How to Read the PSI and NEA Advisory Before Heading to the Pool

By Coach Tan Meng Chew (MC), SG Sink or Swim

Most swimmers in Singapore check the weather before a pool session. They look at the rain radar, glance at the sky, and maybe wonder about thunder. Far fewer check the air.

Singapore usually enjoys good air quality. But every so often, usually during the dry Southwest Monsoon months, smoke from regional fires drifts across the region and the haze returns. On those days, a swim lesson or training session that looked routine on the calendar becomes a real judgement call.

I'm Coach MC of SG Sink or Swim, and I coach swimmers across the west side of Singapore, from first-time Nursery-level kids to adults working through SwimSafer stages. The most common haze question I hear is some version of "The PSI says 90-something. Can we still swim?" The second most common is "The app says one number, the NEA site says another, so which one do I believe?"

This guide answers both. It explains what the PSI and the NEA advisory actually say, how the different numbers relate to each other, and how to turn them into a calm, sensible decision before you pack your swim bag.

Why Air Quality Matters for Swimmers

Swimming is often recommended as a gentle, joint-friendly form of exercise, and it is. But it is also an aerobic activity. When you swim, your breathing rate goes up, and you take in air in rhythm with your strokes. In freestyle, for example, you inhale quickly and deeply every couple of strokes, often through the mouth.

That matters because the main health concern during haze is fine particulate matter, known as PM2.5. These are particles so small that they can travel deep into the lungs. The harder and longer you breathe in polluted air, the more of it you take in. This is why NEA's advice is tied not just to the air quality reading, but to how strenuous and how prolonged your outdoor activity is.

Two more points before we get to the numbers:

• Indoor is not the same as outdoor. An enclosed, air-conditioned indoor pool is a different environment from an open-air pool. But even for indoor pools, you still have to travel there, walk from the bus stop or car park, and wait around in corridors and entrance areas. Indoor facilities reduce your exposure but do not necessarily remove it.

• People are not equally affected. Healthy adults, young children, older adults, pregnant women, and people with chronic heart or lung conditions all fall into different advice categories. The right decision for a fit 25-year-old is not automatically the right decision for a six-year-old learning to float or a 70-year-old with asthma.

The Two Numbers You Need to Know

When people talk about "the PSI," they are usually talking about one of two different readings. NEA publishes both, and the difference between them is the single most useful thing to understand.

1. The 24-hour PSI

The Pollutant Standards Index (PSI) is Singapore's official air quality index. It is calculated from several pollutants, and the reading you see is a 24-hour average, meaning it reflects the past day rather than this exact minute. During haze episodes, PM2.5 is usually the pollutant that drives the number.

Because it is an average, the 24-hour PSI is the reading NEA's health advisory is written against, and it is the number used for broader planning: whether to schedule tomorrow's outdoor session, whether a school should stay open, whether a training block should go ahead.

The downside of averaging is lag. If the air turned bad two hours ago, the 24-hour PSI may still be carrying the cleaner air from earlier in the day. It can under-report what you can smell and see outside right now. The reverse is also true. On a day when the air is clearing, the 24-hour PSI may still look worse than actual conditions.

2. The 1-hour PM2.5 reading

NEA also publishes the 1-hour PM2.5 concentration, measured in micrograms per cubic metre (µg/m³) and updated every hour. It is a snapshot of fine particle levels right now, focused on a single pollutant.

This is usually the first number to move when haze rolls in, and it is the better guide for immediate decisions such as "Should I head out to swim in the next hour?"

A simple rule of thumb I share with parents and adult students:

• Use the 24-hour PSI to plan your day or your week.

• Use the 1-hour PM2.5 to decide what to do in the next hour.

Checking both takes under a minute, and together they give you a much more complete picture than either one alone.

Reading the PSI Bands

The 24-hour PSI is grouped into five bands. Here is what each one means and how NEA's advice generally reads for healthy people and for more sensitive groups.

