Swimming in Haze: What Happens to Your Lungs When You Breathe Hard Outdoors
By Coach Tan Meng Chew (MC), SG Sink or Swim
Every coach knows the moment. A swimmer finishes a hard set, hangs on to the wall and breathes in big, fast gulps. Their chest rises and falls, their shoulders lift, and for a few seconds all they can do is recover. In clean air, that is just effort. In hazy air, parents and students ask me a fair question: "What exactly is happening in my lungs right now, and is it a problem?"
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. I spend my days watching people breathe, because breathing is the engine of swimming. That gives me a coach's feel for the question, but it does not make me a lung specialist. I am a swim coach, not a doctor, so in this article I will lean on official health sources for the biology, keep my own observations clearly labelled, and tell you where the evidence runs out.
The goal is simple: to explain, in plain language, what happens when you breathe hard in hazy air, so that you can make sensible decisions about swimming. I do not want to frighten you, and I do not want to wave the concern away.
The short answer
When you exercise hard in hazy air, you breathe in much more air than at rest, a larger share of it comes through the mouth rather than the nose, and the tiniest particles in the haze can travel deep into the lungs, where they can cause irritation and inflammation. For most healthy people, the short-term effects are usually mild, such as irritated eyes, nose and throat, and tend to settle on their own. For children, people with asthma and others with chronic heart or lung conditions, the effects can be more serious.
Two honest caveats come with that:
I could not find research that measures what happens in swimmers' lungs in haze specifically. Most of what we know comes from studies of exercise and air pollution in general.
Official sources say there is little data on the long-term effects of short, episodic haze like Singapore's. I will not claim lasting harm, and I will not claim there is none.
Let me take you through the journey of a single breath.
The journey of a breath
What is in the haze?
The haze we get in Singapore is mostly smoke. In recent weeks, news reports citing the National Environment Agency (NEA) have described smoke plumes from fires in Sumatra and Kalimantan drifting towards Singapore. The smoke carries tiny solid and liquid particles, which is why air quality reports talk about PM2.5: particulate matter whose particles are 2.5 micrometres across or smaller, far thinner than a human hair.
Step 1: The nose, the body's built-in filter
When you breathe at rest through your nose, the air passes through nasal passages that warm it, moisten it and trap some larger particles before they travel further. It is not a perfect filter, but it does real work.
Step 2: The throat and the airways
The air then moves down the windpipe and into a branching network of airways, like an upside-down tree. The lining of these airways is coated with mucus and tiny hair-like structures that sweep trapped particles upward so they can be coughed out or swallowed. Larger particles tend to be caught in the upper airways. The United States Environmental Protection Agency (EPA) says that larger particles, generally those over about 5 micrometres, deposit mainly in the upper respiratory tract.
Step 3: The deep lung
Here is the part that matters for haze. The EPA says that smaller particles in the PM2.5 size range "can reach the lower respiratory tract including the alveolar region." The alveoli are the tiny air sacs at the ends of the airways, where oxygen passes into the blood and carbon dioxide comes out. They are delicate, they have a vast surface area, and they are a long way from the nose.
In short, PM2.5 is small enough to slip past many of the body's first lines of defence and reach the deep lung.
What changes when you exercise
Now put the swimmer in the picture. The EPA says where particles end up in the lungs depends on several things, including how much is in the air and for how long, and, importantly, physical activity and breathing pattern, specifically whether you breathe through the nose or through the nose and mouth together. That is the key to the exercise question.
You breathe a lot more air
A California Air Resources Board (CARB) page on air pollution and athletes says that breathing speeds up during exercise, and that exercisers can exchange "up to 10 or 20 times more air" than at rest. So the amount of pollutant inhaled goes up as well. A research study I read on exercise and air pollution, published in 2018 in the International Journal of Environmental Research and Public Health, modelled the same idea with numbers: in its participants, ventilation rose from around 8 litres a minute at rest to around 63 litres a minute at 65% of maximal effort, and the modelled inhaled doses of particulate matter were significantly higher during exercise than at rest.
Please keep the limits of that study in mind. Its participants were young, healthy men, the doses were calculated rather than measured, and its authors say the data should not be extrapolated to susceptible groups. It was about exercise in general, in cities with very different pollution levels. It was not about swimming, children or Singapore haze. But it illustrates a principle that official advice relies on: the harder you breathe, the more air, and whatever is in it, passes through your lungs.
More of the air comes through your mouth
When you exercise hard, you usually switch from nose breathing to breathing through your mouth, or your nose and mouth together. CARB notes that a larger share of the air enters through the mouth, bypassing the nasal filter. An Ohio State University health page says the same: breathing through the mouth means pollution bypasses the nose, which normally filters air before it reaches the lungs.
