On 5 November 2026, Lahore's air quality index (AQI) hit 355 on IQAir's live reading, the worst of any city in the world that morning. Healthy air is under 50. Most of us feel smog in our eyes and throat. The bigger question is what years of breathing it do to the heart and brain.
Quick answer: long-term exposure to smog is linked to more heart attacks and strokes. In the PURE study, which followed 157,436 adults in 21 countries, including Pakistan, for about nine years, every 10 μg/m³ rise in fine dust (PM2.5) was linked to 5% more major heart and stroke events. Stroke showed the strongest link: people in the most polluted communities had a 75% higher stroke risk than those in the cleanest. Lahore's 2025 average was 88.9 μg/m³, above the level of the study's most polluted group.
This breakdown explains what smog is, what the study did and found, where Lahore sits, what the study can't tell us, and what WHO advises on the worst days.
What is smog, and what is PM2.5?
Smog is a mix of smoke, dust and gases trapped close to the ground. The part that matters most for health is PM2.5: particles 2.5 micrometres across or smaller. They are small enough to travel deep into the lungs, which is why researchers measure them when they study air pollution and health.
The AQI you see on weather apps turns pollutant levels, usually led by PM2.5, into a single score. IQAir reports the US AQI scale:
| AQI | Category |
|---|---|
| 0 to 50 | Good |
| 51 to 100 | Moderate |
| 101 to 150 | Unhealthy for sensitive groups |
| 151 to 200 | Unhealthy |
| 201 to 300 | Very unhealthy |
| 301 and above | Hazardous. Lahore's 355 on 5 November 2026 sits here |
Source: AirNow, AQI basics
A single day's AQI is a snapshot. Health research like the PURE study looks at the long-term average, measured in micrograms of PM2.5 per cubic metre of air (μg/m³). WHO's guideline for the yearly average is 5 μg/m³.
The study: PURE, 157,436 adults in 21 countries
The Prospective Urban and Rural Epidemiology (PURE) study is one of the largest long-term health studies of its kind, and one of the few that includes low and middle income countries like Pakistan. This analysis, published by Hystad and colleagues in The Lancet Planetary Health in 2020, asked a simple question: do people who live with dirtier air for years have more heart attacks and strokes?
| Detail | What the study did |
|---|---|
| Who | 157,436 adults in 21 countries, including Pakistan |
| How long | Followed for a median of 9.3 years |
| Exposure | Long-term PM2.5 estimated for each person's community |
| Outcomes | Major cardiovascular events: fatal and non-fatal heart disease and stroke |
Source: Hystad et al., The Lancet Planetary Health, 2020
What the study found
| Finding | Result |
|---|---|
| Every 10 μg/m³ more long-term PM2.5 | 5% more major heart and stroke events (hazard ratio 1.05, 95% CI 1.03 to 1.07) |
| Stroke, most vs least polluted fifth of communities | 75% higher risk (hazard ratio 1.75, 95% CI 1.31 to 2.32) |
| Most polluted fifth | PM2.5 above 77.9 μg/m³ |
| Cleanest fifth | PM2.5 below 17.3 μg/m³ |
Source: Hystad et al., The Lancet Planetary Health, 2020
What a 5% rise means
Five percent sounds small. But it applies to every 10 μg/m³, and it applies to everyone who breathes that air, every day, for years. Small increases in risk across a whole city add up to many more heart attacks and strokes.
Where Lahore sits
| Air | Yearly PM2.5 |
|---|---|
| WHO guideline | 5 μg/m³ |
| Cleanest fifth in PURE | Below 17.3 μg/m³ |
| Most polluted fifth in PURE | Above 77.9 μg/m³ |
| Lahore, 2025 average (IQAir) | 88.9 μg/m³ |
Sources: WHO air quality guidelines, PURE study, IQAir
According to IQAir, Lahore's 2025 average was 17.8 times WHO's guideline, making it the 8th most polluted city in the world. That yearly average is above the level of the study's most polluted group. The worst days go far higher: on 2 November 2024, IQAir recorded an hourly AQI of 1,067 in Lahore.
So the real problem is not one bad day. It is years of exposure, and the link to heart attacks and strokes is clear.
What the study can't tell us
- It shows a link, not proof: the study can't say smog caused any single heart attack or stroke. People in polluted places can differ in other ways too.