24-hour PSI	Band	General advice
0–50	Good	No health concern. Normal activities.
51–100	Moderate	Normal activities for most people. Sensitive individuals should watch for symptoms.
101–200	Unhealthy	Healthy people should reduce prolonged or strenuous outdoor exertion. Children, the elderly, pregnant women and people with heart or lung disease should minimise it.
201–300	Very Unhealthy	Everyone should avoid prolonged or strenuous outdoor activity. Sensitive groups should minimise outdoor activity and stay indoors as much as possible.
Above 300	Hazardous	Minimise outdoor activity.

Let's translate this into pool-going decisions.

PSI 0–100: Good to Moderate

This is the green light range. For healthy swimmers and for vulnerable groups, NEA's advice is normal activities. Go and swim, at an outdoor or indoor pool.

One caveat: "Moderate" is not a reason to panic, but if you or your child has asthma or another respiratory condition, it is still worth keeping an eye on how you feel. If someone starts coughing, feels chest tightness, or has irritated eyes or throat, stop and get out of the air.

PSI 101–200: Unhealthy

This is where judgement begins. The advice shifts from "carry on" to "reduce prolonged or strenuous outdoor exertion."

For swimmers, the practical reading is:

• Outdoor pools: A hard training session, a long lesson, or a full swim set is exactly the kind of prolonged, strenuous activity the advice is about. Consider shortening the session, lowering the intensity, or moving it indoors or to another day.

• Indoor pools: A lighter session in an enclosed facility is generally a more reasonable option, but keep the travel and waiting time outdoors to a minimum.

• Children, older adults, pregnant swimmers, and people with chronic lung or heart conditions: The advice for these groups is stricter. Minimise prolonged or strenuous outdoor exertion. For a young child in a group lesson, that usually means postponing an outdoor session.

PSI 201–300: Very Unhealthy

At this level, the answer for outdoor swimming is straightforward: don't. Everyone is advised to avoid prolonged or strenuous outdoor activity, and sensitive groups should stay indoors as much as possible. For most of us, that means cancelling or rescheduling outdoor sessions. If you have a well-ventilated indoor facility that remains open, talk to your coach or the operator, and think hard about whether the trip is necessary.

PSI Above 300: Hazardous

This is rare, and everyone should minimise outdoor activity. Swimming should not be a priority on a day like this.

Reading the PM2.5 Bands

The 1-hour PM2.5 reading is grouped into four bands.

1-hour PM2.5 (µg/m³)	Band	General advice
55 and below	Normal	Normal activities.
56–150	Elevated	Healthy people should reduce strenuous outdoor activity for the next hour. Sensitive groups should avoid it.
151–250	High	Avoid strenuous outdoor activity. Sensitive groups should minimise outdoor activity.
251 and above	Very High	Everyone should minimise or avoid outdoor activity.

The "for the next hour" wording is important. This reading is designed to guide short-term decisions. If PM2.5 is Elevated at 3:00 pm and your session starts at 4:00 pm, check again before you leave. Conditions can improve or worsen quickly.

Here is how I think about it as a coach:

• Normal band: Proceed as planned.

• Elevated band: Ask how strenuous and how outdoors the session is. A relaxed skills lesson in an indoor pool is a very different thing from a 90-minute outdoor endurance set. Consider trimming the intensity.

• High and Very High bands: Postpone outdoor sessions. Make indoor decisions carefully and prioritise vulnerable swimmers' safety.

When the Two Readings Disagree

This is where people get confused, and it is worth working through an example.

Imagine you check at 2:00 pm and see:

• 24-hour PSI: 85 (Moderate)

• 1-hour PM2.5: 120 µg/m³ (Elevated)

Which one do you trust?

Both are right, and they are telling you different things. The 24-hour PSI is still averaging in cleaner hours from earlier in the day, so it looks reassuring. The 1-hour PM2.5 is telling you that the air is worse right now. Haze often arrives this way: PM2.5 climbs first and PSI follows later.

In this case, I would treat the situation as "haze is building." A short, light indoor session is likely fine. A long outdoor session is probably not the best plan. And I would check again in an hour rather than assuming the situation is stable.

