PM2.5: The Particle That Bypasses Every Defence Your Body Has

Your body has spent millions of years building filters against what you inhale.

PM2.5 bypasses all of them.

How Small Is 2.5 Micrometres?

A human hair is ~70 µm wide. PM2.5 is 2.5 µm 28 times smaller than a human hair.

Your nose filters particles above 10 µm. Your throat catches some down to 5 µm. Below 2.5 µm — nothing stops it.

PM2.5 reaches your alveoli, the tiny sacs where oxygen enters your blood. Once there, it doesn't come back out easily.

What Happens Next

Your immune system detects a foreign particle and attacks it. This creates inflammation in your airways the same response that fights infection.

• For healthy adults: mild airway tightness hours after exposure, slightly suppressed SpO₂, slower recovery from exercise.

• For people with asthma: bronchospasm can begin within 2 hours. You feel fine during the run. The tightness hits later.

• Long-term (months, years): reduced lung function, increased cardiovascular risk, COPD in non-smokers. PM2.5 doesn't just damage lungs ultrafine particles can cross into the bloodstream.

The Numbers in India

WHO safe annual limit: 5 µg/m³

• Delhi: ~100 µg/m³ - 20x the limit

• Mumbai: ~50 µg/m³ - 10x the limit

• Bangalore: ~30 µg/m³ - 6x the limit

There is no major Indian city that meets WHO guidelines year-round. The question isn't whether you're exposed. It's how much, and when.

What You Can Do

You can't change the air. But you can change your timing. Running when PM2.5 is 40 µg/m³ instead of 80 µg/m³ is a real and meaningful difference — roughly the difference between 1 and 2 cigarettes of exposure.

Check before you go.