Air Quality Index (AQI) Explained: Levels, Colors, Health Risks and How to Read It

Air pollution is measured in units most people rarely encounter in everyday life: micrograms per cubic metre, parts per billion, parts per million and pollutant-specific averages calculated across hours or days.

Those measurements are scientifically useful, but they do not immediately answer the question most people actually have:

Is the air safe today, and should I change what I am doing?

The Air Quality Index, usually shortened to AQI, exists to answer that question.

An AQI converts measurements of pollutants such as PM2.5, PM10, ozone and nitrogen dioxide into a simpler numerical scale, usually accompanied by colours and health guidance. Higher numbers generally indicate greater pollution and greater potential health concern.

But an AQI is not itself a pollutant and it is not a universal physical measurement like temperature.

It is a communication index created according to rules chosen by a government or air-quality authority.

That distinction explains several things that often confuse people: why two apps sometimes show different AQIs, why AQI systems differ between countries, why the same AQI number can be caused by different pollutants and why today's “current AQI” can change rapidly even before a full daily average has been calculated.

The most useful way to read AQI is therefore to ask four questions:

What is the AQI value? Which system produced it? Which pollutant is driving it? What health advice applies to me?

What Does AQI Mean?

AQI stands for Air Quality Index.

It is a numerical system designed to communicate how polluted the outdoor air is and what that pollution may mean for health.

Instead of asking the public to interpret a PM2.5 concentration of 37 µg/m³ or an ozone concentration measured in parts per billion, an air-quality authority converts the pollutant concentration into an index.

The index then places conditions into categories such as Good, Moderate, Unhealthy or Hazardous, depending on the system being used.

In practical terms:

Low AQI = generally cleaner air and lower short-term health concern.

High AQI = more polluted air and greater reason to reduce exposure.

That is the basic idea.

The details become more complicated because different countries define those categories differently.

AQI Levels at a Glance

The U.S. Environmental Protection Agency's AQI is one of the most widely recognised examples.

U.S. AQI	Category	What it generally means
0–50	Good	Air quality is satisfactory and pollution poses little or no risk
51–100	Moderate	Generally acceptable, although unusually sensitive people may experience risk
101–150	Unhealthy for Sensitive Groups	Sensitive groups may experience health effects
151–200	Unhealthy	Some people in the general population may experience effects; sensitive groups face greater risk
201–300	Very Unhealthy	Health alert; risk increases for everyone
301+	Hazardous	Emergency-level health warning; everyone is more likely to be affected

The colours associated with these categories are green, yellow, orange, red, purple and maroon respectively.

The essential rule is straightforward:

The higher the AQI, the greater the health concern.

But the table applies specifically to the U.S. AQI system. Other countries use different thresholds and category names.

What Is a Good AQI?

Under the U.S. system, an AQI between 0 and 50 is classified as Good.

That does not mean the air contains absolutely no pollution.

It means measured concentrations fall within the range EPA communicates as posing little or no short-term risk to the general population.

An AQI between 51 and 100 is classified as Moderate. Most people can usually continue normal activities, although unusually sensitive individuals may need to pay closer attention to symptoms and pollutant-specific guidance.

Once the AQI rises above 100, health guidance becomes progressively more cautious.

What AQI Is Considered Unhealthy?

Under the U.S. system:

101–150 is Unhealthy for Sensitive Groups.

151–200 is Unhealthy.

201–300 is Very Unhealthy.

301 and above is Hazardous.

The distinction matters because an AQI of 110 does not carry the same population-level warning as an AQI of 210.

At lower unhealthy levels, advice may initially focus on people particularly vulnerable to the pollutant involved. As AQI rises, recommendations increasingly apply to everyone.

Which Pollutants Are Included in AQI?

The U.S. EPA establishes AQI values for five major categories of pollutants regulated under the Clean Air Act:

• ground-level ozone;

• particle pollution, including PM2.5 and PM10;

• carbon monoxide;

• sulfur dioxide; and

• nitrogen dioxide.

Each pollutant is measured differently.

PM2.5 and PM10 are usually expressed as mass concentrations in micrograms per cubic metre.

Ozone and gaseous pollutants may be measured using parts per billion or parts per million.

Because those measurements cannot simply be compared directly, AQI converts each pollutant into a common index.

