surface air can limit vertical mixing. Pollution can then become concentrated closer to where people breathe.

Wind can disperse local pollution but can also transport wildfire smoke, dust, and other pollutants from distant locations.

Seasonal and Regional Differences

Pollution patterns differ across the United States.

Western states may experience significant wildfire-smoke episodes. Sunny summer conditions can increase ozone concerns in many areas. Some valleys experience winter inversions, while major metropolitan areas may face traffic-related pollution throughout the year.

This is why a yearly “most polluted city” ranking should never replace checking today’s local conditions.

How Does Poor Air Quality Affect Health?

Health effects depend on the pollutant, concentration, exposure duration, activity level, and individual susceptibility.

Short-Term Effects

Elevated pollution can irritate the respiratory system and worsen symptoms in susceptible people. Effects and sensitivity differ between pollutants and individuals.

If you have a medical condition, use guidance from your healthcare professional alongside official AQI recommendations rather than relying on a generic internet cutoff.

Long-Term Exposure

Long-term exposure to air pollution is associated with important respiratory and cardiovascular health risks. WHO identifies air pollution as a risk factor for conditions including ischemic heart disease, stroke, chronic obstructive pulmonary disease, asthma, and cancer.

WHO’s guidelines reflect evidence that adverse health effects can occur at lower concentrations than previously understood.

Who Is More Sensitive to Air Pollution?

Sensitivity varies by pollutant.

Groups requiring additional attention can include:

• Children
• Older adults
• People with asthma or other lung disease
• People with cardiovascular disease
• People who spend extended periods exercising or working outdoors

EPA also identifies pregnant people among groups that may benefit particularly from cleaner-air strategies during wildfire-smoke events.

How to Check Air Quality Near You

Don’t stop after looking at one number. A useful check takes only a few moments.

Step 1 — Check the Current AQI

Start with AirNow or your state/local air agency.

Note both the number and its category.

Step 2 — Identify the Main Pollutant

Look for what is driving the reading.

If PM2.5 is elevated because of wildfire smoke, your response may focus heavily on particle exposure and indoor filtration. If ozone is the main issue, the relevant AirNow activity guidance differs.

Step 3 — Check the Forecast

A current reading is a snapshot, not a guarantee for the rest of the day.

Conditions can improve or worsen because of wind, smoke movement, sunlight, temperature, or changing emissions.

Step 4 — Consider Your Activity and Sensitivity

A short walk and a two-hour run do not create the same exposure.

Consider:

AQI + pollutant + personal sensitivity + exercise intensity + duration

Step 5 — Recheck Before Going Outside

Recheck when conditions are changing quickly, particularly during wildfire smoke or other pollution episodes.

30-second checklist:

• ☐ Current AQI and category
• ☐ Main pollutant
• ☐ Forecast
• ☐ Personal sensitivity
• ☐ Activity intensity
• ☐ Time outdoors

Is It Safe to Exercise Outside When Air Quality Is Poor?

There is no single AQI cutoff that works for every person and every pollutant. AirNow provides separate activity guides for particle pollution and ozone.

Why Exercise Can Increase Pollution Exposure

During harder exercise, you breathe more air per minute. That can increase the amount of pollution you inhale.

Duration matters too. A brief walk is different from running, cycling, playing a full outdoor game, or working outside for hours.

When to Modify Outdoor Exercise

When AQI moves into unhealthy ranges, check the AirNow guidance for the pollutant involved.

Possible adjustments include:

• Shortening the activity
• Reducing intensity
• Taking more breaks
• Rescheduling for a period with better conditions
• Moving the activity to a cleaner indoor environment

At Code Orange, AirNow advises pollutant-specific sensitive groups to reduce prolonged or heavy outdoor exertion. At Code Red, exposure reduction becomes relevant to everyone, with particular importance for sensitive groups.

Indoor Exercise Alternatives

Moving inside is useful only if indoor conditions are cleaner.

During wildfire smoke, keep smoke from entering and use appropriate filtration where possible. A gym with doors repeatedly opening to smoky outdoor air may not necessarily provide the same protection as a well-filtered indoor environment.

How to Improve Indoor Air Quality

A practical approach has three parts:

Source control → Ventilation → Filtration

Control Pollution at the Source

Preventing pollution can be more effective than trying to remove it afterward.

Useful measures include:

• Avoid smoking indoors
• Use appropriate kitchen exhaust when cooking
• Address persistent moisture and mold problems
• Maintain combustion appliances
• Avoid unnecessary candles, fireplaces, or other particle-producing activities during smoke events.
• Keep outdoor smoke from entering when pollution is severe

EPA specifically recommends avoiding indoor activities that generate additional particles during wildfire smoke events.

Ventilate When Outdoor Conditions Are Suitable

Opening windows can help remove indoor-generated pollutants when outdoor air is cleaner.

But ventilation can work against you when outdoor smoke or particle levels are high. The correct choice therefore depends on the pollution source.

Use Appropriate Air Filtration

Portable air cleaners and HVAC filters can reduce airborne particles when properly selected and operated.

EPA recommends a MERV 13 filter, or the highest efficiency a particular HVAC system can accommodate, for improved particle filtration. Compatibility matters because not every system is designed for every filter.

This is also where readers considering a new HVAC installation should ask how filter efficiency, airflow, filter dimensions, and fan operation will work together rather than choosing a filter rating in isolation.

Should You Open Windows or Use an Air Purifier?

The answer depends on whether the main pollution problem is inside or outside.

When Opening Windows Can Help

Imagine the outdoor AQI is Good, but cooking has created smoke and particles inside your kitchen.

