PSI, PM2.5 and Swimming Pool Decisions: How to Read the Numbers Before Lessons
By Coach Tan Meng Chew (MC), SG Sink or Swim
During haze season, two sets of numbers dominate the news and the group chats: the PSI and the PM2.5. Parents send me screenshots of one or the other and ask, "Is this bad? Should we still come for lesson?" Often the two numbers seem to disagree. The PSI says "Unhealthy," the PM2.5 says "Normal," or the other way round, and nobody is sure which one to believe.
I'm Coach Tan Meng Chew, known to most people as MC. I run SG Sink or Swim, a swimming school and coaching brand serving the west side of Singapore, teaching from Nursery and Level 1 (Baby & Toddler) through to SwimSafer Stages 1 to 6. Reading these numbers has become part of my daily routine in haze season, because they help me decide whether a lesson should go ahead, be adjusted or be cancelled.
The good news is that the two numbers are not rivals. They do different jobs, and once you understand the jobs, the confusion largely disappears. In this article I will explain what each number means, what the official bands are, how Sport Singapore (SportSG) uses them for facilities, and how I would combine them into a practical decision about a swimming lesson. I am a swim coach, not a doctor or an air-quality scientist, so I will stick to published information, mark my own judgement clearly, and send you to the official channels for live readings.
The short answer
The 24-hour PSI is the planning number. It gives the overall picture for the day, and the official health advice and facility phases are built around it.
The 1-hour PM2.5 is the right-now number. It shows what the air is like at this moment, and it helps with immediate decisions, such as whether to start a lesson now.
Use both, in that order: plan with the PSI, then check the PM2.5 shortly before you leave, then check what your venue has decided.
The venue's decision comes first. If a facility closes or suspends activities, the readings no longer matter for that lesson.
Your child's health comes before any number. If your child is unwell, or has asthma or another breathing condition, follow your doctor's advice.
Let me take you through each of these.
Two numbers, two jobs
Here is the simplest way to see the difference.
24-hour PSI | 1-hour PM2.5 | |
What it tells you | The overall air quality picture over a day | What the air is like right now |
Time scale | A 24-hour measure | A one-hour reading |
Best used for | Planning ahead, such as tomorrow's activities | Immediate decisions, such as whether to start a lesson now |
What it covers | An index; one reporting source I read says it includes both PM10 and PM2.5 | Fine particles (PM2.5) only, in micrograms per cubic metre |
How steady it is | Changes slowly | Can swing up and down within hours |
NEA's haze updates put it neatly: "Refer to the 1-hr PM2.5 for immediate activities, and the 24-hr PSI forecast to plan ahead." HealthHub and Sport Singapore describe the same split. Sport Singapore's haze page says facility decisions are primarily based on the 24-hour PSI forecast for planning, with 1-hour PM2.5 readings used as supplementary information for immediate activity decisions.
Think of it as the difference between a weather forecast and looking out of the window. You need both.
What does PM2.5 actually mean?
PM2.5 stands for particulate matter that is 2.5 micrometres across or smaller. These particles are far thinner than a human hair, and they are small enough to reach deep into the lungs, according to the United States Environmental Protection Agency (EPA). In Singapore's haze, they come mostly from smoke. News reports citing the National Environment Agency (NEA) describe smoke plumes from fires in Sumatra and Kalimantan drifting towards Singapore on prevailing winds.
The 1-hour PM2.5 reading is reported in micrograms per cubic metre (µg/m³), meaning the weight of those fine particles in each cubic metre of air.
The official bands
24-hour PSI bands
According to the Sport Singapore haze poster and NEA advisories I read, the 24-hour PSI is grouped like this:
Band | 24-hour PSI | Advice (general) |
Good | 0 to 50 | Normal activities |
Moderate | 51 to 100 | Normal activities |
Unhealthy | 101 to 200 | Advice depends on group |
Very unhealthy | 201 to 300 | Advice depends on group, stricter |
Hazardous | Above 300 | Advice depends on group, strictest |
The activity advice, as set out in the NEA advisory document and the Ministry of Health (MOH) FAQ, is as follows. (The text of the NEA PDF came out jumbled when I read it, so I mapped its table by order. MOH's FAQ states the same in prose. Please confirm on haze.gov.sg.)
