Can You Swim When the PSI Is High? A Simple Guide for Singapore Swimmers
By Coach Tan Meng Chew (MC), SG Sink or Swim
Every time the haze rolls in, my phone fills with the same message from parents and adult students: "PSI is high today. Can we still swim?"
It's a fair question, and the honest answer is: it depends. It depends on how high the number is, which number you're looking at, whether the pool is indoors or outdoors, how hard you plan to swim, and who is getting in the water. That answer is not very satisfying, so this guide breaks it down into something simple you can use.
I'm Coach MC of SG Sink or Swim, a swimming school and coaching brand serving Singapore's west side. We teach everyone from Nursery-level toddlers to adults working through the SwimSafer stages, so I've had to think carefully about haze decisions for swimmers of every age and ability. Here is how I approach it.
The Short Answer
If you want the quick version, here it is:
PSI 0–100 (Good to Moderate): Go ahead and swim as normal.
PSI 101–200 (Unhealthy): Think twice about outdoor swimming, especially long or hard sessions. Indoor, lighter sessions are more reasonable for healthy swimmers. Children, older adults, pregnant swimmers, and anyone with heart or lung conditions should be more cautious still.
PSI 201–300 (Very Unhealthy): Skip outdoor swimming. Think carefully about whether any trip to the pool is necessary.
PSI above 300 (Hazardous): Stay in. Swimming is not the priority today.
That is the headline. But the details matter, and so does the second number most people forget to check. Let's go through it.
First, Which "PSI" Are You Looking At?
When people say "the PSI is high," they usually mean the number on the NEA haze page or on a weather app. But Singapore's National Environment Agency (NEA) publishes two different readings, and they answer two different questions.
The 24-hour PSI: "How has the day been?"
The Pollutant Standards Index (PSI) is Singapore's official air quality index. The headline figure is a 24-hour average, calculated from several pollutants. During haze, the pollutant that usually drives the number is PM2.5, the very fine particles that can travel deep into the lungs.
The 24-hour PSI is the figure NEA's health advisory is written against. It's the number for planning ahead, such as tomorrow's lessons or this week's training.
Because it's an average, it lags. If the air turned bad an hour ago, the 24-hour PSI may not have caught up yet.
The 1-hour PM2.5: "What is the air like right now?"
NEA also publishes the 1-hour PM2.5 reading, in micrograms per cubic metre (µg/m³), updated hourly. It's a snapshot of fine particle levels at this moment, and it's usually the first number to move when haze arrives.
For the question "Should I swim in the next hour?", this is the more useful number.
My rule of thumb: use the 24-hour PSI to plan, and use the 1-hour PM2.5 to decide. Look at both before you leave home.
The Bands, in Plain English
24-hour PSI bands
PSI | Band | What it means for outdoor activity |
0–50 | Good | Normal activities |
51–100 | Moderate | Normal activities for most; sensitive people should watch how they feel |
101–200 | Unhealthy | Reduce prolonged or strenuous outdoor exertion; sensitive groups minimise it |
201–300 | Very Unhealthy | Avoid prolonged or strenuous outdoor activity; sensitive groups stay indoors |
300+ | Hazardous | Minimise outdoor activity |
1-hour PM2.5 bands
PM2.5 (µg/m³) | Band | What it means for the next hour |
55 and below | Normal | Normal activities |
56–150 | Elevated | Healthy people reduce strenuous outdoor activity; sensitive groups avoid it |
151–250 | High | Avoid strenuous outdoor activity |
251 and above | Very High | Everyone should minimise or avoid outdoor activity |
Notice how much the advice depends on the words "strenuous" and "prolonged." That's where swimmers need to think a little differently from someone who is simply walking to the MRT.
Why Swimming Deserves Its Own Thinking
Swimming feels gentle, and for your joints it is. But for your lungs, it's real exercise.
You breathe hard. Swimming a length of freestyle means a steady rhythm of quick, deep breaths, often through the mouth. That is more air, drawn in faster, than you'd take in strolling around a park.
Sessions can be long. A coached lesson might last 45 minutes to an hour. A training session can be longer. The advisory language about "prolonged" activity applies directly.
The pool deck counts too. Even if you spend most of your time in the water, you're also standing on the pool deck, walking between changing rooms, and waiting for your turn. At an outdoor pool, all of that is outdoor exposure.
The journey counts. Indoor pool or not, you still travel there. A short, direct trip is very different from a long walk in poor air.
