Hourly PM2.5 readings returned to the normal band across all five regions of Singapore at 8am on Sunday (25 October), ranging from 43 to 55 micrograms per cubic metre.
However, the 24-hour Pollutant Standards Index (PSI) remained between 103 and 138, within the unhealthy range islandwide.
Both readings can be correct at the same time. They cover different periods, which matters when deciding whether to head out now or organise a longer outdoor activity.
Why PM2.5 Can Be Normal While PSI Remains Unhealthy
The one-hour PM2.5 reading measures the average concentration of fine particles in the air over one hour. It provides a more immediate indication of air quality during haze episodes.
NEA classifies readings from zero to 55 micrograms per cubic metre as normal. Readings from 56 to 150 fall within the elevated band.
The 24-hour PSI uses pollutant concentrations averaged over a much longer period. It covers six pollutants, including PM2.5, with the highest pollutant sub-index determining the overall PSI.
That means earlier pollution remains part of the calculation even after hourly conditions improve. A normal hourly reading can therefore sit alongside an unhealthy 24-hour PSI. The two numbers also use different scales: PM2.5 is a particle concentration, while PSI is an index.
What This Means for Sunday Plans
NEA’s advisory issued on Saturday forecast a PSI of 80 to 130 for Sunday, spanning the moderate and unhealthy bands.
The agency advises using its PSI forecast and accompanying health advisory for planning next-day outdoor activities, then checking the latest hourly PM2.5 when deciding whether to proceed with immediate activities.
Under the official hourly guide, people can continue normal activities when PM2.5 is in the normal band.
However, hourly concentrations can fluctuate during haze episodes. Sunday’s 8am reading therefore should not be treated as an assurance about conditions throughout the afternoon. Check the latest reading before a walk, run or other outdoor activity, even if you checked earlier in the morning.
Longer or Strenuous Activities Need More Care
For advance planning when the PSI forecast is unhealthy, the Ministry of Health advises healthy people to reduce prolonged or strenuous outdoor physical exertion.
Older people, pregnant women and children should minimise such exertion. People with chronic lung or heart disease should avoid it.
For immediate decisions, if hourly PM2.5 rises into the elevated band, healthy people should reduce strenuous outdoor activity for the next hour. Vulnerable people should avoid strenuous outdoor activity during that period.
MOH also stresses that individual responses vary. People who feel unwell, especially children, older people and those with chronic heart or lung conditions, should seek medical advice.
The latest readings and activity guidance are available on NEA’s haze website and the myENV app.