Using effective kitchen exhaust and bringing in cleaner outdoor air can help remove indoor pollution.

When Keeping Windows Closed Makes More Sense

Now imagine wildfire smoke has pushed outdoor particle levels into an unhealthy range.

Opening every window would bring more smoke indoors. EPA advises keeping windows and doors closed during wildfire-smoke events when it is safe to do so, while using air conditioning and filtration appropriately.

Heat safety still matters. If the building becomes dangerously hot without open windows or air conditioning, EPA advises seeking a cooler location rather than creating another health risk.

HEPA Air Cleaner vs. HVAC Filter vs. Ventilation

Method	Best suited to	Main limitation
Ventilation	Replacing polluted indoor air when outdoor conditions are better	Can introduce outdoor pollution
Portable HEPA cleaner	Filtering airborne particles in a room	Does not solve every gas or source
HVAC filtration	Filtering recirculated air across served spaces	Depends on system compatibility and operation
Source control	Preventing pollution where it originates	Some sources cannot be immediately eliminated

Portable HEPA cleaners primarily target particles. EPA notes that gaseous pollutants require filters/media designed for gases, such as certain activated-carbon systems, and CADR ratings apply to particles rather than gases.

What CADR and Room Size Mean

Clean Air Delivery Rate (CADR) indicates how much filtered air a cleaner delivers. In practical terms, a larger room generally needs a higher CADR.

For wildfire smoke, EPA currently recommends choosing a portable cleaner with a tobacco-smoke CADR of at least two-thirds of the room’s area in square feet, assuming roughly an 8-foot ceiling. A 120-square-foot room, for example, would call for a smoke CADR of at least 80 under that rule of thumb. Higher ceilings may require more capacity.

Don’t buy a purifier solely because the box says “HEPA.” Check whether its airflow capacity fits the space where you will actually use it.

Why Do Different Air Quality Apps Show Different AQI Numbers?

Seeing 82 in one app and 104 in another does not necessarily mean one is broken.

Differences can result from:

• Different monitoring locations
• Regulatory monitors versus lower-cost sensors
• Different update times
• Different averaging periods
• Forecast readings versus observations
• Different AQI systems
• Sensor placement
• Different data-processing or correction methods

U.S. AQI vs. Other AQI Systems

“AQI” is not one universal worldwide formula.

Different countries may use different pollutants, averaging periods, breakpoints, and index scales. Therefore, two numbers from different national systems should not automatically be compared as if they represent identical pollution concentrations or health categories.

Which Reading Should U.S. Readers Use?

For health-related decisions in the United States, start with official sources such as AirNow and your state or local air agency.

Consumer sensors and apps can provide useful additional context, especially for hyperlocal conditions, but first understand where their data comes from and which AQI scale they use.

AQI vs. WHO Air Quality Guidelines: Why They Are Different

AQI, WHO guidelines, pollutant concentrations, and U.S. regulatory standards answer different questions.

• Read more: Air Quality

Concept	Main purpose
AQI	Communicates current/daily pollution-related health concerns
Pollutant concentration	Measures the amount of a pollutant
WHO guideline	Provides health-based concentration guidance
U.S. standard	Defines a regulatory air-quality standard

WHO’s 2021 guideline for PM2.5 is 5 µg/m³ as an annual average and 15 µg/m³ as a 24-hour average.

The U.S. regulatory standard is different. EPA strengthened the primary annual PM2.5 National Ambient Air Quality Standard to 9.0 µg/m³ in 2024 while retaining the 24-hour PM2.5 standard of 35 µg/m³.

These values should not be substituted for one another because they serve different purposes.

Daily vs. Annual Exposure

An annual average tells you about pollution over a long period. A 24-hour guideline addresses a much shorter averaging period. AQI, meanwhile, is designed to communicate daily conditions and associated health concerns.

So you should not take WHO’s annual PM2.5 value of 5 µg/m³ and treat it as an AQI cutoff.

Common Air Quality Mistakes to Avoid

Several common mistakes can turn useful data into a poor decision:

• Confusing AQI with PM2.5 concentration. One is an index; the other is a pollutant measurement.
• Checking only the color. Read the category and number too.
• Ignoring the main pollutant. Ozone and particle pollution can require different activity decisions.
• Assuming a clear sky means clean air. Some pollutants are not visually obvious.
• Assuming indoor air is always cleaner. Cooking, smoke, combustion, mold, and other indoor sources matter.
• Opening windows during heavy outdoor smoke. This can increase particle infiltration.
• Assuming every purifier removes everything. Particle filters do not automatically remove gases.
• Buying an undersized purifier. CADR and room size matter.
• Using annual city rankings to judge today. Current conditions can be very different from annual averages.
• Checking once and forgetting about it. Smoke, ozone, wind, and weather can change conditions during the day.

A 30-Second Air Quality Decision Method

Instead of memorizing dozens of rules, use this five-step process.

1. Check

Look up the current AQI and its category.

2. Identify

Find which pollutant is driving the reading.

3. Assess

Consider whether you are in a sensitive group and what you plan to do.

A 10-minute walk and a two-hour run should not be treated as equivalent exposures.

4. Act

Choose an appropriate response: continue normally, shorten outdoor activity, reduce exertion, move indoors, limit outdoor pollution entering the home, ventilate when outside conditions are better, or use suitable filtration.

5. Recheck

Check again when conditions are changing.

Wildfire smoke can move, ozone can vary during the day, and a morning reading does not guarantee the same conditions that afternoon.

Frequently Asked Questions