Particles are carried deeper
CARB also says that, with faster, deeper breathing, pollutants are carried more deeply into the lungs. Ohio State puts it plainly: during outdoor exercise, "we take more frequent and deeper breaths, inhaling even more" of the pollution in the air.
Putting it together: during hard exercise in poor air, you breathe in more air, more of it by mouth, and it travels further down. That is the basic physiology behind the official advice to reduce prolonged or strenuous exertion as haze worsens.
What the particles can do once they are in the lungs
The EPA describes three pathways by which inhaled PM2.5 can cause effects in the body:
Inflammation and oxidative stress. Particles in the airways and deep lung can trigger an inflammatory response, the body's reaction to an irritant. Inflamed airways can become swollen, produce more mucus and react more easily.
Disruption of the autonomic nervous system. The autonomic nervous system controls things like heart rate and many automatic body functions.
Movement of particles from the alveolar region into the blood, where they can affect other organs, such as the heart.
The EPA says the health effects of PM2.5 are well established by decades of research on everyday outdoor exposure, and cites respiratory and cardiovascular emergency department visits and hospital admissions as examples from population studies.
Please note that this is about particulate pollution in general, and much of that research looks at exposure over days, months or years. It does not tell us the size of the effect from one swim session in Singapore haze.
What people actually feel
On the ground, the effects people notice are usually these. Ohio State's page lists headaches; eye, nose and throat irritation; lung inflammation; chest tightness; and airway inflammation, with people with chronic lung disease particularly affected. HealthXchange, a Singapore health information site, describes how haze can irritate the nose, lungs and eyes of otherwise healthy children, causing itchy, watery or red eyes, a runny or blocked nose, sneezing, a sore or dry throat, a dry irritant cough, breathing difficulty and reduced exercise tolerance.
The Ministry of Health (MOH) says that for healthy people, short-term exposure over a few days mainly causes irritation of the eyes, nose and throat that usually resolves on its own. HealthHub says the same, and adds that health effects can lag exposure by one to three days.
That delay is worth underlining. A swimmer can feel fine in the water, and then cough or feel tight the next day. If you are only watching for problems during the lesson, you can miss the connection.
Why swimmers deserve a specific look
Everything above applies to any exercise in hazy air. Is swimming different? Here is my honest answer: I do not know, and I could not find a study that compares them. What I can offer is coaching observation and reasoning, clearly labelled.
Swimming is mouth-breathing exercise. In most strokes, a swimmer takes in air through the mouth during short, timed breaths, and breathes out underwater. That means the nasal filter is largely bypassed when it matters most. This is simply how swimming works.
Swimmers breathe hard and rhythmically for long periods. A trained swimmer can sustain high breathing effort through a long set, and a beginner may breathe fast and shallowly when nervous. Either way, the lungs are doing a lot of work.
The air above the water is the air they breathe. Swimming in water does not clean the air above it. In an open outdoor pool, a swimmer breathes outdoor air throughout the lesson. In an enclosed, air-conditioned pool hall, they may breathe air that has been through the building's ventilation, which could mean less exposure, though I cannot judge that for any particular venue. Sport Singapore's Haze Management Plan (April 2026) treats air-conditioned facilities more leniently than outdoor and non-air-conditioned ones, but, as I read it, it does not say how swimming pools are classified.
Swimming has its own considerations for sensitive airways. Some people find the humid, chemical-scented air around pools irritating, and a health information page I read notes that in elite swimmers, intense training and repeated exposure to chlorine may contribute to the development of asthma. That is separate from haze, but it is a reminder that a swimmer's airways are exposed to more than one thing.
On the other side of the ledger, swimming is a wonderful form of exercise, and regular activity is good for health. A page from Ohio State makes the same point about outdoor exercise generally: healthy people do not have to give up their routines on poor air days in most places it was written about, though people at risk of exercise-induced symptoms, such as those with asthma or COPD, should check air quality daily and try to stay indoors on poor air days. That advice was written with a different context in mind, not Singapore haze, so the better guide for us is the official Singapore activity guidance below. But it is a good reminder that the aim is to adjust, not to give up swimming.
What Singapore's official guidance says
The NEA's activity guidelines for the 24-hour PSI group people into healthy persons; elderly, pregnant women and children; and people with chronic lung or heart disease. As I read them (the text of the NEA file came out jumbled when extracted, so I mapped the table by order, and MOH's FAQ states the same bands 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 (Unhealthy) | Reduce prolonged or strenuous outdoor exertion | Minimise prolonged or strenuous outdoor exertion | Avoid prolonged or strenuous outdoor exertion |
201 to 300 (Very unhealthy) | Avoid prolonged or strenuous outdoor exertion | Minimise outdoor activity | Avoid outdoor activity |
Above 300 (Hazardous) | Minimise outdoor activity | Avoid outdoor activity | Avoid outdoor activity |
Notice the focus on prolonged or strenuous exertion. That is exactly the physiology we have just walked through: the more you breathe, and the longer you breathe it, the more you take in.
