How do you read PM2.5 and AQI figures?

When fine particulate levels are high, the numbers on apps and signboards do not all mean the same thing. It is worth reading them correctly before deciding anything.

How do PM2.5 and AQI differ?

PM2.5 is the concentration of particles up to 2.5 microns, reported in micrograms per cubic metre.

AQI, the air quality index, converts each pollutant into a sub-index and reports the highest of them. Each sub-index is calculated over a defined averaging period — 24 hours for PM2.5 under the Thai index — so an AQI figure is not a direct particle concentration, and not a snapshot of the air right now.

The Pollution Control Department publishes both the AQI and the underlying PM2.5 concentrations, with the averaging period stated alongside.

Why does the averaging period matter?

A 1-hour average reflects the situation right now. A 24-hour average reflects the day as a whole.

If the 24-hour figure looks moderate but the morning reading spiked, planning a morning run from the daily average will mislead you.

Which location's reading should you use?

Readings vary considerably between districts, and between a main road and a side street. The monitoring station nearest to where you actually are gives a truer picture than a province-wide average.

If home and work are in different districts, check both.

How do you choose a mask?

Masks differ in what they filter. Check whether the product is specified for fine particles.

Fit matters as much as type: gaps around the edge let air bypass the filter.

What can you do indoors?

Keep windows closed while outdoor readings are high, and ventilate when they fall.

An air purifier with a particulate filter works better in a closed room. Check the room size it is rated for, and change the filter on the manufacturer's schedule.

Reduce indoor sources too, such as burning incense or candles, and cooking smoke without extraction.

What about when you get home?

Change clothes, wash your hands and face, and keep outdoor clothing off the bed. None of that needs equipment.

When should you see a doctor?

If something is interfering with daily life during this period, see a doctor. An article cannot stand in for an examination.