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PM2.5 is a measured concentration of very fine particles in air, usually reported in micrograms per cubic meter (µg/m³). AQI is a separate, normalized index that converts pollutant measurements into a number, color and health-concern category. On the U.S. EPA/AirNow scale, higher AQI means more pollution and greater potential health concern; AQI above 100 is the point at which the warning first applies to sensitive groups.
What do PM2.5 and AQI mean?
PM2.5 is a pollutant measurement
PM2.5 means particulate matter with an aerodynamic diameter of 2.5 micrometers or less. These particles are also called fine particle pollution. For perspective, 2.5 micrometers is far smaller than the width of a human hair. A PM2.5 reading describes how much of that material is present in a volume of air, normally in micrograms per cubic meter (µg/m³). It is a concentration, not a score.
The U.S. Environmental Protection Agency (EPA) defines the particle and its measurement in its AirData basic information. A reading of 8 µg/m³, for example, means the monitor estimated eight micrograms of PM2.5 in each cubic meter of sampled air during its stated measurement period.
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AQI is a communication index
The U.S. EPA/AirNow Air Quality Index (AQI) translates a pollutant concentration into an index value from 0 upward, a color and a plain-language health category. It is designed to communicate the likelihood of health effects, not to report the mass of particles in the air. The EPA describes the relationship this way: “The higher the AQI value, the greater the level of pollution and the greater the health concern.” See Communicating Air Quality Conditions: The Air Quality Index.
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Because they measure different things, an AQI of 50 is not “the same amount” as 50 µg/m³ of PM2.5. Use the applicable breakpoint table to translate between them; never compare the two numbers as though they share units.
How the U.S. PM2.5 AQI scale works
The following are the current U.S. PM2.5 breakpoints. The revised values became effective May 6, 2024. The detailed EPA table is dated October 12, 2026, at AQI Breakpoints.
| U.S. AQI category | AQI value | PM2.5 concentration breakpoint | Color | General meaning |
|---|---|---|---|---|
| Good | 0–50 | 0.0–9.0 µg/m³ | Green | Air quality is satisfactory; pollution poses little or no risk for the general population. |
| Moderate | 51–100 | 9.1–35.4 µg/m³ | Yellow | Generally acceptable, although unusually sensitive people may face some risk. |
| Unhealthy for Sensitive Groups | 101–150 | 35.5–55.4 µg/m³ | Orange | Sensitive groups may experience health effects; others are less likely to be affected. |
| Unhealthy | 151–200 | 55.5–125.4 µg/m³ | Red | Some of the general public may experience effects, with more serious effects possible for sensitive groups. |
| Very Unhealthy | 201–300 | 125.5–225.4 µg/m³ | Purple | The risk of health effects is increased for everyone. |
| Hazardous | 301 and higher | 225.5 µg/m³ and higher | Maroon | Emergency-condition warning; everyone is more likely to be affected. |
These are U.S. reporting definitions, not a diagnosis of how one particular person will respond. People with asthma or other respiratory disease, heart disease, older adults, children and people who are pregnant can have different risks, and individual advice should come from a clinician.
How do I interpret the AQI numbers?
AQI 0–50: Good
Green readings indicate that the air is considered satisfactory under the U.S. AQI description. Routine outdoor activities generally do not need to change because of the AQI alone.
AQI 51–100: Moderate
Yellow means air quality is generally acceptable. People who are unusually sensitive to air pollution may notice symptoms or need to reduce prolonged, strenuous exposure, especially if they already know that smoke or particles trigger problems.
AQI 101–150: Unhealthy for Sensitive Groups
Orange is the first category officially classified as unhealthy. The caution is directed primarily at sensitive groups, while the general public is less likely to experience effects at this range. If you are in a sensitive group, consider shortening or moving strenuous outdoor activity indoors when the reading is near the top of the range, and follow your existing medical plan.
