Air quality and the AQI
The Air Quality Index (AQI) is a number that tells us how clean or dirty the air is. A low number is good. It is worked out from the amount of harmful things in the air. The most important are PM2.5 (tiny dust and smoke particles, about 30 times thinner than a hair, which can go deep into the lungs and blood), PM10 (bigger dust), ozone and gases from vehicles and fires.
A usual six-band scale is: 0–50 good, 51–100 moderate, 101–150 unhealthy for sensitive people, 151–200 unhealthy, 201–300 very unhealthy, above 300 hazardous. Countries use slightly different scales and colours, so use the one on your local air-quality app or website.
Common sources: vehicle smoke, factories, burning waste or crop stubble, construction dust, cooking with wood, fireworks and wild fires.
Why exercise and dirty air are a double hit
At rest you breathe about 6 to 8 litres of air a minute. In hard running you breathe 40 litres or more, which is roughly 6 times as much. The nose filters some dust, but hard exercise makes us breathe through the mouth, which filters less.
A simple model: dust breathed in ≈ (AQI ÷ 50) × breathing multiplier. Take resting in clean air (AQI 50) as 1 unit. Resting at AQI 150 gives 3 units. Running at AQI 150 gives 3 × 6 = 18 units. This is a simple way to compare, not an exact measurement.
Who must be extra careful? Children (their lungs are still growing), older people, pregnant women and anyone with asthma, heart or lung problems.
Smart choices on bad-air days
- Check the AQI before you go out, ideally the hourly value.
- Good or moderate: exercise as usual. If you are sensitive, go a bit easier.
- Unhealthy (above 150): make it lighter: walk instead of run, or shorter time. Choose a park away from busy roads.
- Very unhealthy (above 200): exercise indoors: skipping, yoga, body-weight workouts. Keep windows closed on the worst hours.
- Masks: a well-fitted N95 (or equal) mask filters most tiny particles. Cloth and loose masks help much less. A mask makes breathing harder, so do not run hard with one.
- Stop and rest if you cough, wheeze, feel chest tightness or dizziness.
Cleaner places are usually away from traffic, and air is often cleaner in the afternoon than in the early morning in winter smog, when cold air traps the dust near the ground. Check the app for your city.
Exercise in the heat
When we exercise, muscles make heat. The body cools down by sweating: sweat on the skin takes heat away as it dries. In high heat, and especially in humid air, sweat does not dry well, so we overheat and lose water and salts.
- Water: drink before, during and after. A runner can lose around 0.5 to 1 litre of sweat in an hour.
- Shade and clothes: light, loose, light-coloured clothes and a cap.
- Early or late: exercise in the early morning or evening, not at noon.
- Build up slowly: the body adapts to heat in about 1 to 2 weeks of gradual practice.
Warning signs: dizziness, headache, nausea, cramps, very heavy sweating. Stop, move to shade, sip water. Emergency: hot, dry skin, confusion or fainting. Call for medical help at once and cool the person with water.
Cold, humid and high places
Cold: warm up for longer, wear layers you can remove, cover your nose and mouth with a scarf, and keep hands and ears warm. Cold air can make asthma worse. You still need water. Humid weather feels hotter because sweat cannot dry; slow down. High places have thinner air with less oxygen, so tire sooner; go up slowly and rest. In all weather, listen to your body and adapt the plan, not just push through.
Try it
Predict, then check in the 3D: set AQI 150 and Run. How many units of dust? (3 × 6 = 18.) Now tick Indoors. Is it less than the walk outside at the same AQI? Compare Walk (3 × 2.5 = 7.5) outdoors with Run indoors.
At home: for one week, look up the AQI and the temperature in your city before school sports. Write the best time of the day to exercise and one change you would make.
Key formulas and definitions
- Dust breathed in ≈ (AQI ÷ 50) × breathing multiplier
- Breathing multiplier: rest 1, walk about 2.5, run about 6 (simple model)
- AQI bands: 0–50 good; 51–100 moderate; 101–150 sensitive; 151–200 unhealthy; 201–300 very unhealthy; 300+ hazardous
- Heat safety: Water, Shade, Early
- Warm-up longer in the cold; slow down in humid heat
Worked examples
1. Using the simple model, find the dust units for walking (multiplier 2.5) when AQI is 100.
Units = (100 ÷ 50) × 2.5 = 2 × 2.5 = 5 units.
2. Rahul runs (multiplier 6) at AQI 150. His sister rests at the same place. How many times more dust does Rahul breathe in?
Rahul: (150 ÷ 50) × 6 = 18. Sister: (150 ÷ 50) × 1 = 3. 18 ÷ 3 = 6 times more.
3. Outdoors AQI is 200. The indoor air is about 15% as dirty. What is the indoor AQI, and what are the units for a run indoors?
Indoor AQI = 0.15 × 200 = 30. Units = (30 ÷ 50) × 6 = 0.6 × 6 = 3.6. Outdoors it would be (200 ÷ 50) × 6 = 24 units. So indoors is nearly 7 times less.
4. A player loses about 600 mL of sweat in an hour. How much does she lose in 90 minutes of practice, and how much water should she plan to drink?
90 minutes = 1.5 hours. 600 × 1.5 = 900 mL lost. She should drink roughly that much over and around the session, in small sips, and a little more when it is very hot.
5. The AQI app shows 85 at 6 a.m. and 210 at 6 p.m. In which bands are they, and when should the school run the 5 km race?
85 is moderate and 210 is very unhealthy. Run the race at 6 a.m. (or indoors if the evening is the only time). If it is also hot, the early time is better for heat too.
Common mistakes
- Thinking that exercise in the open is always healthier. On bad-air days, hard outdoor exercise can do more harm than good.
- Forgetting that running multiplies the dust we breathe in, not just the AQI.
- Wearing a thin cloth mask and thinking it filters tiny PM2.5.
- Exercising at noon in summer and not drinking water until you feel very thirsty.
- Ignoring dizziness, chest tightness or wheezing and pushing on.