24-hour PSI | Healthy persons | Elderly, pregnant women, children | Chronic lung or heart disease |
100 or below | Normal activities | Normal activities | Normal activities |
101 to 200 | Reduce prolonged or strenuous outdoor exertion | Minimise prolonged or strenuous outdoor exertion | Avoid prolonged or strenuous outdoor exertion |
201 to 300 | Avoid prolonged or strenuous outdoor exertion | Minimise outdoor activity | Avoid outdoor activity |
Above 300 | Minimise outdoor activity | Avoid outdoor activity | Avoid outdoor activity |
The Sport Singapore poster explains the three action words: Reduce means do less, Minimise means do as little as possible, and Avoid means do not do. It also stresses that the advice is about prolonged or strenuous exertion.
1-hour PM2.5 bands
The one-hour PM2.5 bands, as set out in the Sport Singapore poster and a 2016 report on NEA's introduction of the bands, are:
Band | 1-hour PM2.5 (µg/m³) |
Normal | 55 and below |
Elevated | 56 to 150 |
High | 151 to 250 |
Very high | Above 250 |
A recent Singapore news report and a human-resources magazine article that summarised NEA's advice describe the activity guidance for each band roughly as follows:
Normal: continue usual activities.
Elevated: the public should reduce strenuous outdoor activity, and vulnerable people should avoid it.
High: the public should avoid strenuous outdoor activity, and vulnerable people should avoid outdoor activity.
Very high: all outdoor activity should be minimised, and vulnerable people should avoid outdoor activity.
I should flag a wrinkle here. When NEA introduced the PM2.5 bands in June 2016, it said they were not tied to health advisories, because the evidence for sub-daily PM2.5 effects was not yet strong enough, and that health advisories applied only to the 24-hour PSI. The more recent reports I read describe activity advice at each PM2.5 band, so it seems the guidance has evolved since then. I have only seen the newer advice second-hand, so please check NEA's own channels, such as haze.gov.sg, for the current wording before relying on it.
Why the two numbers can disagree
A few real examples from this haze season show how.
4 September 2026. In a report on the day the 24-hour PSI first entered the Unhealthy range this year, the Central region's 24-hour PSI was 104 (Unhealthy) while the islandwide 1-hour PM2.5 was 44, in the Normal band.
13 October 2026. According to a news report of that morning's readings at 9 am, the 24-hour PSI was in the Unhealthy range in every region: west 172, central 169, east 154, south 134 and north 127. The 1-hour PM2.5 told a different story region by region: east 119, central 101 and west 97, all in the Elevated band, north 66 and south 50. The south was in the Normal band, down from 65 an hour earlier.
Here is what these examples teach.
The PSI reflects the day; the PM2.5 reflects the hour. A 24-hour index stays high if the air has been bad for much of the past day, even if the current hour is better. A one-hour reading can improve or worsen quickly.
Regions differ. The west, which is where SG Sink or Swim and many of our families are based, was the highest in the 24-hour PSI on 13 October. Always read your own region, not the islandwide headline.
Conditions can change within hours. Afternoon showers on 10 October slightly improved the readings, according to a news report, though air quality remained unhealthy. Sport Singapore's haze plan says short spikes in PM2.5 may improve within one to two hours.
Neither number is wrong. They are measuring different things over different time spans. That is why using both is better than trusting either alone.
How Sport Singapore uses them for facilities
Many Singapore families swim at ActiveSG facilities, and many lessons by independent coaches and swim schools take place there. So it helps to know how Sport Singapore uses these numbers.
Its Haze Management Plan (April 2026) sets out phases by 24-hour PSI. As I read it:
24-hour PSI | Outdoor or non-air-conditioned facilities | Air-conditioned facilities |
0 to 50 | Open | Open |
51 to 100 | Open | Open |
101 to 200 | Operate with conditions | Open |
201 to 300 | Close | Operate with conditions |
Above 300 | Close | Close |
In the 101 to 200 phase, organisers must carry out a risk assessment before continuing activities, and anyone feeling unwell must stop and seek medical attention. At 201 to 300, outdoor activities are suspended and air-conditioned facilities may continue with risk management measures. Above 300, all facilities close and organised activities are suspended.