None of this means you should avoid swimming. It means that on a bad air day, swimming should be thought about like any other strenuous outdoor exercise.
Indoor vs Outdoor Pools: Does It Matter?
Yes, it matters a lot, but not in a simple "indoor is safe, outdoor is not" way.
Outdoor pools
At an outdoor pool, you're breathing outdoor air the whole time, and you're doing it with elevated breathing during your swim. This is the situation the health advisory is most concerned with. Once the PSI reaches the Unhealthy band, or the 1-hour PM2.5 moves into Elevated or above, outdoor sessions deserve a second look. In the Very Unhealthy band or above, I'd generally postpone.
Indoor pools
An enclosed indoor pool reduces your exposure compared with an open-air one. In many cases, a lighter session indoors during an Unhealthy day is a reasonable option for healthy swimmers.
But "indoor" isn't a magic shield. Ask yourself:
How long is the trip to the pool, and how much of it is outdoors?
Is the facility properly enclosed, or are there large open sections?
Is everyone in your group healthy, or are there sensitive swimmers?
Do you need to wait outside before or after the session?
If you're unsure about a facility, ask the operator. They can tell you how their pool is set up and whether they have any haze-specific policies.
A Scenario Guide
Let's put the numbers into real situations.
Scenario 1: PSI 60, PM2.5 30
Everything is in the green. Swim as planned, indoors or outdoors.
Scenario 2: PSI 90, PM2.5 110
This one is tricky. The 24-hour PSI looks fine, but the hourly PM2.5 is Elevated, which suggests the air is getting worse right now. Haze often arrives like this: PM2.5 climbs first, and PSI catches up later.
What I'd do: for an indoor lesson, proceed but keep outdoor time short. For a hard outdoor session, consider shortening or softening it. For young children or sensitive swimmers, be more cautious. Recheck the reading before you leave.
Scenario 3: PSI 130, PM2.5 140
Both readings say the same thing: the air is poor. The advice is to reduce prolonged or strenuous outdoor exertion.
What I'd do: postpone outdoor sessions, especially for children and vulnerable swimmers. Healthy adults might consider a shorter, easier indoor swim. Anyone with asthma or other conditions should follow their doctor's guidance.
Scenario 4: PSI 130, PM2.5 35
The 24-hour PSI is still carrying the bad air from earlier, but the current hour looks clear. This can happen when haze is clearing.
What I'd do: look at the trend. If the hourly readings have been falling steadily, conditions are improving and a moderate session may be reasonable for healthy swimmers. If the readings are jumping around, stay cautious. Give extra weight to the needs of children and sensitive swimmers.
Scenario 5: PSI 220, PM2.5 200
This is Very Unhealthy on the PSI and High on the PM2.5. Skip outdoor swimming. Think carefully about whether a trip to an indoor pool is necessary either. It's a day to reschedule.
Who Is Swimming? Match the Advice to the Most Vulnerable Person
NEA's advice differs for different groups, and I always tell parents and adult students to plan around the most sensitive person in the group.
Young children. Children breathe faster relative to their size and tend to be very active. In lessons, they're often moving constantly. I'm more conservative with young learners. Once the PSI moves into the Unhealthy band, I'd generally lean toward postponing outdoor lessons. A missed lesson can be made up, and small lungs deserve the extra caution.
Older adults. Aquatic exercise is a wonderful, gentle activity for older adults. But this group is also in the more sensitive category. On poor air days, an indoor, lighter session or a rest day is usually the wiser choice.
Pregnant swimmers. Pregnant women are also in the more cautious group. Follow the conservative advice, and talk to your doctor about what is right for you.
Swimmers with asthma or chronic lung or heart conditions. For these swimmers, the guidance is stricter at every step. Please speak with your doctor about a personal plan for haze days: when to skip, what medication to bring, and what symptoms mean you should stop. General articles like this can't replace that advice.
Healthy adults and competitive swimmers. Fit adults tend to feel they can push through. But hard training means deep, rapid breathing, which is exactly when exposure adds up. On days with elevated readings, dial the intensity down, shorten the sets, or move indoors. One easy session costs you almost nothing. A chest infection can cost you weeks.
Your Two-Minute Pre-Swim Routine
Here is the routine I recommend. It takes about two minutes.
Step 1: Check the 24-hour PSI. Open haze.gov.sg or the myENV app, and look at the reading for your region. On Singapore's west side, check the West reading rather than only the national headline.
Step 2: Check the 1-hour PM2.5. What is the air like right now, and is it rising or falling?