Now reverse it. It's 8:00 am after a bad night:

• 24-hour PSI: 130 (Unhealthy)

• 1-hour PM2.5: 40 µg/m³ (Normal)

Here the PSI is still carrying last night's bad air, while the current hour looks clear. The right move is to look at the trend over the last few hours. If PM2.5 has been steadily falling, conditions are improving, and a moderate session may be reasonable, especially for healthy adults. If the hourly readings are bouncing around, be cautious, particularly with children and sensitive swimmers.

The point is not to memorise a formula. It is to understand what each number is designed to tell you, and to look at the direction things are heading.

A Note on Third-Party Apps and AQI

Many phone weather apps and global air quality sites show an AQI (Air Quality Index) instead of, or alongside, NEA's PSI. These are different scales with different calculations and different bands. An AQI of 120 on an international app is not the same thing as a PSI of 120 from NEA.

If you are using an app, check whether it is displaying NEA's official PSI or a converted figure. For decisions based on NEA's advisory, go to the source. NEA's haze website (haze.gov.sg) and the myENV app show the official readings, regional breakdowns, and health advisories.

Speaking of regions: Singapore's readings are reported for North, South, East, West, and Central. If you swim on the west side, as many of our SG Sink or Swim students do, check the West reading rather than relying only on the national headline figure. Conditions can differ slightly across the island.

A Practical Pre-Pool Checklist

Here is the routine I recommend to swimmers and parents. It takes about two minutes.

The night before, or the morning of:

1. Open haze.gov.sg or the myENV app.

2. Check the 24-hour PSI for your region and the NEA health advisory that goes with it.

3. Note whether it is Good, Moderate, Unhealthy, or worse, and whether the forecast suggests improvement or deterioration.

About one to two hours before your session:

4. Check the 1-hour PM2.5 reading.

5. Compare it to the 24-hour PSI. Are they telling the same story?

6. Ask yourself three questions:

   • Is the session indoors or outdoors?

   • How strenuous and how long is it?

   • Who is swimming? Are there children, older adults, pregnant swimmers, or anyone with a heart or lung condition?

Right before you leave:

7. Take one last look at the PM2.5 reading. It updates hourly, and a lot can change.

8. Pack what you need, including any reliever medication for swimmers with asthma or other conditions. Do not leave it at home because the sky looked clear that morning.

At the pool:

9. Keep outdoor waiting time short. Use indoor waiting areas where you can.

10. Watch for symptoms such as coughing, wheezing, chest tightness, unusual breathlessness, eye or throat irritation, or dizziness. If anyone experiences these, stop the session, move indoors, and seek medical help if symptoms persist or are severe.

Special Considerations for Different Swimmers

Young children

Children breathe faster relative to their body size, and they are often more active than adults during play. That is why NEA's guidance puts children in a more cautious category than healthy adults. For young children in Nursery to early SwimSafer levels, I lean toward postponing outdoor lessons once the PSI moves into the Unhealthy band, and I keep a closer eye on PM2.5. A missed lesson can always be made up. Lungs are not something to gamble with for the sake of a schedule.

Older adults

Many older swimmers take up aquatic exercise for its low-impact benefits, and that is a wonderful thing. But older adults are also a group NEA advises to be more cautious. If you are in this group and swim outdoors, be conservative once air quality worsens. An indoor, lighter session or a rest day is often the wiser choice.

Swimmers with asthma or other chronic conditions

For people with chronic lung or heart disease, the advice becomes more conservative at each step. Talk to your doctor about a personal plan for haze days, including when to skip a session, what medication to carry, and what symptoms should prompt you to stop. Nothing in a general article like this can replace that conversation. Also, if you are considering wearing a mask, speak to your doctor first, especially if you have a serious condition or are pregnant.

Competitive and training swimmers

Fit adults may feel invincible, but hard training means deep, rapid breathing, which is exactly when exposure adds up. On days with elevated PM2.5, consider dialling down intensity, shortening sets, or shifting to indoor facilities. One skipped hard session will not derail your season, but a chest infection might.