On PM2.5, the plan says the 1-hour reading is supplementary information for immediate activity decisions, and that it cannot override facility advisories based on the 24-hour PSI. Sport Singapore's haze page says the same.
When PM2.5 triggers a stricter step
Recently, ActiveSG announced that "due to the elevated PM2.5 readings," SportSG would take "additional precautionary measures to temporarily close all outdoor and non-air-conditioned facilities" in its ActiveSG Sport Centres in the West, including Jurong East, Jurong West, Bukit Batok, Choa Chu Kang and Clementi. As I read it, that is a stricter step than the plan's baseline for the 101 to 200 band, which is "operate with conditions," and the notice itself calls it an "additional" measure. So PM2.5 can prompt a more cautious decision than the PSI band alone would require. I do not have the current status of that closure, so please check go.gov.sg/activesgfacilitiesclosure and the Sport Centre's Facebook page on the day.
An important caveat I keep repeating, because it matters: the documents I read do not say how swimming pools are classified as outdoor, non-air-conditioned or air-conditioned. Neither the plan nor the notice names swimming pools. So the numbers tell you about the air, but only the venue can tell you what it is doing about its pool.
A practical way to decide: plan, confirm, check
Here is the three-step routine I use. It takes a couple of minutes.
Step 1: Plan with the 24-hour PSI
Look at the 24-hour PSI and forecast for your region. This tells you what kind of day it is, and which official advice and facility phase apply. If the PSI is in the Very unhealthy or Hazardous range, the answer is simple: outdoor and non-air-conditioned facilities are closing or closed under Sport Singapore's plan, and I would skip.
Step 2: Confirm with the 1-hour PM2.5
Shortly before you leave, look at the latest 1-hour PM2.5 for your region. Is it Normal, Elevated, High or Very high? Is it rising or falling compared with the previous hours? If it is rising, think twice. If it has dipped, that may be a sign conditions are improving, though one reading is not a trend.
Step 3: Check the venue and your child
Has the venue closed, suspended or adjusted lessons? Is the pool outdoors, covered or enclosed? And is your child well, with no unusual cough, eye irritation or breathing difficulty? The venue's decision and your child's health override everything else.
Putting the two numbers together: my coaching guide
Below is a simple matrix I use to think through lessons for children. Please read it carefully: this is my own coaching framework, not an official protocol, and it is for generally healthy children. If your child has asthma or another chronic condition, your doctor's advice comes first, and the official guidance is more cautious for them at every level.
24-hour PSI | 1-hour PM2.5 | What I would typically do |
0 to 100 | Normal | Go as normal |
0 to 100 | Elevated or higher | A short-term spike. Check the venue, consider delaying, and keep intensity low if the lesson goes ahead |
101 to 200 | Normal | The day has been smoky but the air is currently cleaner. Check the venue; if open, consider a short, gentle lesson, preferably in an enclosed pool; skip for sensitive children |
101 to 200 | Elevated | Be cautious. Outdoor or open-sided pool: I lean towards skipping or a very light session. Enclosed pool and venue operating: a short, gentle lesson may be reasonable for a healthy child |
101 to 200 | High or very high | Skip outdoor lessons. Official advice for the public at these PM2.5 levels is to avoid strenuous outdoor activity |
201 or above | Any | Skip. Outdoor and non-air-conditioned facilities close under Sport Singapore's plan, and official advice for children is to minimise or avoid outdoor activity |
Two things stand out when you lay it out like this. First, the PM2.5 reading is what separates a "go gently" day from a "skip" day within the same PSI band. Second, the more outdoors the pool, the more weight I give to the numbers. For an enclosed pool, venue decisions and the child's health tend to matter more.
The pool itself matters as much as the numbers
The readings describe the outdoor air. What you breathe at the pool depends on the pool.
Fully open outdoor pool. Swimmers breathe outdoor air throughout, so the readings apply most directly. This is the type I am most cautious with.
Covered but open-sided pool. A roof helps with sun and rain but not necessarily with air quality, because air moves freely. I would treat it as close to outdoor unless the venue says otherwise.
