Singapore Haze: How Smoke Travels and Which Air-Quality Reading to Check

Smoke from vegetation and peat fires can travel to Singapore on prevailing winds. But a smoky smell, a hazy view and an air-quality reading do not measure the same thing. To judge conditions, check the reading for your region—and choose the one that matches when you plan to be outdoors. 279

How does smoke reach Singapore, and why does haze vary by area?

Fires in the surrounding region can send smoke plumes towards Singapore when winds blow in its direction. NEA has reported plumes from fires in Kalimantan and Sumatra during haze episodes. Shifting winds and showers can change conditions over time and across the island, which is why Singapore publishes regional readings rather than relying only on an island-wide figure. 582033

What is in the haze?

Fire smoke contains particles of different sizes, including fine PM2.5 and larger PM10, as well as gases and compounds that can produce an odour. PM2.5 refers to particles with diameters below 2.5 micrometres; NEA identifies it as the predominant pollutant during transboundary haze. 89

Singapore monitors six pollutants for its Pollutant Standards Index (PSI): PM2.5, PM10, sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide. That list describes what the index tracks—not every substance in smoke or every cause of a smoky smell. 827

Why might it smell smoky when PSI has barely changed?

A smell does not tell you the concentration of the pollutants used to calculate PSI. Timing matters, too: a short-lived change may be more apparent in a one-hour PM2.5 reading than in the longer-period PSI. Reduced visibility is not a PSI measurement either; particles can affect what you see, while moisture and weather can also alter visibility. Treat smell and appearance as reasons to check the regional readings, not as substitutes for them. 78912

How are the two readings calculated?

The 24-hour PSI is an index reported from pollutant measurements. Each of the six monitored pollutants is assigned a sub-index using specified concentration breakpoints; the highest applicable sub-index determines the PSI. Although it is called the 24-hour PSI, the calculation uses different measurement periods for different pollutants, including 24-hour particulate concentrations and shorter periods for some gases. It is updated hourly and represents longer-period air quality, not the PM2.5 concentration in the last hour. 82731

The one-hour PM2.5 reading is the average concentration of fine particles measured over the past hour, expressed in micrograms per cubic metre (µg/m³). It is not a one-hour PSI. Its bands are Normal at 0–55 µg/m³, Elevated at 56–150, High at 151–250 and Very High at 251 or above. Because conditions can change, NEA advises checking it when planning activities over the next few hours. 71234

Which reading should guide outdoor plans?

Use the 24-hour PSI and its forecast for longer exposure and plans for the next day. Its categories are Good (0–50), Moderate (51–100), Unhealthy (101–200), Very Unhealthy (201–300) and Hazardous (above 300). At Unhealthy PSI, healthy people should reduce prolonged or strenuous outdoor exertion; older people, pregnant women, children and people with chronic heart or lung disease should follow the more cautious advice for their group. 111228

For a run or another activity in the next few hours, check your region’s one-hour PM2.5 and its accompanying personal guide, then recheck if conditions change. At an Elevated reading, NEA advises healthy people to reduce strenuous outdoor activity for the next hour; its advice for vulnerable groups is more cautious. The two readings answer different questions, so neither should be treated as a replacement for the other. 371234