AQI 151–200: Unhealthy
Red means some people in the general population may experience effects, and sensitive groups may experience more serious effects. Reduce prolonged or heavy exertion outdoors, particularly near the upper end of the range, and pay attention to symptoms.
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AQI 201–300: Very Unhealthy
Purple signals that the risk of effects is increased for everyone. Postpone strenuous outdoor exercise when possible, keep indoor air cleaner during smoke events, and use official local guidance for schools, workplaces and events.
AQI 301 and higher: Hazardous
Maroon is an emergency-condition warning. Everyone is more likely to be affected. Avoid outdoor exposure unless authorities or emergency services advise otherwise, and follow local instructions during wildfire smoke or other severe pollution episodes.
The AQI is a population-level communication tool describing effects that may occur within hours or days. It cannot predict exactly how an individual will feel, and the category is not a substitute for urgent medical care.
Why a PM2.5 concentration and an AQI can appear inconsistent
They use different units and purposes
PM2.5 is expressed in µg/m³. AQI is an index value with no concentration unit. A weather app may display both because one is the underlying pollutant measurement and the other is the public-facing health scale.
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The short-term PM2.5 standard used for AQI interpretation is based on a 24-hour average. A reading from a consumer monitor may represent one minute, five minutes, an hour or a rolling window. An instantaneous concentration therefore cannot be compared directly with a daily AQI as if both were equivalent observations.
AirNow can show a near-real-time NowCast AQI, which uses recent hourly observations to support current decisions. That helps the index respond to changing smoke or pollution while the underlying PM2.5 standard remains based on a daily/24-hour measure. The EPA’s 2024 fact sheet explains this distinction at Final Updates to the Air Quality Index (AQI) for Particulate Matter.
The displayed pollutant may not be PM2.5
AQI is calculated for several pollutants, including ozone. If an app labels a value simply “AQI,” check which pollutant is driving it. A PM2.5 concentration can only be translated with the PM2.5 breakpoint row, not with an ozone or other-pollutant table.
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How to read an AQI badge or monitor: a practical method
- Identify the index system. Confirm that the badge uses the U.S. EPA/AirNow AQI. Other countries and apps may use different scales, colors and breakpoints.
- Find the pollutant. Look for PM2.5, ozone or another pollutant name. Do not assume every AQI number represents fine particles.
- Check the location. An official station across town, a regional forecast and an indoor sensor measure different environments. Smoke can vary sharply over short distances.
- Read the time stamp and averaging period. Note whether the value is a current NowCast, a forecast, a 24-hour average or a short sensor snapshot.
- Use the category before the raw number. Match the value to Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy or Hazardous, then apply the category’s caution.
- Compare like with like. When monitoring a trend, compare the same pollutant, location, units, time window and index system from one reading to the next.
- Recheck during changing conditions. Wind shifts, traffic, heating, wildfire smoke and indoor activities can change PM2.5 quickly. Use official updates for decisions affecting health or public activity.
Examples: translating PM2.5 into the U.S. AQI categories
| Displayed PM2.5 concentration | Applicable breakpoint | U.S. category | Interpretation |
|---|---|---|---|
| 6.0 µg/m³ | 0.0–9.0 | Good (0–50) | Within the green range. |
| 9.1 µg/m³ | 9.1–35.4 | Moderate (51–100) | Just inside the yellow range under the revised scale. |
| 35.5 µg/m³ | 35.5–55.4 | Unhealthy for Sensitive Groups (101–150) | The first orange breakpoint; sensitive groups receive the warning. |
| 70 µg/m³ | 55.5–125.4 | Unhealthy (151–200) | The red category applies. |
| 230 µg/m³ | 225.5 and higher | Hazardous (301+) | In the maroon emergency-warning range. |
The table identifies the category range; an official AQI calculation also applies the EPA’s interpolation and reporting rules to the relevant concentration and averaging period. A raw sensor value should not be treated as an official AQI unless it has been processed and quality-controlled for that purpose.
