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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesPM2.5 and AQI describe the same air-quality problem in different ways. PM2.5 is fine particle pollution and a measured concentration, usually shown in micrograms per cubic meter (µg/m³). The U.S. Environmental Protection Agency (EPA) and AirNow convert pollutant measurements into the Air Quality Index (AQI), a 0-and-up communication scale that indicates how much health concern is associated with the air. They are related, but a PM2.5 number is not an AQI number.
For the U.S. scale, AQI 0–50 is Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, 151–200 Unhealthy, 201–300 Very Unhealthy, and 301 or higher Hazardous. The higher the AQI, the greater the level of pollution and the greater the health concern. The breakpoints below are the revised EPA values that took effect May 6, 2024.
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What do PM2.5 and AQI mean?
PM2.5 is a particle-size category and a concentration
PM2.5 means particulate matter with particles no larger than 2.5 micrometers in diameter. EPA also calls it fine particle pollution. A monitor reports how much of this material is present in a volume of air, most commonly in micrograms per cubic meter (µg/m³). That value is a physical measurement of concentration.
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EPA’s AirData Basic Information page explains the size definition, AQI purpose and category descriptions. AirNow’s AQI Basics page provides the public-facing U.S. categories and colors.
AQI is a normalized communication scale
The U.S. AQI translates a pollutant level into an index value and a plain-language category. It is designed to communicate pollution and associated health concern consistently, rather than to report a mass of particles. An AQI of 80 and a PM2.5 concentration of 80 µg/m³ are not equivalent measurements.
EPA describes the purpose this way: “The higher the AQI value, the greater the level of pollution and the greater the health concern.” The AQI is a population-level communication tool about effects that may occur within hours or days; it is not an individual diagnosis or a prediction of exactly what one person will feel.
This article uses the U.S. EPA/AirNow AQI. Other countries and apps may use different index names, formulas, colors or breakpoints, so confirm which system a badge represents before interpreting it.
How PM2.5 concentrations map to the U.S. AQI
For the current U.S. PM2.5 AQI, use the following breakpoint table. The concentration ranges are expressed in µg/m³ and correspond to the AQI ranges shown alongside them.
| U.S. AQI category | AQI value | PM2.5 concentration breakpoint | Color |
|---|---|---|---|
| Good | 0–50 | 0.0–9.0 µg/m³ | Green |
| Moderate | 51–100 | 9.1–35.4 µg/m³ | Yellow |
| Unhealthy for Sensitive Groups | 101–150 | 35.5–55.4 µg/m³ | Orange |
| Unhealthy | 151–200 | 55.5–125.4 µg/m³ | Red |
| Very Unhealthy | 201–300 | 125.5–225.4 µg/m³ | Purple |
| Hazardous | 301 and higher | 225.5 µg/m³ and higher | Maroon |
These are the revised EPA PM2.5 AQI breakpoints listed by the EPA Air Quality System. AQI is an index value derived from the applicable breakpoint; do not treat the concentration and index columns as interchangeable units. If an app shows only a PM2.5 concentration, use the table to identify the corresponding range, while remembering that the averaging period must also match.
How to read the boundaries
The decimal values are reporting breakpoints, not extra precision about your personal exposure. Under the current table, 9.0 µg/m³ is still in Good and 9.1 µg/m³ is in Moderate. Likewise, 35.4 is the top of Moderate and 35.5 begins Unhealthy for Sensitive Groups. A device that rounds to a whole number can hide a boundary, so check the unrounded value or the category supplied by the reporting service when a reading is close to a cutoff.
The AQI category is the useful communication layer. The number gives more detail within that layer, but an AQI of 100 and 101 are not intended to be treated as identical: 101 is the first value in the next health-concern category.
How do I interpret the AQI numbers?
Good: AQI 0–50
Air quality is satisfactory, and pollution poses little or no risk according to the general U.S. AQI description. A PM2.5 value within 0.0–9.0 µg/m³ falls in this band under the current breakpoints.
Moderate: AQI 51–100
Air quality is generally acceptable. Some unusually sensitive people may face some risk even though the category is not an alert for most people. The current PM2.5 range is 9.1–35.4 µg/m³.
