Is it OK to exercise outdoors on high PM2.5 days?

Whether you can exercise outdoors depends on the Thai AQI colour that day. Thailand's Pollution Control Department (PCD) advises the general public to carry on with outdoor activities as normal on blue and green days, cut down on strenuous outdoor exercise on yellow days, limit it and wear a PM2.5 mask on orange days, and stop outdoor activities on red days. On high-dust days, a lighter, shorter workout, a better time of day or a session indoors are the main options.

Can I exercise outdoors when PM2.5 is high?

What PCD advises for each colour

PCD's advice under the 2023 Thai Air Quality Index separates the general public from people in at-risk groups. The table shows what it says about outdoor activity and exercise.

In short: a normal workout on blue and green days, less time on hard outdoor exercise on yellow days, and on orange days keep hard outdoor exercise brief and wear a mask (at-risk groups should avoid it). On red days there are no outdoor activities, so exercise indoors. The EPA's guide, which uses the US scale, suggests that sensitive groups consider rescheduling or moving activities indoors from its "Unhealthy" level, and everyone else from its "Very Unhealthy" level. For how the colours and numbers are calculated, see How do you read PM2.5 and AQI figures?

Why exercise matters more than walking to the shop

When you exercise you breathe faster and harder. The US Environmental Protection Agency (EPA) explains that higher breathing rates during exercise increase the dose of particles that are taken in and deposited in the airways. The EPA also says that the chances of being affected increase the longer a person is active outdoors and the more strenuous the activity. That is why the advice is about strenuous exercise, not about stepping outside.

Who needs to take more care

PCD's advice has a separate column for at-risk groups. The EPA's air quality guide for particle pollution lists groups it considers at greater risk, including:

• older adults
• children and teenagers, because they breathe more air for their body weight than adults
• pregnant people
• outdoor workers
• people under a doctor's care for an ongoing health problem

If you are in one of these groups, follow the at-risk column, and any advice your doctor has already given you.

How do I decide on the day?

Look at the hourly reading, not only the daily one

The big number in the Air4Thai app's face icon is a 24-hour average from the station nearest to you, updated every hour. PCD explains that tapping the bar graph shows the 1-hour average as well, and that the hourly figure lets people plan outdoor activities based on current air quality. So check the hourly figure before you head out as well. If the hourly figure is climbing, keep the session short or move it indoors.

Change the time, the intensity or the route

The EPA suggests four ways to keep exercising while breathing in less dust:

How to tell if a workout counts as hard

The NHS gives a simple test. During moderate activity, such as brisk walking or cycling, you can still talk, but not sing. During vigorous activity, such as running, fast cycling or football, you cannot say more than a few words without pausing for breath. On yellow and orange days, swapping a run for a brisk walk is one way to lower the intensity.

Is it better to exercise indoors?

Indoor air is not always clean

On red days, indoors is the place to exercise, but indoor air can also carry dust. A 2019 study by Kliengchuay and colleagues at Mahidol University measured PM2.5 inside and outside 17 homes in Bangkok and Chiang Mai:

In Chiang Mai during the haze, indoor levels were close to those outside. The authors linked this to the way houses are built and used, and advised closing doors and windows. The study covered only 17 homes, so other homes may differ. The EPA says the same in its guide: when particle levels are high outdoors, they can be high indoors too.

Keep cool as well

The EPA's guide also notes that staying inside with the windows closed can be dangerous in extremely hot weather if there is no air conditioning. In Thailand's hot season, if your room gets too hot, exercise somewhere with air conditioning, such as a gym or a community sports centre, or keep the session short and light.

Simple exercises for a small room

The NHS describes strength exercises that need only a sturdy chair or a wall. Use a solid chair without wheels that will not slip.

The NHS suggests doing these at least twice a week and gradually increasing the repetitions. Its weekly guideline for adults is at least 150 minutes of moderate activity or 75 minutes of vigorous activity, spread over 4 to 5 days or every day, plus strengthening activities on at least 2 days. Moving some sessions indoors during haze season keeps you within that weekly pattern. If you sit most of the day, see I sit at work all day and barely exercise. Where do I start?

What do people in Thailand do on high-dust days?

Staying in and limiting outdoor activity

A 2024 online survey by Luangwilai and colleagues at Navamindradhiraj University asked 921 people in urban Bangkok and Chiang Mai about PM2.5. Most agreed that they were susceptible to PM2.5 and that its effects were serious. The protective steps they reported were staying indoors, limiting outdoor activities, using air purifiers and wearing N95 masks. The survey covered city residents only and relied on what people reported themselves.

Reading the numbers is still hard

A survey of caregivers at child development centres in Chiang Mai by Phuanukoonnon and colleagues found that 97.0% were aware of the risks of PM2.5 for children, but only 9.7% could correctly interpret AQI levels, and 28.2% used air quality apps. The survey covered one province. Checking the colour and the hourly reading before you exercise is a habit worth building.

About masks

PCD advises wearing a PM2.5 mask every time you go outside from orange days (and from yellow days for at-risk groups). Which masks filter PM2.5 and how to fit them are covered in How do you read PM2.5 and AQI figures? If your eyes itch during haze season, see Itchy, red eyes in PM2.5 haze season: what can I do?

When should I stop exercising and see a doctor?

The EPA describes coughing or shortness of breath during activity as signs to take it easier. PCD lists unusual signs to watch for, especially in at-risk groups:

• frequent coughing
• difficulty breathing, rapid breathing or feeling unable to breathe
• wheezing
• chest tightness or chest pain
• palpitations
• nausea
• unusual tiredness
• dizziness

If you notice any of these, stop exercising and rest somewhere with cleaner air. PCD advises seeing a doctor promptly if anything unusual happens. In Thailand, you can see a doctor at a hospital or clinic. If you are already under a doctor's care, follow their advice about exercise during haze season.