Last Updated on September 24, 2026 by Jeff Tomas
Seeing an AQI or PM2.5 reading in Thailand and not knowing what to do with it is a common problem. The number can help you decide whether to exercise outdoors or limit time outside, but its meaning depends on the scale, the pollutant, and the averaging period.
Thailand’s AQI is a unitless index, while PM2.5 is measured as a concentration in micrograms per cubic meter. Once you know which figure you’re looking at, you can read it in context and make safer daily choices. For local guidance, see understanding Chiang Rai AQI and PM2.5 levels.
Key Takeaways
- Thailand’s AQI is a unitless health index, while PM2.5 is a measured concentration in micrograms per cubic meter (µg/m³).
- On Thailand’s scale, AQI 0 to 25 is very good, 26 to 50 is good, 51 to 100 is moderate, 101 to 200 begins to affect health, and 201 or higher affects health.
- A 24-hour PM2.5 average of 37.5 µg/m³ corresponds to AQI 100 in Thailand; 75.1 µg/m³ reaches the red category.
- As readings rise, reduce outdoor exertion, wear a well-fitting PM2.5 mask when needed, and follow local health advice, especially during Chiang Rai’s burning season.
- AQI numbers can differ between Thai and U.S. apps because their scales use different breakpoints, so check the system behind each reading with the U.S. Embassy’s AQI guide.
Reading Thailand’s AQI: What PM2.5 Numbers Mean
PM2.5 and AQI describe related parts of air quality, but they are different measurements. Thailand’s Pollution Control Department (PCD) calculates an AQI from pollutant-specific breakpoints. PM2.5 is one of six pollutants in the system, alongside PM10, ozone, carbon monoxide, nitrogen dioxide, and sulfur dioxide.
AQI and PM2.5 concentration are different readings
PM2.5 refers to fine particles with a diameter of 2.5 micrometers or smaller. Their concentration is measured in micrograms per cubic meter, written as µg/m³. AQI, by contrast, is a unitless index that turns a pollutant reading into a scale that is easier to interpret.
For example, a monitor might report a PM2.5 concentration of 40 µg/m³. That figure describes the amount of fine particles in a volume of air. The corresponding AQI is a separate number, calculated by comparing the concentration with Thailand’s breakpoint ranges. The PCD uses a 24-hour average for PM2.5 and breakpoint interpolation to calculate its sub-index, so a single live concentration is not automatically the same as the daily AQI.
What Thailand’s AQI color bands tell you
Thailand’s PCD categories group AQI values by air-quality level. The labels and color descriptions can vary between summaries, particularly at the highest readings. The EANET overview of Thailand’s AQI categories provides one reference for interpreting the scale.
| Thailand AQI | Category |
|---|---|
| 0 to 25 | Excellent |
| 26 to 50 | Good or Satisfactory |
| 51 to 100 | Moderate |
| 101 to 200 | Unhealthy |
| 201 or higher | Very Unhealthy or hazardous-style upper band |
Since apps and reports may use different category names or scales, check the scale and health advice shown by your source. For local context, see the Thailand air quality report and PM2.5 levels.
How a PM2.5 reading maps to an AQI category
These simplified 24-hour PM2.5 breakpoints show how concentrations map to AQI bands:
| PM2.5 concentration | Corresponding AQI |
|---|---|
| 0 to 25 µg/m³ | 0 to 25 |
| 26 to 37 µg/m³ | 26 to 50 |
| 38 to 50 µg/m³ | 51 to 100 |
| 51 to 90 µg/m³ | 101 to 200 |
| 91 to 120 µg/m³ | 201 to 300 |
Higher AQI bands continue above these ranges. Because the calculation uses interpolation, a concentration near a breakpoint can move into another category as levels change. Check whether a service shows a current sensor reading or the 24-hour average before comparing its figures.
How to judge a Thailand PM2.5 reading without mixing up scales
A PM2.5 concentration tells you how much fine-particle pollution is in the air. An AQI turns that concentration into a health communication scale, but the result depends on the system and averaging period used. Check both before comparing app readings or deciding what a category means.
Why Thai AQI and US AQI may show different numbers
Thailand and the United States use different AQI breakpoints and public-health categories. As a result, the same PM2.5 concentration can produce different index numbers or labels. Thailand’s official scale uses five color bands, while the US EPA’s AQI uses a 0-to-500 scale with six health categories.
For example, the EPA places PM2.5 concentrations of 9.1 to 35.4 µg/m³ in its “Moderate” band. Thailand uses its own thresholds, so an app showing a Thai AQI and another showing US AQI may display different numbers for similar air conditions. The indices are not interchangeable.