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Unhealthy for Sensitive Groups: AQI 101–150
AQI 101 is the practical point at which the U.S. scale first labels air unhealthy. People in sensitive groups may experience health effects, while the general public is less likely to be affected at this range. On the PM2.5 scale, 35.5–55.4 µg/m³ maps here.
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Unhealthy: AQI 151–200
Some of the general public may experience effects, and sensitive groups may experience more serious effects. This category covers PM2.5 concentrations from 55.5 to 125.4 µg/m³.
Very Unhealthy: AQI 201–300
The risk of health effects is increased for everyone. The PM2.5 breakpoint range is 125.5–225.4 µg/m³.
Hazardous: AQI 301 and higher
This is an emergency-condition warning. Everyone is more likely to be affected, and the PM2.5 concentration is at least 225.5 µg/m³. The scale has no stated upper AQI limit in this category.
The category describes the likelihood of effects for groups of people, not a guarantee that every person will have symptoms. Follow the health message supplied with the official AQI for the location and time you are considering.
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Why an instant PM2.5 reading may not match an AQI badge
AQI uses a time window
The PM2.5 short-term standard used for AQI interpretation is based on a 24-hour average. A sensor showing a one-minute or otherwise short snapshot is measuring a different time window. A brief spike can be much higher than the day’s average, while a recent clean period can make an instantaneous value look lower than a daily value that still includes earlier pollution.
For that reason, do not compare an instantaneous concentration directly with a daily AQI as though both were the same kind of reading. First identify the monitor’s averaging period and whether the AQI is a current estimate or a completed daily value.
NowCast supports current decisions
AirNow may display a near-real-time NowCast AQI calculated from recent hourly data. NowCast helps people make decisions about current conditions, while the standard PM2.5 AQI basis remains a daily, 24-hour measure. The two displays can therefore differ without either being erroneous.
When an app shows “current AQI,” check whether it identifies NowCast, a forecast, a daily observation or another provider’s calculation. Keep the label and timestamp with the number when you interpret it.
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What to record with a reading
A useful air-quality note includes the pollutant, value, units, location, timestamp and averaging period. Also record whether the number is an observation, a NowCast estimate or a forecast. Without those labels, a number copied from a phone screen can be mistaken for a different kind of measurement later. This is especially important when comparing an outdoor regional badge with a monitor inside a room or near a roadway.
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What changed in the AQI in 2024?
The revised breakpoints became effective May 6, 2024
EPA revised the U.S. PM2.5 AQI thresholds effective May 6, 2024. The most visible change is the Good-to-Moderate boundary: PM2.5 enters Moderate at 9.1 µg/m³, so Good now ends at 9.0 µg/m³. The earlier boundary was 12.0 µg/m³. EPA also changed several upper-category breakpoints.
The revised PM2.5 24-hour standard remains 35 µg/m³. The changes reflect updated health science and the agency’s revised standards, not a claim that local pollution suddenly became worse.
Why old charts can disagree
Charts, calculators and screenshots created before May 6, 2024 may show the former breakpoints. If an older chart places a concentration in a different category, check its publication date and replace it with the current values in the EPA’s PM2.5 AQI fact sheet and common questions or the current Air Quality System table. A difference caused by the revised scale is not evidence that the monitor changed or that air quality deteriorated.
How to compare two air-quality readings correctly
Use this checklist before deciding that one reading is higher, safer or worse than another:
- Identify the pollutant. Confirm that both readings are PM2.5. A PM2.5 value cannot be compared numerically with an AQI based on another pollutant.
- Confirm the geography. Check whether the numbers describe the same monitor, neighborhood, city, county or regional reporting area. A regional AQI can differ from a personal monitor beside a road or indoors.
- Check the units. PM2.5 is normally shown in µg/m³; AQI is an index number. Do not rank them as if they shared a unit.
- Check the averaging period. Note whether the PM2.5 value is one minute, an hourly average, a rolling estimate or a 24-hour average, and whether the AQI is NowCast or another period.