Enclosed, air-conditioned pool hall. Exposure may be lower, depending on sealing, ventilation and filtration, and Sport Singapore's plan treats air-conditioned facilities more leniently. But "indoor" is not a guarantee, and the official guidance asks for strenuous indoor activity to be minimised for young children when the air is Unhealthy. For example, NEA's announcement on measures said preschools limit long outdoor play and minimise strenuous indoor physical activities for children in that range.
These are my reasoning, not official classifications. Ask the venue how it treats its pool.
Why swimming is worth being careful about
Official advice focuses on prolonged or strenuous exertion because of how breathing works. The EPA says that where particles end up in the lungs depends on several things, including physical activity and whether you breathe through your nose or your mouth. A California Air Resources Board page says exercisers breathe faster, take more of the air in through the mouth, and carry pollutants deeper into the lungs, exchanging "up to 10 or 20 times more air" than at rest. Swimming is largely mouth-breathing exercise, so the same logic applies, though I could not find a study measuring swimmers specifically.
For children, the picture is a little more delicate. HealthXchange explains that children's lung defences are still developing, and that haze can irritate the nose, lungs and eyes of otherwise healthy children. MOH notes that effects can lag exposure by one to three days, so a child who seems fine at the pool may cough the next day.
I have written about the lungs in more detail in a separate article. The point here is that the numbers matter because they tell you how much poor air a swimmer might breathe while working hard.
Common mistakes I see
Looking at only one number. A PSI of 90 with a PM2.5 spike, or a PSI of 150 with a clean-looking hour, each tells half the story.
Using the islandwide headline. Regions differ. On 13 October, for example, the west and south differed noticeably in PM2.5. Read your own region.
Relying on yesterday's reading. Conditions can change within hours. Check close to the lesson.
Assuming a pool is open or closed. The numbers do not tell you what a venue has decided. Check the venue.
Assuming "indoor" means safe. Ventilation varies, and official guidance still asks for strenuous indoor activity to be minimised for young children when the PSI is Unhealthy.
Judging by the sky. Haze can look milder or worse than it is. The readings are more reliable than your eyes.
Ignoring the child in front of you. No number replaces looking at your child. A cough, itchy eyes or an unusual mood matters.
Forgetting about the days after. Effects can lag by one to three days, so keep watching.
Who should be extra careful, whatever the numbers say
Based on the official sources, the groups to be most careful with are children, the elderly, pregnant women and people with chronic heart or lung conditions, such as asthma. Official guidance asks for more caution from these groups at lower levels than for healthy adults. If your child has asthma, please speak to the doctor who treats them, follow their asthma action plan, and let your coach know. HealthXchange says children with asthma should follow their asthma action plan if the PSI is above 100 and they have a cough, wheezing or shortness of breath.
In an emergency, such as severe difficulty breathing, call 995.
A quick routine you can copy
Morning: check the 24-hour PSI forecast for your region.
Midday: check for notices from your venue, such as ActiveSG's facility closure page and the Sport Centre's Facebook page, and from your swim provider.
Before leaving home: check the 1-hour PM2.5 for your region, and its trend.
Look at your child: are they well? Any coughing, eye irritation or breathing difficulty?
Decide: go, adjust or skip.
At the pool: tell the coach if anything feels off, and keep any prescribed medication within reach.
Afterwards: watch for symptoms over the next one to three days.
How SG Sink or Swim uses the numbers
Here is how I approach this at SG Sink or Swim. We serve the west side of Singapore and teach many young children, who are among the groups official guidance asks us to be most careful with. These are principles, and details depend on the venue and the day.
The venue decides first
If a venue closes or suspends activities, our lessons there are off. The numbers help me understand why, but they do not override a venue's decision.
I read my region, not the headline
Because we are on the west side, I look at the western readings, and I check them close to lesson time.
I use PSI to plan and PM2.5 to decide
I look at the 24-hour PSI to understand the day, and the 1-hour PM2.5 to decide about a lesson that is about to start. If PM2.5 is climbing, I would rather wait or reschedule than start and stop.
I lower the intensity before I cancel, where it is safe
On borderline days, with the venue open and a healthy child, shorter, gentler sessions focused on water confidence, floating, kicking and relaxed breathing can keep progress ticking over without demanding hard breathing. On poor days, we skip.
