AQI Numbers Explained: What 150, 300 and 500 Mean for Your Day
AQI (Air Quality Index) turns a messy pollution reading into one number from 0 to 500: under 50 is fine for everyone, above 150 means everyone should cut outdoor effort, and above 300 means stay in. Here is what each band means for your walk, the school run, open windows, children, older parents and pregnancy, plus why two apps rarely agree.
What does the AQI number actually measure?
Think of AQI as a thermometer for air. Behind it sit real measurements, mostly of PM2.5 (the fine smoke and dust that makes smog), plus PM10, ozone, nitrogen dioxide and a few other gases. Each one gets converted to the same 0 to 500 scale, and the app shows you whichever pollutant is worst right now. In a Punjab winter that is almost always PM2.5.
So an AQI of 300 doesn’t mean “300 micrograms of dust”. It means the air sits in the very unhealthy band. Some apps show the raw PM2.5 figure in µg/m³ as well; that is the ingredient, AQI is the grade.
Quick facts
- Scale: 0 to 500, higher is worse
- Main smog pollutant: PM2.5
- Worst hours: late evening, night and early morning
- Bands used here: the US-EPA style ranges most apps show
- Numbers guide activity: they don’t diagnose or replace a doctor
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AQI bands and what to do at each one
These are the US-EPA style bands that most air apps in Pakistan and India display. Some Indian government boards use a slightly different national scale, so check which one your app uses before comparing.
| AQI | Band | Healthy adults | Children, elderly, asthma, pregnancy |
|---|---|---|---|
| 0 to 50 | Good | Anything goes | Anything goes |
| 51 to 100 | Moderate | Normal day | Very sensitive people ease long hard exercise |
| 101 to 150 | Unhealthy for sensitive groups | Normal, shorter hard workouts | Shorten outdoor play and walks |
| 151 to 200 | Unhealthy | Cut long or hard outdoor effort | Keep outdoor time short, exercise indoors |
| 201 to 300 | Very unhealthy | Move exercise indoors | Stay in as much as possible |
| 301 and above | Hazardous | Only essential trips | Stay in, windows shut, mask if going out |
The 150 line
Below 150, healthy adults carry on. Above it, everyone’s lungs notice. A brisk morning walk at AQI 180 means hundreds of deep breaths of fine dust, which is why the walk moves indoors or shrinks to ten minutes.
The 300 line
Above 300, the air is hazardous for everybody. This is the zone where Lahore and Delhi schools have closed in recent winters. Errands only, a fitted N95 for the trip, and the house kept shut through the worst hours.
What about 500?
The US scale tops out at 500, and some apps print “500+” or even higher numbers. Past 300 the advice doesn’t change much: stay in, keep the air inside as clean as you can, and watch anyone vulnerable closely.
The number tells you how hard to breathe outside. Your body tells you whether it’s coping.
A day planner: walks, school run and windows
- Morning (6 to 9 am): check the reading before the walk. Winter mornings are often the worst of the day because cold air traps smoke near the ground. Above 150, walk later or indoors.
- School run: above 200, keep the car windows up with the fan on recirculate, and a child walking to school wears a child-size mask that seals. Take the quieter street, even if it’s longer.
- Midday (11 am to 3 pm): the sun warms the ground, air mixes upward, and readings often dip. This is the best slot to air the house for 10 to 15 minutes and for any outdoor exercise.
- Evening (6 pm onward): readings climb again as traffic, cooking fires and cooling air meet. Shut windows before sunset on bad days.
- Night: keep bedroom windows shut when AQI is high, but never seal a room that has a gas heater running.
Why the app near you says something different
Two phones in the same room can show 180 and 260. Neither is necessarily lying.
- Different sensors. Government stations use large reference instruments. Many apps pull from cheap laser sensors on rooftops, which read high in fog and humidity because water droplets look like dust to them.
- Different places. A sensor beside a busy chowk reads worse than one in a park two kilometres away. Smog is patchy.