- It isn't a Lahore risk score: the study covered 21 countries, so we can't turn these numbers into a risk figure for Lahore alone.
- Exposure was estimated: PM2.5 was estimated for where people lived, not measured on each person. IQAir's city average is measured differently, so the comparison above is a guide, not an exact match.
How to protect yourself on high-smog days
WHO's guidance for severe or hazardous pollution alerts focuses on cutting how much polluted air you breathe:
| Step | What WHO says |
|---|---|
| Stay indoors | Limit time outside on severe and hazardous days |
| Close windows | Keep windows and doors shut while the air is worst |
| Air purifier | Run one indoors if you have it. WHO stops short of recommending purifiers as a public measure |
| Exercise | Keep exercising, but move it indoors or to a time and place with cleaner air. Avoid heavy exertion outside |
| Masks | Scarves and dust masks don't protect you. A well-fitted N95 may help if you must be outside for long periods |
Sources: WHO air pollution alert fact sheet, WHO Q&A, October 2024
Do N95 masks work against smog?
WHO's 2024 guidance does not recommend routine mask use for the public, because real-world evidence is limited. Its alert advice is narrower: if you have to spend long periods outside on a hazardous day, a properly fitted N95 may help. Cloth, scarves and loose dust masks do not filter PM2.5.
When to get help
Signs of a heart attack or stroke need emergency care straight away. In Pakistan, call 1122:
- Chest pain, pressure or tightness, especially spreading to the arm, jaw or back
- Sudden shortness of breath, cold sweat or fainting
- Face drooping on one side, or a weak or numb arm or leg
- Slurred or confused speech
- Sudden severe headache, loss of vision or loss of balance
If you have heart disease, lung disease or diabetes, or you are pregnant, ask your doctor how to manage smog season. Coughing, wheezing or breathlessness that keeps coming back also needs a check.
Final word: smog is a long-term problem
One hazardous day hurts. Years of living in polluted air is what the PURE study links to more heart attacks and strokes, and Lahore's yearly average sits above the study's most polluted group. You can't fix the city's air alone, but on the worst days you can cut how much of it you breathe: stay in, shut the windows, filter the air indoors and move your workout.
FAQs
How does smog affect the heart?
In the PURE study, long-term exposure to fine dust (PM2.5) was linked to more heart attacks and strokes: 5% more major cardiovascular events for every 10 μg/m³ increase. The study shows a link rather than proof of cause.
Can smog cause a stroke?
Stroke had the strongest link in the PURE study. People in the most polluted fifth of communities had a 75% higher stroke risk than those in the cleanest fifth. The study can't prove smog caused individual strokes.
What is a safe AQI level?
On the US AQI scale used by IQAir, 0 to 50 is good. Above 150 is unhealthy for everyone, and above 300 is hazardous. For the long-term average, WHO's guideline is 5 μg/m³ of PM2.5.
How bad is the air in Lahore?
Lahore's 2025 average PM2.5 was 88.9 μg/m³, 17.8 times WHO's guideline, according to IQAir, making it the 8th most polluted city in the world. On 5 November 2026 its live AQI reached 355, in the hazardous range.
How can I protect myself from smog?
On severe or hazardous days, WHO advises staying indoors with windows closed, using an air purifier if you have one, and avoiding heavy exercise outside. Keep exercising, but indoors or at a cleaner time. Scarves and dust masks don't protect you; a fitted N95 may help if you must be outside for long.
Sources
- Hystad P, et al. Associations of outdoor fine particulate air pollution and cardiovascular disease in 157,436 individuals from 21 high-income, middle-income, and low-income countries (PURE). The Lancet Planetary Health, 2020. Read the paper
- IQAir. Lahore among the top 10 most polluted cities in the world, 2026. IQAir newsroom
- Daily Pakistan, citing IQAir live data, 5 November 2026. Read the report
- Dawn, 2 November 2024. Read the report
- WHO. WHO global air quality guidelines, 2021
- AirNow (US EPA). AQI basics
- WHO SEARO. What to do when there is an air pollution alert. Fact sheet (PDF)
- WHO. Air pollution: personal interventions and risk communication, October 2024. WHO Q&A
Disclaimer: Control Group is an initiative by Vitalis Living. This breakdown is for educational purposes only and does not constitute medical advice. If you have a heart, lung or other medical condition, speak to your doctor about managing smog exposure.