- Verify the index system. Make sure the badge uses the U.S. EPA/AirNow AQI. A foreign or proprietary scale may use different breakpoints and colors.
- Read the category and health caution. Once the systems and time windows match, use the category—Good through Hazardous—to understand the intended public-health message.
For a raw PM2.5 value, use the current EPA breakpoint table rather than mental arithmetic or an old online chart. The table supplies the category ranges; it does not make a short-term personal reading equivalent to an official daily AQI.
How to use AQI information in everyday decisions
Start with the category, not just the number
The category is the communication layer designed for quick decisions. AQI 100 and AQI 101 are numerically adjacent, but 101 begins the Unhealthy for Sensitive Groups category and carries a different caution. At values above 150, the scale places the general public as well as sensitive groups in categories where effects may occur.
Use the official health message for the location and period shown. The AQI communicates population-level risk over hours or days; it cannot account for every person’s medical history, activity, indoor environment or exposure duration.
Use trends without over-reading them
Successive readings from the same source and time window can show whether conditions are improving or worsening. A single value, however, cannot explain why the level changed or prove that two locations have the same exposure. Weather, smoke movement, traffic and monitor placement can all produce different local readings. Treat the trend as context, then check the official category and timestamp.
Keep the timestamp attached to the reading
Smoke, traffic and weather can change particle levels quickly. Record the location, time, pollutant, units and averaging period when saving a reading. This prevents a current snapshot from being mistaken for a day-long condition or a forecast from being mistaken for an observation.
Do not infer a diagnosis from a number
An AQI category is not a clinical test. If someone develops concerning symptoms, use appropriate medical advice rather than trying to diagnose the cause from a sensor value. The EPA categories indicate when health concern is greater across a population; they do not establish an individual exposure limit or guarantee a particular response.
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What consumer PM2.5 sensors can—and cannot—tell you
Useful for local snapshots
A personal or low-cost PM2.5 sensor can show short-term changes at a particular location, such as a room, school, workplace or neighborhood. That local information may help with activity planning or with noticing patterns that a regional monitor cannot resolve.
Not a substitute for official AQI
Most low-cost sensors do not meet the same data-quality standards as AirNow monitors. EPA recommends AirNow for health and air-quality information, while treating sensor readings as complementary local snapshots. A sensor’s value can be affected by its placement, humidity, calibration, maintenance and time window; a displayed number alone does not establish regulatory data quality.
EPA’s What Do My Sensor Readings Mean? Sensor Scale Pilot Project explains the distinction between one-minute personal sensor data and official AQI. Do not present a consumer monitor as necessary to understand the AQI, and do not use a brand-specific accuracy claim to override the official regional reading.
Common sensor-comparison mistakes
Comparing a personal monitor with AirNow is meaningful only after checking location and time window. A one-minute value beside a cooking source may rise sharply while a regional NowCast changes slowly. Moving the device, placing it near an open window or comparing it during different weather can also change the result. These differences do not by themselves show that either instrument is broken. They show that the devices are answering different measurement questions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A quick interpretation example
Suppose a display reports PM2.5 at 40 µg/m³. On the current U.S. breakpoint table, 40 falls in the 35.5–55.4 µg/m³ range, which is the AQI 101–150 Unhealthy for Sensitive Groups band. That conclusion is valid only if the 40 µg/m³ value uses the relevant AQI averaging basis and the display is using the U.S. breakpoints.
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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →If another display reports AQI 120 for the same area, the two values are not numerically contradictory: one is a concentration and the other is an index. If a personal sensor reports 40 µg/m³ for one minute while AirNow shows a lower NowCast value, the different time windows and locations may explain the difference. Compare the metadata before judging either reading.
As a second boundary example, a concentration of 9.0 µg/m³ remains in Good, while 9.1 µg/m³ enters Moderate under the revised table. If an app rounds both to “9,” rely on its stated category and averaging period rather than trying to reconstruct a category from the rounded display.
Frequently asked questions
Is PM2.5 the same thing as AQI?
No. PM2.5 identifies fine particles and reports their concentration in µg/m³. AQI is an index that translates a pollutant level into a health-concern category. Use the EPA breakpoints to relate them, but do not treat their numbers as the same unit.