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EPA revised the PM2.5 AQI breakpoints after updating its health standards and reviewing health science. The most visible change is that the Good-to-Moderate boundary moved from 12.0 to 9.0 µg/m³. Several upper-category breakpoints changed as well. The revised PM2.5 24-hour standard remains 35 µg/m³.
Older screenshots, printed charts and software documentation may therefore show a different category for the same concentration. Use the current EPA breakpoint table rather than an undated chart. EPA’s common-questions document directly addresses the question, “Did EPA update the AQI because my air quality is getting worse?” The answer is no: the change reflects updated understanding of health impacts and revised standards, not a claim that local pollution suddenly increased. See the EPA fact sheet and common questions.
How to compare two air-quality readings correctly
Before deciding that one reading is higher, lower or safer, verify all five comparison axes:
- Pollutant: PM2.5, ozone and other pollutants use different breakpoints.
- Geography: A neighborhood sensor, a citywide station and a regional forecast may not describe the same air.
- Units: µg/m³ is a concentration; AQI is an index. Do not rank them on one numerical scale.
- Averaging period: One-minute, hourly, NowCast and 24-hour values answer different questions.
- Index system: U.S. EPA/AirNow colors and thresholds may not match another country’s AQI.
After those checks, compare the AQI category and its health caution. If you only have a PM2.5 concentration, use the current EPA breakpoints to identify the corresponding category rather than guessing from the number’s size.
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A low-cost monitor can provide a useful local snapshot: for example, it may reveal that cooking, candles, a fireplace or an open window is temporarily raising indoor particles. EPA’s Sensor Scale Pilot Project explains how personal sensor readings differ from official AQI data.
- Use a sensor to observe short-term changes at its specific location and to plan activities around those changes.
- Check the device’s units, averaging window, calibration information and placement before interpreting a value.
- Do not present a low-cost sensor as equivalent to an AirNow regulatory monitor; most do not meet the same data-quality standards.
- For health decisions, wildfire smoke alerts and community-wide conditions, use AirNow and local public-health guidance.
A sensor’s one-minute reading can be much more variable than a 24-hour AQI. That difference is expected, not proof that either display is necessarily wrong.
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Questions to ask when readings disagree
Is the number a measurement, an estimate or a forecast?
Start with the label and timestamp. A monitor reading is an observation for a particular place and time. A regional app value may be a forecast, while an AirNow NowCast uses recent hourly observations to represent current conditions. A 24-hour value smooths short spikes that a one-minute sensor exposes. None is automatically “the right number” for every purpose; each answers a different question.
Can I convert any PM2.5 number directly into AQI?
Only when the number is clearly PM2.5 in µg/m³ and its averaging period is appropriate. Select the current PM2.5 breakpoint row, identify the surrounding AQI range and use an official calculation that applies EPA interpolation and reporting rules. A sensor’s raw display is not an official AQI merely because its value falls within a breakpoint range.
Why can two nearby places show different categories?
Geography matters. A station across town, an indoor monitor and a regional forecast sample different air. Smoke, traffic, cooking, heating and ventilation can produce sharp local differences. Compare locations only when they represent the exposure you are trying to understand.
What should I do when the AQI is changing quickly?
Check the timestamp and refresh official information rather than relying on an old screenshot. Wind shifts and wildfire smoke can change concentrations rapidly. For schools, workplaces, events or other shared decisions, use AirNow and local public-health instructions, not a single personal sensor reading.
Who is included in “sensitive groups”?
EPA’s descriptions are population-level categories. People with asthma or other respiratory disease, heart disease, children, older adults and people who are pregnant may need more caution, but personal risk varies. Follow an existing clinician’s plan, and seek medical care for concerning symptoms rather than waiting for an AQI category to change.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common mistakes to avoid
- Treating AQI as a concentration: AQI 100 does not mean 100 µg/m³.