Before comparing readings, check the app’s settings or information panel for the scale it uses. You can also compare the reported PM2.5 concentration in µg/m³, provided both readings use a comparable averaging period. The EPA’s PM2.5 AQI breakpoint table shows how concentrations map to US categories.
How Thailand’s AQI bands compare with WHO PM2.5 guidance
The World Health Organization’s 2021 air quality guidelines recommend PM2.5 levels of 5 µg/m³ for the annual mean and 15 µg/m³ for the 24-hour mean. These are health-based guideline values, not Thailand AQI categories. See the WHO air quality guidelines for their definitions and context.
A Thai reading labeled “Good” or “Moderate” can still be above the WHO 24-hour guideline. That difference doesn’t make the Thai category incorrect: the systems answer different questions and use different thresholds. Also, the WHO 24-hour guideline is based on the 99th percentile of daily averages over a year, so it is not a simple daily safe-or-unsafe cutoff.
Why one reading may not tell the whole story
PM2.5 can change by hour and location. A monitor beside a busy road may report different conditions from one in a residential area, and neither station describes every nearby street or home. Apps may also draw on different monitors or show different averaging periods, such as a live estimate versus a rolling 24-hour value.
Check the reading’s timestamp, monitor location, data source, and averaging window. Then look at the recent trend rather than relying on one number. For added context, compare official reporting with local PM2.5 monitoring in Chiang Rai.
What to do when PM2.5 rises in Thailand
A high PM2.5 reading is a cue to check current local conditions and adjust your plans. Use official updates for the local picture, then follow health guidance that fits your needs.
Check Air4Thai and other trusted updates
Air4Thai is the Pollution Control Department’s public website and app for checking current air-quality readings. It provides station readings, maps, and available graphs or forecasts. You can find the app on Google Play. For advice on reducing health risks, also check updates from the Ministry of Public Health’s Department of Health.
Before acting on a reading, check when it was updated and, when possible, choose a monitoring station near your home or destination. Conditions can vary across short distances, so a faraway station may not reflect the air where you are. Other apps can help you compare trends, but treat them as additional references. Check their data source, station location, and AQI scale before comparing their numbers with Air4Thai.
Match your plans to the reading and your health
When local advice recommends caution or pollution is high, reduce strenuous outdoor activity or move it indoors. You might shorten a run, choose an indoor workout, or reschedule outdoor work. Check the latest local guidance before making plans, since readings can change during the day.
Children, older adults, pregnant people, and anyone with heart or lung disease may need extra care. If you develop symptoms, follow medical guidance and contact a healthcare professional when needed. A reading can help you plan, but it cannot diagnose a health problem or replace advice from your clinician.
Plan ahead for northern Thailand’s haze season
Seasonal haze, including smoke from fires and open burning, can push PM2.5 higher across northern Thailand. Levels may change quickly, and conditions can differ between towns, neighborhoods, and monitoring stations. Before outdoor travel or exercise, check the forecast as well as recent readings to see whether pollution is rising or easing.
During the haze season, build flexibility into your plans. If conditions worsen, consider changing your route or postponing outdoor activity rather than relying on yesterday’s reading. Local measures can also change; for example, check current details about Chiang Rai’s burning restrictions alongside air-quality updates.
Frequently Asked Questions
Even after you understand Thailand’s AQI and PM2.5 readings, you may still have practical questions about daily decisions. These answers cover what the numbers can and cannot tell you.
Can AQI tell me whether pollution is rising?
A single AQI value describes conditions for its reported time period, so it may not show whether pollution is getting worse. Check the app’s trend graph or forecast, along with the timestamp, before planning a long outdoor activity. A sharp increase in PM2.5 can also matter even before the AQI category changes.
Why is the AQI high when PM2.5 looks low?
Thailand’s AQI can reflect pollutants besides PM2.5. The overall index is based on the highest pollutant sub-index, so ozone or another measured pollutant may drive the displayed value. The Thailand AQI sub-index method explains how pollutant-specific readings contribute to the overall score.
Will staying indoors protect me from PM2.5?
Indoors can offer some protection, but outdoor particles may enter through open windows, doors, and ventilation. During a pollution episode, keep windows closed when practical and use a properly sized air purifier with a HEPA filter if available. Avoid adding indoor smoke, such as from smoking or burning incense.
When should I wear an N95 or KN95 mask?
Consider a well-fitting N95 or KN95 when you need to be outdoors during elevated pollution, especially for extended periods or strenuous activity. The seal matters: gaps around the nose or cheeks let particles bypass the filter. Follow local health advice, and review these PM2.5 health precautions during pollution alerts.
What should I do if I feel unwell when the AQI is low?
A low AQI does not rule out symptoms or explain their cause. People can respond differently to pollution, and other conditions may also cause breathing or eye irritation. If symptoms persist or worsen, contact a healthcare professional rather than relying on the app reading alone.