- Different averages. Some show the last hour, others a 24-hour average. The hourly figure jumps around; the daily one lags behind what you’re breathing now.
- Different scales. US-style AQI and some national scales give different numbers for the same dust.
How to read any app: pick one, find the nearest sensor to your home, and watch its pattern for a week. You’re looking for your worst and best hours, not a perfect number. The trend is more useful than any single reading.
PM2.5 versus AQI
If your app shows both, PM2.5 is the concentration in micrograms per cubic metre, and AQI is that figure turned into a grade. They rise and fall together, but not in a straight line. A PM2.5 of around 35 µg/m³ already sits in the 100 to 101 region on the US scale, and roughly 55 µg/m³ lands near 150. Lahore winters often go well past 150 µg/m³, deep in the very unhealthy band.
The word “smog” was coined in the early 1900s by joining smoke and fog. The Punjab winter version is exactly that: crop and traffic smoke caught under a lid of cold, still air, which is why a windy or rainy day can clear it in hours.
Children, elderly parents and pregnancy
Children breathe faster than adults and take in more air for their size, so the sensitive-group column applies to them from AQI 101. Older adults with heart or lung disease belong in the same column, and so do pregnant women. None of this means panic at 120. It means shorter outdoor play, exercise moved to the middle of the day or indoors, and a closer eye on coughs.
For keeping the house itself cleaner, the honest look at air-purifying plants in smog season saves you money, and the indoor workout options are in smog season indoor exercise. More seasonal guides live in Season Shift.
AQI guides what you do outside. It doesn’t decide treatment. Wheeze, chest tightness or a cough that keeps someone up at night at any AQI means following the asthma action plan the doctor wrote and calling that doctor. Get emergency care for blue or grey lips, very fast or laboured breathing, the chest sucking in under the ribs, or being unable to speak in full sentences. Chest pain on a smog day in someone with heart disease is an emergency too. The signs that separate smog cough from asthma or infection are covered in smog cough, asthma or chest infection.
AQI questions, answered
What AQI is good for health?
0 to 50 is good for everyone, and 51 to 100 is acceptable for nearly everybody. Above 100, people with asthma, children and older adults should start cutting long outdoor exercise.
Is AQI 150 dangerous?
AQI 150 sits at the top of the “unhealthy for sensitive groups” band. Healthy adults can carry on with shorter workouts, while children, older people and those with asthma should keep outdoor time short.
What does AQI 300 mean?
300 is the top of “very unhealthy” and the doorstep of “hazardous”. Move all exercise indoors, keep windows shut in the worst hours and limit trips out to what’s necessary.
Why is AQI highest in the morning and at night?
Cold air near the ground at night acts like a lid and traps smoke and dust. The midday sun warms the ground and mixes the air upward, so readings often fall in the early afternoon.
Why do AQI apps show different numbers?
They use different sensors, locations, time averages and sometimes different scales. Pick one app, use the nearest sensor, and follow its pattern rather than comparing apps.
How accurate is the AQI in Pakistan today on phone apps?
Good enough to show the pattern and the band, not precise to the unit. Low-cost sensors can read high in fog. Use the band and the trend to plan your day.
Should windows be opened when AQI is high?
Keep them shut in the evening, night and early morning. Air the house briefly around midday when readings dip, and always ventilate any room with a gas heater, whatever the AQI.
Can children play outside when AQI is above 150?
Keep outdoor play short and gentle, and move running games indoors. Above 200, children are best kept inside apart from the school run.
What AQI is safe during pregnancy?
Treat pregnancy as a sensitive group: from 101, shorten outdoor walks, and above 150 walk indoors. Ask your doctor or midwife about any breathlessness.
Is the AQI forecast reliable?
Forecasts a day or two ahead are rough but useful, especially for wind and rain that clear the smog. Plan the week with the forecast, and the day with the live reading.
Does rain lower the AQI?
Usually yes. Rain washes particles out and wind blows them away, so readings often drop sharply after a shower. They build again once the air goes still.