- Using an old breakpoint chart: The PM2.5 thresholds changed on May 6, 2024.
- Ignoring the pollutant label: A displayed AQI may be driven by ozone rather than PM2.5.
- Comparing a spot reading with a daily index: Match the averaging period first.
- Assuming green or yellow guarantees zero risk: Categories describe population-level likelihoods; unusually sensitive people can react at lower values.
- Turning a category into personal medical advice: Follow a clinician’s plan and seek care for concerning symptoms.
- Assuming one indoor monitor represents an entire home or city: Particle levels depend on room, activity, ventilation and outdoor conditions.
Quick reference for everyday decisions
| If the U.S. AQI is… | Practical reading |
|---|---|
| 0–50 | Green; routine activity is generally appropriate. |
| 51–100 | Yellow; most people can continue normally, while unusually sensitive people should watch for symptoms. |
| 101–150 | Orange; sensitive groups should reduce prolonged or strenuous exposure when needed. |
| 151–200 | Red; some of the general population may be affected, so reduce heavy or prolonged outdoor exertion. |
| 201–300 | Purple; everyone’s risk is increased, so avoid strenuous outdoor activity when possible. |
| 301+ | Maroon; treat as an emergency-condition warning and follow official instructions. |
For a current U.S. reading, start with AirNow’s AQI Basics, then check the pollutant, time window and local advisory. The most reliable interpretation combines the category with those details rather than relying on a bare number.
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Frequently Asked Questions
Is PM2.5 the same thing as AQI?
No. PM2.5 is a concentration of fine particles, usually in µg/m³. AQI is a unitless U.S. EPA/AirNow index that translates pollutant data into a number, color and health-concern category.
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What AQI is considered unhealthy?
On the U.S. scale, AQI 101 begins Unhealthy for Sensitive Groups. AQI 151 begins Unhealthy for the broader population, followed by Very Unhealthy at 201 and Hazardous at 301.
Did EPA change the AQI because air pollution suddenly got worse?
No. Revisions effective May 6, 2024 reflect updated health science and standards. The Good-to-Moderate PM2.5 breakpoint moved from 12.0 to 9.0 µg/m³, so older charts may classify the same concentration differently.
Why does my monitor disagree with the weather app?
The displays may measure different locations, pollutants or time windows. A personal sensor may show a one-minute indoor snapshot, while an app may show an AirNow NowCast or forecast. Low-cost sensors also do not meet the same data-quality standards as AirNow monitors.
Can I use a low-cost PM2.5 sensor to make health decisions?
Use it for local short-term patterns and activity planning, but rely on AirNow and local public-health guidance for wildfire smoke, community conditions and major health decisions. Check the device’s units, averaging window, calibration information and placement.
Does a PM2.5 reading represent a 24-hour average?
Not necessarily. AQI interpretation uses a 24-hour PM2.5 standard, but consumer monitors may report one-minute, hourly or rolling values. AirNow NowCast uses recent hourly data for near-real-time communication, so always read the timestamp and averaging period.
Which people should be extra cautious?
EPA categories identify sensitive groups rather than diagnosing individuals. People with asthma or other respiratory disease, heart disease, children, older adults and people who are pregnant may face different risks and should follow their clinician’s plan.
Do AQI colors and breakpoints apply everywhere?
No. This article uses the U.S. EPA/AirNow AQI. Other countries and services can use different scales, colors, pollutants and breakpoints, so confirm the index system before comparing numbers.
The Bottom Line
Bottom line: PM2.5 is the measured amount of fine particles in air; AQI is the U.S. EPA/AirNow health-communication scale built from pollutant data. Check the pollutant, location, units, averaging period and index system before interpreting a number. Use the current breakpoints effective May 6, 2024, treat AQI above 100 as the point where health concerns begin for sensitive groups, and use AirNow or local public-health guidance when conditions are serious or rapidly changing.
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