The 24-hour PSI is used for planning, while the 1-hour PM2.5 reading helps with immediate decisions, according to HealthHub and Sport Singapore. And Sport Singapore's plan says facilities adjust their operations by haze phase: outdoor and non-air-conditioned facilities operate with conditions at PSI 101 to 200, close at 201 to 300, and all facilities close above 300. ActiveSG has also recently announced a temporary closure of outdoor and non-air-conditioned facilities in its western Sport Centres because of elevated PM2.5 readings, so please check the current status with your venue.
What we know and what we do not
I think it is important to be straight about the limits.
What is well supported:
Fine particles can reach the deep lung, and exercise increases how much air, and how much of it by mouth, enters the lungs. (EPA, CARB, Ohio State)
Particulate pollution can cause inflammation and respiratory symptoms, and can worsen existing heart and lung conditions. (EPA, MOH)
Children, the elderly, and people with chronic lung or heart disease are more sensitive. (MOH, HealthXchange)
Effects can be delayed by one to three days. (MOH, HealthHub)
What I could not find:
Studies measuring lung effects in swimmers exposed to haze.
A rule for how much haze exposure from a single swim session is "too much."
What official sources say is unknown:
The MOH FAQ says there is little robust data on the longer-term effects of Singapore's episodic haze, and that there is no local or international data on the long-term health impact of short-term episodic haze exposure. It notes that overseas studies of continuous, multi-year exposure to fine particles have linked it to problems such as reduced lung development and childhood asthma. HealthXchange says long-term effects usually do not occur unless exposure is prolonged and intense over many months or years. (The MOH FAQ is dated 21 November 2016, so it is worth checking for an update.)
So my honest summary is that breathing hard in hazy air plausibly increases your exposure and can cause short-term irritation and, in sensitive people, more serious symptoms, while the lasting effects of occasional hazy swims are unknown.
Who is most affected
The people to be most careful with, according to the official sources, are:
Children. HealthXchange explains that their immune systems and lung defences are still developing.
People with asthma and allergic conditions. HealthXchange lists haze among asthma triggers and says it can cause attacks needing emergency care. The MOH FAQ cites a local study that found a 3.1% increase in asthma hospital admissions for each 10-unit rise in the 24-hour PSI. If your child has asthma, please also read my separate article on asthma and swimming lessons during haze, and speak to your child's doctor.
People with chronic heart or lung disease.
The elderly.
Pregnant women. MOH advises them to reduce haze exposure for the sake of the unborn baby.
Anyone already unwell, such as with a cold, cough or fever.
Warning signs: what to watch for in the water and afterwards
Stop the activity and seek help if you notice:
A cough that persists or worsens.
Wheezing, a whistling sound when breathing.
Difficulty speaking in full sentences because of breathlessness.
Chest tightness or chest pain.
Unusual paleness, bluish lips, dizziness or faintness.
A swimmer who looks distressed or cannot catch their breath.
MOH's advice is that anyone not feeling well should seek medical attention, and that people with chronic heart or lung conditions should keep their medications readily available. In an emergency, such as severe breathing difficulty, call 995.
Keep watching in the following days too. Because effects can be delayed, a cough or tightness that appears one to three days after a hazy swim may be connected. If it persists or worries you, see a doctor.
Common myths about haze and swimming
Myth: "The water filters the air, so swimming is safer than running." The water does not clean the air above it. A swimmer breathes the air at the pool, so the quality of that air is what matters. Whether the pool is open or enclosed is the more relevant question.
Myth: "Deep breathing helps clear the haze out." Breathing deeply in poor air does the opposite of protecting you, because it draws in more of it. That is why the guidance focuses on reducing prolonged or strenuous exertion.
Myth: "I am fit and healthy, so haze won't affect me." Healthy people can still get irritation of the eyes, nose and throat. The official guidance for healthy people is to reduce, and at higher levels avoid, prolonged or strenuous outdoor exertion.
Myth: "A mask will let me swim safely." Nobody can swim in a mask. 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 and that surgical masks do not filter fine particles well. The real decisions for swimmers are whether, where and how hard to swim.
Myth: "If I feel fine in the pool, there is no problem." Effects can lag by one to three days, so feeling fine in the moment does not tell the whole story.
Myth: "Haze means I must stop swimming for the whole season." Not necessarily. The aim is to adjust intensity and choices according to the readings, the venue and your health, and to return to regular swimming when the air clears.
Practical steps to lower your exposure
Based on the official advice and my coaching approach, here is what I would do.