The pollutant producing the highest AQI sub-index generally determines the headline AQI.

This pollutant is sometimes described as the dominant, primary or responsible pollutant, depending on the system.

Why the Dominant Pollutant Matters

Imagine two days when the AQI is 160.

On the first day, the high value may be caused by PM2.5 from wildfire smoke.

On the second, it may be caused by ground-level ozone produced during hot, sunny weather.

The headline AQI is similar, but the atmospheric problem is different.

The likely sources are different.

The way concentrations change through the day may be different.

The groups most vulnerable can differ.

And some exposure-reduction strategies may differ.

This is why checking only the large AQI number can leave out useful information.

When possible, look for:

AQI value → category → dominant pollutant → health guidance

rather than reading the AQI number alone.

How Is AQI Calculated?

The underlying calculation is more technical than the final number suggests.

First, air-quality monitors measure the concentration of individual pollutants.

Each pollutant has a set of breakpoints that connect concentration ranges to AQI ranges.

A measured concentration is placed between two concentration breakpoints. The corresponding AQI is then calculated between the index values assigned to those breakpoints.

The U.S. system uses a linear interpolation equation:

AQI = [(Ihigh − Ilow) ÷ (BPhigh − BPlow)] × (C − BPlow) + Ilow

where:

C is the measured pollutant concentration;

BPlow and BPhigh are the concentration breakpoints surrounding that measurement; and

Ilow and Ihigh are the corresponding AQI values.

You do not normally need to calculate this yourself. Air-quality agencies and apps do it automatically.

But understanding the process reveals something important:

AQI is not a raw measurement. It is a converted health-communication value.

A Simple PM2.5 AQI Example

The current U.S. PM2.5 breakpoints provide an easy example.

For a 24-hour PM2.5 concentration:

0.0–9.0 µg/m³ corresponds to AQI 0–50: Good.

9.1–35.4 µg/m³ corresponds to AQI 51–100: Moderate.

35.5–55.4 µg/m³ corresponds to AQI 101–150: Unhealthy for Sensitive Groups.

Higher concentrations move into increasingly unhealthy AQI categories.

These PM2.5 breakpoints were revised after EPA strengthened its particulate-matter standard, which is why older charts may show a different Good-range threshold.

This is another reason to use current official AQI information rather than relying indefinitely on old graphics circulating online.

Why AQI 100 Is Important

In the U.S. system, an AQI value of 100 generally corresponds to the concentration of the relevant short-term national ambient air-quality standard.

Values at or below 100 are therefore generally presented as satisfactory from the perspective of the index.

Above 100, health concern increases—first particularly for sensitive populations, then more broadly as pollution rises.

This does not mean there is a magical biological boundary between AQI 99 and AQI 101.

Health risk exists on a continuum.

AQI categories simplify that continuum so millions of people can use pollution data without needing specialist training in environmental epidemiology.

AQI Is Not the Same in Every Country

One of the most important facts about AQI is also one of the most frequently overlooked:

There is no single worldwide AQI system.

Different countries can use different:

• pollutants;

• concentration breakpoints;

• averaging periods;

• health standards;

• category names;

• colours;

• calculation methods; and

• maximum index values.

This means an AQI of 150 in one country is not automatically equivalent to an AQI of 150 somewhere else.

The number must be interpreted using the system that produced it.

U.S. AQI vs India AQI

India provides an excellent example of why international AQI numbers must be interpreted carefully.

India's National Air Quality Index, administered through the Central Pollution Control Board, uses six categories:

India AQI	Category
0–50	Good
51–100	Satisfactory
101–200	Moderate
201–300	Poor
301–400	Very Poor
401–500	Severe

The U.S. and Indian systems therefore use noticeably different categories.

For example:

An AQI of 175 is classified as Unhealthy under the U.S. AQI.

An AQI of 175 falls within India's Moderate category.

That does not mean one government believes the same pollution is harmless while the other believes it is dangerous.

The systems are based on different pollutant breakpoints and regulatory frameworks.

Comparing the numbers directly without checking the underlying system can therefore be misleading.

Which Pollutants Are Included in India's AQI?

India's National AQI framework considers eight pollutants:

• PM10;

• PM2.5;

• nitrogen dioxide;

• sulfur dioxide;

• carbon monoxide;

• ozone;

• ammonia; and

• lead.