Pregnant swimmers

Pregnant women are included in NEA's more cautious group. If you swim while pregnant, follow the more conservative guidance and check with your doctor about what is right for you.

Do Masks Belong at the Pool?

You cannot swim in a mask, so this question is really about the trip to and from the pool. Health authorities in Singapore have generally said that N95 masks are not needed for short trips in ordinary haze conditions, and not needed indoors. They have suggested that healthy people who must be outdoors for several hours when the air is at hazardous levels may consider a well-fitted N95. People who are elderly, pregnant, or who have severe lung or heart problems should ask a doctor before relying on one.

For a short walk from the MRT station to an indoor pool, the more useful strategies are usually simple ones: minimise time outdoors, arrive close to your session start, and avoid lingering in open corridors.

Haze Is Not the Only Thing to Check

Air quality is one item on a longer list of pool-day checks. In Singapore, thunderstorms are just as much a part of the picture. Pools typically suspend swimming when lightning is detected nearby, and that rule is much more black and white than haze. Lightning is a clear stop signal, while haze calls for judgement. Follow your facility's instructions on lightning, and treat the haze advisory as one more input into your decision.

Also remember that pool operators and organisations sometimes issue their own notices, especially for outdoor facilities and for programmes involving children. If your pool, school, or coach announces a cancellation or change, follow it.

Making the Decision: A Simple Framework

If you want a compact way to think through the decision, use this four-step approach:

1. Where am I swimming? Indoor and outdoor pools carry different exposure levels.

2. What are the numbers? Check the 24-hour PSI for the day's picture and the 1-hour PM2.5 for the next hour.

3. Who is swimming? Match the advice to the most vulnerable person in the group.

4. What is the plan? A short, light session is not the same as a long, hard one. If in doubt, shorten it, soften it, or reschedule it.

When the answer is unclear, the safe choice is nearly always the right choice. Swimming is a lifelong skill and a lifelong habit, and one rescheduled session costs very little.

Common Questions

"The PSI is 95. Is that safe?"

That is within the Moderate band, where NEA's advice is normal activities for healthy people and for sensitive groups. But check the 1-hour PM2.5 as well. If it is climbing into the Elevated band, the situation may be changing, and sensitive swimmers should be more careful.

"The PSI is 105. Do we cancel?"

Not automatically. At 101–200 the advice is to reduce prolonged or strenuous outdoor exertion, not to shut everything down. An indoor or shorter, gentler session may be fine for healthy swimmers. Outdoor sessions for children and sensitive groups are where I would be most cautious.

"Which number does NEA use for its advisory?"

The health advisory is written against the 24-hour PSI, and the 1-hour PM2.5 is used to guide immediate, short-term decisions.

"My app shows a different number from NEA. Which one is right?"

Use NEA's figures for NEA's advice. Many apps use a different index that is not directly comparable.

"Can I swim if I feel fine?"

Feeling fine is a good sign but not a guarantee, and some people notice symptoms only later. Follow the advisory for the conditions, not just how you feel at the start.

Final Thoughts

You do not need to become an air quality expert to make good pool-day decisions. You need to know two numbers, what they measure, and what the bands mean:

• The 24-hour PSI tells you how the day has been and is what NEA's health advisory is written against.

• The 1-hour PM2.5 tells you what the air is like right now and is the better guide for the next hour.

• The advice changes with who is swimming and how hard, so match your decision to the most vulnerable swimmer in your group.

Haze seasons come and go, and most days in Singapore the air is perfectly fine for swimming. The goal is not to fear the pool, but to build a two-minute habit: check the air, check the numbers, make a sensible call, and get on with the joy of being in the water.

At SG Sink or Swim, our priority is that every swimmer, from a nervous first-timer to a SwimSafer Stage 6 candidate, has a safe and enjoyable experience. Part of that is knowing when to swim, and knowing when the smartest thing you can do is wait for clearer skies.

Stay safe, stay informed, and see you at the pool.

Coach Tan Meng Chew (MC)

SG Sink or Swim