Is a PM2.5 reading of 40 dangerous?
On the current U.S. PM2.5 AQI table, 40 µg/m³ falls in the AQI 101–150 Unhealthy for Sensitive Groups range when the relevant averaging basis is used. The category is a population-level communication, not an individual diagnosis. Check the official health message and your reading’s time window.
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The devices may cover different locations, averaging periods or data-quality standards. A low-cost sensor can provide a useful local snapshot, while AirNow supplies official regional information and may show a NowCast based on recent hourly data. Compare metadata before deciding that one reading is wrong.
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Did EPA update the AQI because my air quality is getting worse?
No. EPA’s May 6, 2024 update reflects updated health information and revised standards. It does not claim that local pollution suddenly worsened. Older charts may simply use the former breakpoints.
What does AQI above 100 mean?
AQI 101–150 is Unhealthy for Sensitive Groups. At 151 and above, the categories indicate that some of the general public may experience effects, with greater concern at higher values. Read the category and official health message rather than relying on the number alone.
Can I compare an AQI number with a PM2.5 number?
Not directly. First confirm the same pollutant, location, averaging period and U.S. index system, then use the breakpoint table. An AQI value and a concentration in µg/m³ are different kinds of measurements.
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Does AQI 301 have a maximum?
The Hazardous category begins at AQI 301 and has no stated upper AQI limit in the category table. Its PM2.5 breakpoint begins at 225.5 µg/m³ and higher, and it is an emergency-condition warning for everyone.
Do I need to buy a PM2.5 monitor?
No. AirNow and other official services provide AQI information without requiring a personal device. A consumer sensor can be useful when you need a local short-term snapshot, but EPA says most low-cost sensors do not meet AirNow’s data-quality standards and should complement, not replace, official AQI information.
Where to verify a U.S. AQI reading
- AirNow AQI Basics for the U.S. category meanings and colors.
- EPA AirData Basic Information for PM2.5, AQI purpose and sensitive-group descriptions.
- EPA Air Quality System AQI Breakpoints for the current detailed concentration ranges.
- EPA’s PM2.5 AQI fact sheet and common questions for the 2024 revision, effective date and NowCast context.
- EPA’s Communicating Air Quality Conditions: The Air Quality Index for the normalized-scale and health-communication explanation.
In short, read PM2.5 as a concentration, read AQI as a U.S. health-communication index, and always align the pollutant, location, units, averaging period and index system before comparing numbers.
Frequently Asked Questions
What do PM2.5 and AQI mean?
PM2.5 is fine particulate matter no larger than 2.5 micrometers, reported as a concentration in µg/m³. AQI is the U.S. EPA/AirNow index that translates pollution levels into health-concern categories.
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How do I interpret the AQI numbers?
Use the category first: 0–50 Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, 151–200 Unhealthy, 201–300 Very Unhealthy and 301+ Hazardous. Then check the timestamp, location, pollutant and averaging period.
Why did the EPA AQI breakpoints change in 2024?
The revised breakpoints took effect May 6, 2024, reflecting updated health information and standards. The Good-to-Moderate PM2.5 boundary moved from 12.0 to 9.0 µg/m³; the change does not mean local pollution suddenly worsened.
Can a one-minute sensor reading be compared with AQI?
Not as if they were identical. A one-minute sensor value and a daily or NowCast AQI use different time windows and may represent different locations. Treat the sensor as a local snapshot and use AirNow for official AQI information.
What does AQI 101 mean?
AQI 101 begins the Unhealthy for Sensitive Groups category. Sensitive people may experience effects, while the general public is less likely to be affected at that range.
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Read the official health message for your location and time. AQI 151 and higher is Unhealthy or worse, where some of the general public may experience effects and sensitive groups may face more serious effects.
The Bottom Line
Bottom line: PM2.5 is a measured fine-particle concentration; AQI is the U.S. EPA/AirNow index that communicates pollution-related health concern. Use the current breakpoint table, keep the averaging period and timestamp attached, and rely on the category and official health message rather than comparing raw PM2.5 and AQI numbers directly.
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