For each pollutant with sufficient monitoring data, a sub-index is calculated.

The highest valid sub-index determines the overall AQI.

India's system therefore follows the same broad communication principle—convert multiple pollutants into a simpler index—but its detailed rules differ from the U.S. system.

Why AQI Values From Different Apps Can Disagree

People often open two weather or pollution apps and discover different AQI numbers for the same city.

That does not automatically mean one of them is fraudulent.

Several factors can create differences.

Different AQI Systems

One app may display the local national AQI while another defaults to the U.S. AQI.

A PM2.5 concentration converted under two different systems can produce different index numbers.

Different Monitoring Stations

One app may use the monitor nearest your location.

Another may use a city average.

A third may combine measurements from several stations with modelling.

Different Pollutants

Not every monitoring station measures every pollutant.

One location may therefore report an AQI based primarily on PM2.5 while another has ozone or nitrogen-dioxide data available.

Different Update Times

One platform may update every hour while another uses older data.

During wildfire smoke or rapidly changing weather, even a one-hour difference can matter.

Different Averaging Methods

AQI calculations depend on pollutant-specific averaging periods.

A platform attempting to describe “current” conditions may use methods different from those used to calculate the official final daily AQI.

Sensors vs Regulatory Monitors

Low-cost air sensors can provide useful local information but may behave differently from reference-grade regulatory instruments.

Humidity, placement, calibration and sensor quality can affect readings.

The best response to conflicting numbers is therefore not simply to choose whichever app looks most alarming.

Check which AQI system, data source, monitoring station and time period each platform is using.

Daily AQI vs Current AQI vs AQI Forecast

These terms sound similar but describe different products.

Daily AQI

The daily AQI summarises pollution using pollutant-specific averaging rules.

It is the official AQI associated with a day once the necessary measurements are available.

Current AQI

People generally need information before the day is over.

Current-air-quality systems therefore estimate conditions using the most recent measurements.

In the United States, EPA's AirNow system uses a method called NowCast for ozone and particle pollution.

The method uses recent hourly measurements and gives greater weight to newer readings when conditions are changing quickly.

AQI Forecast

A forecast predicts future air quality.

Meteorological conditions, pollutant emissions, smoke movement and atmospheric models can be used to estimate whether conditions are likely to improve or deteriorate.

The distinction is similar to weather:

current AQI = what conditions appear to be like now

forecast AQI = what conditions are expected to become

daily AQI = the day's pollution index calculated according to defined rules

What Is NowCast AQI?

Waiting for a complete 24-hour average would make some air-quality information useless for real-time decisions.

Imagine wildfire smoke arriving at 2 p.m.

People deciding whether children should play outside at 3 p.m. cannot wait until tomorrow to learn today's completed daily PM2.5 average.

EPA's NowCast addresses this problem.

It uses recent hourly measurements for ozone and particle pollution and applies weighting that responds to how quickly conditions are changing.

When pollution is stable, several hours of data can contribute meaningfully.

When pollution changes rapidly, recent measurements receive more influence.

This creates a value that better reflects current conditions while remaining connected to the AQI framework.

What Does AQI Tell You About Health?

AQI is designed primarily as a short-term risk communication tool.

As pollution rises, breathing polluted air becomes more concerning.

But the risk is not identical for everyone.

Health effects depend on:

• pollutant;

• concentration;

• duration of exposure;

• breathing rate;

• age;

• underlying disease;

• individual susceptibility; and

• total exposure across different environments.

AQI therefore provides population-level guidance rather than a personal medical diagnosis.

Who Is in a Sensitive Group?

The answer depends partly on the pollutant.

People who may be more vulnerable to air pollution include:

• children;

• older adults;

• people with asthma;

• people with cardiovascular disease;

• people with chronic lung disease;

• pregnant people;

• people who spend long periods outdoors; and

• people performing strenuous outdoor work or exercise.

Outdoor workers and athletes are worth particular attention because exercise increases breathing rate.

A runner may inhale substantially more air during vigorous exercise than someone sitting indoors.

The same outdoor pollution concentration can therefore produce a larger inhaled dose.

What Should You Do at Different AQI Levels?