Check the numbers. Look at the 24-hour PSI for the day's picture and the latest 1-hour PM2.5 reading for immediate decisions.
Follow the venue. If a facility closes or suspends activities, that settles it.
Prefer enclosed spaces on poor days, if the venue is open and your doctor has no objection, though "indoor" is not a guarantee.
Lower the intensity. Shorter, easier sessions mean less hard breathing. On borderline days, focus on technique, drills and relaxed breathing rather than hard sets.
Warm up gently and notice how your breathing feels before anything demanding.
Stay hydrated. Official sources suggest drinking plenty of water, which may help a dry or itchy throat.
Support the air at home. Official advice includes closing doors and windows when outdoor air worsens, keeping air-conditioner filters clean and avoiding smoking indoors.
Keep medication within reach if you have a chronic lung condition, and follow your doctor's plan.
Stop at the first warning sign and rest.
See a doctor for symptoms that persist or recur.
How SG Sink or Swim approaches 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. Because I watch breathing for a living, here is how I handle haze. These are principles, and details depend on the venue and the day.
I follow the official numbers and the venue
If a venue closes or suspends activities, our lessons there are off. I would always rather rely on the latest readings and the venue's notices than on how the sky looks.
I treat breathing as the thing to protect
On borderline days, I reduce intensity before I reduce anything else. Shorter sessions, more rest, and a focus on water confidence, floating, kicking and relaxed breathing keep a swimmer progressing 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.
I watch and I stop
I look at breathing rate, colour, energy and mood. Persistent coughing, wheezing, a tight chest or an unusually quiet child means out of the water, rest and follow your doctor's plan if there is one. I would rather stop unnecessarily than miss it.
I ask parents to tell me what I need to know
Please let me know about asthma, allergies or any breathing concern, and where prescribed medication is kept.
I do not give medical advice
I can follow the plan your doctor has given you, but I do not diagnose or decide on medication. That belongs to you and your doctor.
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 about adapting lessons for a swimmer with breathing concerns, you are welcome to get in touch through sgsinkorswim.com.
Frequently asked questions
Can haze damage my lungs permanently if I swim in it?
I cannot say, and neither can the sources I read. MOH says there is no local or international data on the long-term health impact of short-term, episodic haze exposure, while overseas studies link continuous, multi-year exposure to fine particles with problems such as reduced lung development. For short-term effects, the sources describe mostly irritation that usually resolves on its own, and more serious effects in sensitive groups.
Why does exercise make haze worse for my lungs?
Because you breathe much more air, more of it through your mouth, and it travels deeper. Sources describe exercisers exchanging up to 10 to 20 times more air than at rest, and the EPA says deposition in the lungs depends on physical activity and whether you breathe through your nose or mouth.
Is swimming worse than running in haze?
I do not know. I could not find research comparing them. Both involve hard breathing, and swimming is largely mouth breathing.
Is an indoor pool better for my lungs during haze?
It may offer less exposure than an open outdoor pool, but I cannot confirm how pools are classified in official plans, and indoor air depends on ventilation and filtration. Ask the venue.
What does PM2.5 mean?
It refers to particulate matter with particles 2.5 micrometres across or smaller, small enough to reach the deep lung, according to the EPA.
I feel fine after swimming. Does that mean I am safe?
Not necessarily. Effects can lag by one to three days.
Should I wear a mask to the pool?
N95 masks are not needed for brief trips between buildings or for indoor use, according to HealthHub. Ask your doctor about your own situation or your child's.
My child has asthma. What should we do?
Please speak to your child's doctor and follow their asthma action plan. I have written a separate article on this.
When should I see a doctor?
If you or your child develop breathing difficulties, or symptoms that persist or recur. People with chronic heart or lung conditions should seek medical attention promptly if they become unwell.
The bottom line
Haze is made of tiny particles, and PM2.5 is small enough to reach the deep lung.
Exercise changes the picture: you breathe much more air, more of it through your mouth, and it travels deeper. That is why official advice targets prolonged or strenuous exertion.
In the lungs, particles can cause irritation and inflammation, and in sensitive people, such as children and those with asthma or chronic conditions, the effects can be more serious. Effects can lag by one to three days.
I could not find research on swimmers in haze, and the long-term effects of short, episodic haze are unknown. I will not overstate either the risk or the safety.
Sensible steps are simple: check the readings, follow the venue, lower the intensity, watch for warning signs, tell your coach and see a doctor if symptoms persist.
Haze seasons pass, and a swimmer's journey is long. The people who do best are the ones who stay informed, listen to their bodies and get back into a regular rhythm as soon as the air clears.
At SG Sink or Swim, we teach from a first splash through to SwimSafer Stage 6, and keeping swimmers safe, and breathing well, 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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