Two air-quality apps can show different numbers for Bangkok without measuring the same thing. Before deciding which one to trust, check four labels: pollutant, unit, location and averaging period.
An AQI score is not a dust concentration. An hourly reading is not a daily average. A monitor on one side of the city is not a measurement inside your hotel room. Those distinctions are more useful than choosing whichever screen looks reassuring.
This guide helps you compare the readings. For health precautions, follow current Thai public-health advice and any plan your clinician has given you.
Start with the unit beside the number
Thailand's Pollution Control Department explains that AQI is a unitless index, while PM2.5 concentration is reported in micrograms per cubic metre, usually written µg/m³. Thai AQI can reflect several pollutants; the headline index is not necessarily the PM2.5 concentration. (Thai Pollution Control Department: Air4Thai PM2.5 and AQI)
| Label on your screen | What it represents | What to compare it with |
|---|---|---|
| PM2.5, µg/m³ | Concentration of fine particles | PM2.5 concentration with the same averaging period |
| AQI, Thai AQI, US AQI or another named index | A score calculated under a particular index system | The same index, pollutant basis and period |
| Hourly PM2.5 | A shorter-period concentration | The corresponding hour, location and unit |
| 24-hour PM2.5 | A concentration averaged over a longer window | Another 24-hour value with a matching end time |
| Forecast | A prediction | A forecast for the same place and period, not an observation |
Do not compare “60 AQI” with “35 µg/m³” and conclude that one provider reports almost twice as much pollution. They are different quantities. Those figures are illustrative, not today's Bangkok readings.
“Updated this hour” does not always mean “averaged over one hour”
In its explanation of Air4Thai, the Pollution Control Department distinguishes an hourly PM2.5 figure from a 24-hour average updated with new hourly measurements. A newly refreshed screen can therefore still describe the previous 24 hours. (Thai Pollution Control Department: Air4Thai PM2.5 and AQI)
Other systems can use another method. The US Environmental Protection Agency's AirNow service uses a NowCast calculation involving multiple recent hours for current particle pollution. Its explanation is useful for understanding the concept; it does not establish which method every app uses. (US EPA: AirNow AQI methods)
The practical question is: What time period does this number describe? A timestamp alone does not answer it. Open the information panel or chart legend.
If one app shows an hourly concentration and another a rolling daily average, label that difference before treating their disagreement as an error. Do not simply average the two numbers yourself.
Use a station comparison, not a city ranking
Open Air4Thai and select the Bangkok area. The service is operated by Thailand's Pollution Control Department. Its regional office describes station-based observations, historical information and forecasts. (Thai PCD: Air4Thai stations and forecasts)
Then record the station name, observation time and metric. When checking another app, look for the same station or a clearly identified nearby measurement. A city headline may combine or select observations differently; consult that provider's methodology.
For a day split between Sukhumvit and the riverside, check the relevant locations separately if the service provides them. A map label saying “Bangkok” is not enough to identify which reading you are looking at.
Use this small comparison card:
| Field | Reading A | Reading B |
|---|---|---|
| Provider and station | ||
| Observation date and local time | ||
| Pollutant | ||
| Unit or named AQI scale | ||
| Averaging period | ||
| Observed or forecast | ||
| Advisory shown by the source |
If a row cannot be filled in, the readings are not yet a fair comparison. Save a screenshot with the labels visible if you need to discuss it with your hotel, school or workplace.
Turn the reading into a changeable day plan
Thailand's Department of Health advises checking air quality before going out and reducing or avoiding prolonged outdoor activity during high-pollution episodes, especially for vulnerable groups. Its guidance also covers properly used protective masks and reducing indoor exposure. (Thai Department of Health)
Make the day's plan flexible before leaving:
- Identify which activity involves the longest outdoor exposure.
- Choose a realistic indoor alternative and confirm that it is open.
- Check the journey to that alternative; an “indoor day” can still include long outdoor walks and queues.
- Recheck conditions and official advice before the next outdoor activity.
- Follow individual medical advice where relevant; an app cannot assess your personal tolerance.
For example, a visitor planning an outdoor market followed by a lengthy riverside walk could keep the indoor activity as a substitute, rather than buying another non-refundable outdoor excursion. A resident can discuss a workplace's or school's published air-quality procedure instead of trying to interpret a colour without context.
These are planning examples, not claims that a particular venue has clean indoor air. Ask a property what it actually measures and what filtration it provides.
If you are choosing somewhere to live
An apartment's outdoor location and its indoor conditions are separate questions. Before committing to a long lease, ask:
- Is the advertised purifier actually in the unit you will rent?
- Who maintains it and replaces its filters?
- Are any indoor measurements available, and where and when were they taken?
- What happens when building management issues an air-quality notice?
- Can your usual commute be shortened or changed when public-health advice calls for less time outside?
Treat photographs of a purifier and a city AQI screenshot as starting points for questions, not performance certificates. Use our housing checklist to evaluate the rest of the property, and the Bangkok transport guide to compare the actual journey.
Three conclusions the readings cannot support
“Below a legal standard means zero risk.” WHO air-quality guidelines are health-based recommendations used to inform public policy; they are not a personal guarantee of safety or identical to every country's legal standard. (WHO air-quality guidance)
“Today's low number proves this neighbourhood is always better.” One observation cannot establish a year-round comparison.
“A forecast promises tomorrow's conditions.” A forecast helps you plan, but still needs checking against updated observations and advice.
Keep one clearly labelled source as your routine starting point, investigate discrepancies using the comparison card, and let the current advisory inform your plans. The aim is a defensible decision, not agreement between every number on your phone.