Exact advice should come from the authority issuing your local AQI, because recommendations differ between systems and pollutants.

As a general exposure-management principle:

AQI situation	Practical response
Cleaner / low AQI	Normal outdoor activity is generally appropriate
Moderately elevated	Unusually sensitive people may need to monitor symptoms or reduce prolonged exertion
Unhealthy for sensitive groups	Vulnerable groups should consider reducing prolonged or strenuous outdoor activity
Unhealthy	More people should reduce prolonged or strenuous outdoor exposure
Very unhealthy	Substantial exposure reduction becomes appropriate for the wider population
Hazardous / severe episode	Follow official health advisories and minimise exposure where practical

The goal is not panic.

It is dose reduction.

Air-pollution exposure depends partly on how polluted the air is, how long you breathe it and how much air you inhale.

Reducing exercise intensity, shortening outdoor time, moving activities to a cleaner period or using cleaner indoor air can reduce inhaled pollution.

Should You Exercise When AQI Is High?

Exercise has major health benefits, so the answer is not simply “never exercise when pollution exists.”

Instead, modify the exposure.

If air quality is poor, options can include:

• exercising indoors where air is cleaner;

• moving exercise to a different time;

• reducing duration;

• reducing intensity;

• choosing a location farther from heavy traffic; or

• following local pollutant-specific guidance.

The higher the pollution and the greater your individual vulnerability, the stronger the case for modification.

Someone with asthma may need to change plans at a lower pollution level than a healthy adult.

Does Wearing a Mask Help When AQI Is High?

A well-fitted particulate respirator can reduce inhalation of airborne particles such as PM2.5 when properly used.

However, not every face covering provides the same filtration or fit.

Masks designed mainly for droplets or loose-fitting cloth coverings should not be assumed to provide the same particulate protection as a properly fitted respirator.

Masks also do not provide equivalent protection from every gaseous pollutant.

For example, a particulate respirator designed to filter particles does not remove all ozone or carbon monoxide.

The dominant pollutant therefore matters.

Can an Air Purifier Help?

Indoor air filtration can reduce particle concentrations when a suitable purifier is correctly sized, maintained and operated.

This can be particularly useful during wildfire-smoke episodes or high outdoor PM2.5 conditions.

But an air purifier does not automatically remove every pollutant.

Particle filters are designed primarily for particles. Gases may require different filtration technology.

Indoor sources also matter. Cooking, smoking and combustion can create pollution even when outdoor AQI is relatively good.

AQI should therefore not be interpreted as a complete description of indoor air.

What AQI Cannot Tell You

AQI is useful because it simplifies.

That simplicity also creates limitations.

It Does Not Describe Every Pollutant

An AQI usually covers a defined set of major pollutants.

Other hazardous air pollutants may be present without materially changing the headline AQI.

It Does Not Identify the Source

A high PM2.5 AQI tells you particles are elevated.

It does not tell you whether those particles came primarily from traffic, wildfires, household combustion, industry, agricultural chemistry or a combination.

Answering that requires emissions analysis and source-apportionment studies.

See Sources of Air Pollution Explained for how scientists distinguish those sources.

It Does Not Measure Your Lifetime Exposure

AQI is primarily designed for shorter-term communication.

Someone living for decades beside a heavily trafficked road may have significant chronic exposure even if daily AQI alerts rarely become dramatic.

It Does Not Fully Describe Indoor Air

The AQI shown on your phone generally refers to outdoor air.

Indoor pollution can differ because of ventilation, filtration, cooking, smoking, heating, cleaning products and other sources.

It Cannot Describe Every Street

Pollutant concentrations vary across space.

Near-road nitrogen dioxide can change dramatically within relatively short distances.

Smoke plumes can affect one neighbourhood more than another.

A city-wide number therefore cannot represent the exact air every resident is breathing.

Why Location Matters So Much

Air-quality monitors measure the atmosphere at specific locations.

People do not stay at monitoring stations.

They travel from homes to roads, offices, schools, shops and parks, encountering different pollution concentrations along the way.

A person living beside a major highway may experience a different long-term exposure pattern from someone living several kilometres away even when both receive the same city-level AQI notification.

Industrial sites can create similar local gradients.

This is one reason modern air-quality systems increasingly combine monitoring with modelling, satellite observations and denser sensor networks.