I am more cautious with children and sensitive swimmers
Babies, toddlers, young children and anyone with a known breathing sensitivity get a more conservative approach, whatever the numbers say.
I want to know about your child's health
Please tell me about asthma, allergies or any breathing concern, and where prescribed medication is kept.
I do not give medical advice
I follow the plan your child's doctor has given you, but I do not diagnose or decide on medication.
I keep progress visible through a pause
Our programmes run from Nursery and Level 1, through Level 2 (Beginner) and Level 3 (Intermediate), and on to SwimSafer Stages 1 to 6. Because progress is tied to that pathway, a haze pause does not leave families guessing where their child stands, and we can restart from the right point.
If you would like to talk about how we handle haze, or how lessons would work for your child, you are welcome to get in touch through sgsinkorswim.com.
Frequently asked questions
What is the difference between PSI and PM2.5?
The 24-hour PSI is a planning index for the day; the 1-hour PM2.5 is a short-term reading of fine particles for immediate decisions. NEA's advice is to use the 1-hour PM2.5 for immediate activities and the 24-hour PSI forecast to plan ahead.
Which one should I look at before a swim lesson?
Both. Plan with the 24-hour PSI, then check the 1-hour PM2.5 shortly before you leave, then check your venue's notices.
My PSI says Unhealthy but PM2.5 says Normal. What now?
It usually means the day has been smoky but the current hour is cleaner. Check the venue, consider a short, gentle lesson only if your child is well and the venue is operating, and skip for sensitive children.
What PM2.5 level is too high for swimming?
I could not find an official PM2.5 threshold for swimming. The reports I read describe activity advice by band: at Elevated, the public should reduce strenuous outdoor activity and vulnerable people avoid it; at High, the public should avoid strenuous outdoor activity. Please verify the current wording on NEA's channels.
Is there an official PSI cut-off for swimming lessons?
Not that I could find. Sport Singapore's plan sets facility phases by PSI band, but it does not say how pools are classified. Check with your venue.
Where do I find the readings?
NEA's haze website, haze.gov.sg, publishes the 24-hour PSI and the 1-hour PM2.5 by region.
Where do I check if my pool is open?
Check ActiveSG's facility closure page (go.gov.sg/activesgfacilitiesclosure), the Sport Centre's Facebook page and your provider. The ActiveSG page notes that same-day closures may not appear immediately.
Why was a facility closed when the PSI was only in the Unhealthy range?
ActiveSG's recent notice cited elevated PM2.5 readings and called the closure an additional precautionary measure. That suggests it responded to real-time air quality, which is how I read it.
Do the numbers apply to indoor pools?
They describe outdoor air. How much reaches an indoor pool depends on the building. Ask the venue.
Should my child wear a mask to the pool?
HealthHub says N95 masks are not needed for brief trips between buildings or for indoor use, and HealthXchange notes they may not fit children well. Ask your doctor.
My child has asthma. Do the numbers still apply?
They are a starting point, but your child's doctor's advice and asthma action plan come first. Official guidance is more cautious for people with chronic lung conditions at every level.
The bottom line
PSI plans the day; PM2.5 checks the hour. Use both, in that order.
Read your own region and check close to lesson time. Conditions can change within hours, and regions differ.
The venue's decision comes first, and the pool type matters. The documents I read do not say how pools are classified, so ask.
PM2.5 can prompt stricter steps than the PSI band alone, as ActiveSG's recent western closure notice shows.
Your child's health beats any number. If in doubt, skip, because a rescheduled lesson is easy to recover from.
My matrix is a coaching guide, not an official rule. Verify official advice on haze.gov.sg, and follow your doctor's advice for any breathing condition.
Haze seasons pass, and a child's swimming journey is long. The families who do best are the ones who read the numbers calmly, check with their venue and coach, and get back into a regular rhythm as soon as the air clears.
At SG Sink or Swim, we teach from a child's very first splash through to SwimSafer Stage 6, and keeping swimmers safe is always part of the job. If you have questions about lessons during haze, reach out at sgsinkorswim.com.
Stay safe, breathe easy, and see you at the pool when the air is clear.
Coach Tan Meng Chew (MC) SG Sink or Swim | sgsinkorswim.com
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