Step 3: Compare the two. Are they telling the same story? If PM2.5 is climbing while PSI still looks fine, haze may be building.
Step 4: Ask your three questions.
Is the session indoors or outdoors?
How long and how hard will it be?
Who is swimming?
Step 5: Decide, then recheck. The PM2.5 reading updates hourly, so take one last look before you leave.
Step 6: Pack smart. If anyone uses a reliever inhaler or other regular medication, bring it. Don't leave it at home because the morning looked clear.
What to Watch for at the Pool
Even when the numbers say it's okay to swim, stay alert. Stop the session, get indoors, and seek medical help if symptoms don't settle if you notice:
Coughing or wheezing
Chest tightness or unusual breathlessness
Eye, nose, or throat irritation
Dizziness or unusual fatigue
A child who suddenly seems unwell or wants to stop
Some people are more sensitive than others, and reactions can be different from day to day. Listening to the body is part of good swimming, on hazy days and clear ones.
Coaches and parents, please don't push a swimmer to "just finish the set" if they are showing symptoms. There is always another session.
Common Myths About Haze and Swimming
Myth: "Water protects you from the haze."Being in the water doesn't filter the air you breathe. Your lungs take in the same air as they would anywhere else, and swimming makes you breathe harder.
Myth: "If it's an indoor pool, it's automatically fine."Indoor pools reduce exposure but don't eliminate it, especially if the travel, waiting areas, or facility layout involve outdoor time.
Myth: "If I feel fine, the air must be fine."Feeling fine at the start is good, but it isn't a guarantee. Some people notice symptoms later. Follow the advisory for the conditions, not only how you feel.
Myth: "A mask solves everything."You can't swim in a mask, so masks only matter for the trip. Health authorities in Singapore have generally said N95 masks aren't needed for short trips or indoors. Healthy people who must be outdoors for several hours in hazardous conditions may consider a well-fitted N95, and those who are elderly, pregnant, or have serious lung or heart problems should ask a doctor first.
Myth: "The app number and the NEA number are the same thing."Not always. Many apps use a different air quality index, AQI, which is calculated differently and has different bands. For NEA's advice, check NEA's own figures.
Myth: "One reading tells the whole story."The 24-hour PSI and 1-hour PM2.5 are designed to complement each other. Use both.
Frequently Asked Questions
Can I swim if the PSI is 100?
That is the top of the Moderate band, where NEA's advice is normal activities. It's still worth checking the 1-hour PM2.5 and watching for symptoms, particularly for sensitive swimmers.
Can I swim at an indoor pool if the PSI is in the Unhealthy range?
For healthy swimmers, a lighter indoor session can be a reasonable choice, but keep the outdoor time on the way there to a minimum. For children, older adults, pregnant swimmers, and people with lung or heart conditions, the advice is more conservative. If in doubt, postpone.
Should we cancel children's swim lessons when the PSI is high?
For outdoor lessons, once the PSI reaches the Unhealthy band, I'd lean strongly toward postponing. For indoor lessons, it's a judgement call that depends on the reading, the facility, and each child's health. Always follow your operator's policy as well.
How do I know when it's safe again?
Watch the trend in the 1-hour PM2.5, and check that the 24-hour PSI is coming down into a lower band. When both are in the Normal or Good range, you're back to normal activities.
What about thunderstorms?
That's a separate check. Pools typically suspend swimming when lightning is detected nearby, and that rule is far more black and white than haze. Follow your facility's instructions.
Where do I find the official readings?
NEA's haze website (haze.gov.sg) and the myENV app show the official PSI and PM2.5 readings, regional breakdowns, and health advisories.
My Simple Rule as a Coach
If I could give every swimmer just one piece of advice, it would be this:
Match the swim to the sky. Clear air, swim as planned. Hazy air, swim shorter, softer, and indoors. Very bad air, wait.
Haze days are the exception in Singapore, not the rule. Most days, the air is fine and the pool is waiting. But having a simple plan means that when the haze does arrive, you don't have to guess or worry. You check two numbers, think about who's swimming and where, and make a calm decision.
At SG Sink or Swim, we believe swimming is a lifelong skill, and that a lifelong swimmer is one who knows when to get in the water and when to wait. Whether your child is learning to float, your parent is starting aquatic exercise, or you're working towards a SwimSafer stage, a rescheduled session is a small price for healthy lungs.
Stay informed, stay safe, and see you at the pool.
Coach Tan Meng Chew (MC)
SG Sink or Swim
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