The objective is to estimate pollution between official monitoring points rather than assuming one monitor perfectly represents an entire city.

Why Weather Changes AQI

Pollution levels can change rapidly even when emissions remain similar.

Weather strongly influences how pollutants accumulate and disperse.

Wind

Strong winds can dilute local pollution but may also transport smoke or dust from another region.

Temperature Inversions

An inversion can trap pollution close to the ground rather than allowing it to disperse upward.

Sunlight

Sunlight drives the chemical reactions that create ground-level ozone.

Rain

Rain can remove some particles from the atmosphere.

Heat

Hot, sunny conditions often favour high ozone episodes.

This is why AQI can improve or deteriorate from one day to the next without a corresponding dramatic change in local traffic or industry.

Why PM2.5 Often Drives AQI During Smoke Events

Fine particulate matter is one of the major components of wildfire smoke.

PM2.5 particles are small enough to penetrate deep into the respiratory system, which is why they receive significant attention in health-based air-quality guidance.

During severe wildfire events, concentrations can change rapidly as smoke plumes move.

That makes current monitoring particularly useful.

For a complete explanation of particle size and health significance, see What Is Particulate Matter?

Why Ozone AQI Often Rises on Sunny Days

Ground-level ozone behaves differently from particle pollution.

It is not normally emitted directly.

Instead, it forms when nitrogen oxides and volatile organic compounds react in sunlight.

This can cause ozone concentrations to rise during warm, sunny afternoons.

The highest ozone level may even occur away from the original source of precursor emissions.

For more on this atmospheric chemistry, see What Is Smog and How It Forms?

Is AQI the Same as PM2.5?

No.

This is one of the most common sources of confusion.

PM2.5 is a pollutant concentration.

AQI is an index.

A PM2.5 monitor might report:

22 µg/m³

An AQI system then converts that concentration into an index value according to its rules.

Two countries can receive the same PM2.5 concentration and report different AQI numbers because their breakpoints differ.

When comparing scientific pollution data internationally, the underlying concentration can therefore be more meaningful than comparing national AQI numbers.

AQI vs PM2.5: Which Number Should You Use?

For everyday health decisions:

AQI is usually easier.

It translates pollution into health guidance.

For scientific comparison:

pollutant concentration is often more informative.

Concentrations allow researchers to compare air pollution without first passing the measurements through different national index systems.

Both numbers are valuable.

They answer different questions.

Why “Moderate” Does Not Mean Zero Risk

Air-pollution risk does not suddenly appear when an AQI category changes colour.

Human biology does not operate according to app colour bands.

AQI categories simplify continuous relationships between pollution and health so they can be communicated clearly.

This means “Good” or “Moderate” should not be interpreted as a guarantee that every individual faces exactly zero risk.

Long-term exposure matters as well as short-term peaks.

Sensitive individuals may also experience effects at concentrations tolerated by most of the population.

AQI is therefore best understood as risk guidance, not a declaration that air below one number is perfectly harmless and air one point higher is suddenly dangerous.

Can AQI Be Above 500?

Some AQI systems can mathematically report extremely high values, although normal public-facing scales often concentrate on a range up to 500.

Under the U.S. AQI framework, values above 300 are already classified as Hazardous.

At extreme concentrations, the practical message becomes more important than the precise difference between two enormous numbers:

pollution is dangerously high and official emergency health guidance should be followed.

Very high AQI episodes can occur during intense wildfire smoke, dust storms or severe urban pollution events.

Why You Should Check the Official Local AQI

Third-party apps can be useful, but the official national or local air-quality authority should remain an important reference because it defines the AQI being used.

In the United States, EPA and AirNow provide official AQI information.

In India, the Central Pollution Control Board provides the National AQI and monitoring information through its air-quality network.

Official sources can explain:

• which scale is being used;

• which pollutants are included;

• which station generated the reading;

• whether data are provisional;

• which pollutant is dominant; and

• which health advice applies.

This context becomes especially important during severe pollution episodes.

How to Read AQI in 30 Seconds

When you check air quality, use this sequence:

1. Read the AQI Number

Determine the pollution category.

2. Confirm the AQI System

Is it U.S. AQI, India AQI or another national index?

3. Check the Dominant Pollutant

Is PM2.5, PM10, ozone, NO₂ or another pollutant responsible?

4. Check the Time

Is this a current estimate, completed daily AQI or forecast?

5. Check Your Risk

Do you have asthma, heart disease or another condition that increases vulnerability? Are children or older adults involved?

6. Consider the Activity

A short walk and a two-hour outdoor run create very different inhaled doses.

7. Follow Local Guidance

Use the recommendations provided for the pollutant and AQI category.

That is more useful than simply memorising a colour chart.

Frequently Asked Questions

What does AQI stand for?

AQI stands for Air Quality Index. It converts measured pollutant concentrations into a simpler numerical and colour-coded scale used to communicate air quality and health concern.

What is a good AQI?

Under the U.S. AQI system, 0–50 is classified as Good. Other countries may use different definitions, so always check the local AQI system.

Is AQI 100 bad?

In the U.S. system, AQI 100 is at the upper end of the Moderate category. Values above 100 begin the Unhealthy for Sensitive Groups category. India's AQI system classifies numbers differently.

Is AQI 150 unhealthy?

Under the U.S. system, AQI 150 is at the top of Unhealthy for Sensitive Groups. AQI 151 begins the Unhealthy category. Under India's AQI, 150 is within the Moderate range.

Is AQI 200 dangerous?

In the U.S. system, AQI 200 is classified as Unhealthy, with sensitive groups at greater risk and some members of the general population potentially experiencing health effects. In India, an AQI of 200 is at the upper boundary of the Moderate category. Local guidance should therefore always be used.

What is the difference between AQI and PM2.5?

PM2.5 is a type of fine particulate pollutant measured as a concentration. AQI is an index that converts PM2.5 and other pollutant concentrations into a health-communication scale.

Why is my AQI different on different apps?

Apps may use different AQI standards, monitoring stations, sensors, update times, averaging methods or modelling systems. Check the data source and AQI system before comparing values.

Which pollutant determines the AQI?

AQI systems generally calculate a sub-index for each available pollutant. The pollutant with the highest sub-index determines the overall AQI.

Does AQI measure indoor air quality?

Usually no. Public AQI systems primarily describe outdoor ambient air. Indoor air can be cleaner or more polluted depending on outdoor infiltration, cooking, smoking, ventilation, filtration and other indoor sources.

Can AQI change within an hour?

Yes. Pollution can change rapidly because of smoke movement, traffic, atmospheric chemistry and weather. Current-air-quality systems use recent observations to provide more timely guidance.

Why is AQI high when the sky looks clear?

Many harmful air pollutants are invisible at concentrations important to health. Clear skies therefore do not guarantee clean air.

Why can AQI improve after rain?

Rain can remove some airborne particles, while changing winds and atmospheric conditions can also disperse pollution. The effect varies by pollutant.

Should I stay indoors when AQI is high?

It depends on the AQI level, pollutant, your health and whether indoor air is actually cleaner. During severe particulate-pollution episodes, reducing outdoor exposure and using cleaner indoor air can reduce inhaled dose. Follow local health guidance.

Is air quality worse in the morning or evening?

There is no universal answer. Traffic, atmospheric mixing, temperature inversions and ozone chemistry can create different daily patterns. Ozone commonly peaks later in the day, while particulate pollution may behave differently.

Can AQI predict long-term health risk?

Not by itself. AQI is mainly a short-term communication tool. Long-term health risk depends on cumulative exposure across months and years.

AQI Is a Warning System, Not the Air Itself

The Air Quality Index succeeds because it turns complicated atmospheric measurements into information ordinary people can act on.

But the simplicity of the number can create a false impression that AQI is itself a physical measurement.

It is not.

Behind every AQI value lies a chain of information:

pollutant concentration → averaging period → health-based breakpoint → pollutant sub-index → overall AQI → health guidance

Understanding that chain explains why AQI systems differ between countries, why different apps can disagree and why the pollutant behind the number matters.

So the best question is not simply:

“What is the AQI?”

Ask instead:

“Which AQI system is this, which pollutant is driving it, how current is the reading, and what does the official guidance recommend for someone like me?”

That turns AQI from a coloured number on a phone screen into what it was designed to be: a practical tool for understanding pollution and reducing exposure.