What Is PM2.5? The Complete Guide to Fine Particulate Matter

PM2.5 is the air pollutant that keeps scientists, doctors, and public health experts up at night — and for good reason. It's invisible, pervasive, and directly linked to millions of premature deaths worldwide every year.

🔬 What Exactly Is PM2.5?

PM2.5 stands for Particulate Matter with a diameter of 2.5 micrometres or less. To put that in perspective:

• A human hair is about 70 µm in diameter
• PM2.5 particles are 30 times thinner than that
• They're invisible to the naked eye — you can't see, smell, or taste them

These particles are a complex mixture of tiny solids and liquid droplets: soot, dust, metals, organic chemicals, acids, and soil particles. Their microscopic size is what makes them so dangerous — they're small enough to slip past every defence your body has.

💀 Why PM2.5 Is the Most Dangerous Pollutant

When you breathe in PM2.5, the particles travel deep into your respiratory system:

1. Past the nose and throat — unlike larger dust, PM2.5 isn't filtered by nasal hairs or mucous membranes 2. Into the bronchi and bronchioles — the branching airways of your lungs 3. Down to the alveoli — the tiny air sacs where oxygen enters your blood 4. Into the bloodstream — where they travel to every organ in your body

Health Effects of PM2.5 Exposure

| Duration | Health Impact | |----------|-------------| | Hours | Eye, nose, and throat irritation; aggravated asthma; reduced lung function | | Days–Weeks | Bronchitis; increased respiratory infections; cardiovascular stress | | Months–Years | Chronic lung disease (COPD); heart disease; stroke; lung cancer | | Lifetime | Reduced life expectancy (5+ years in heavily polluted Indian cities) |

The WHO estimates that ambient PM2.5 pollution causes 4.2 million premature deaths globally each year. In India alone, PM2.5 is the leading environmental cause of death.

📊 PM2.5 Levels: What the Numbers Mean

India's National Air Quality Index (NAQI) uses these breakpoints for PM2.5 (24-hour average):

| NAQI Category | PM2.5 (µg/m³) | Health Implication | |---------------|---------------|-------------------| | Good | 0–30 | Minimal risk | | Satisfactory | 31–60 | Minor discomfort for sensitive people | | Moderate | 61–90 | Breathing problems for asthma/lung patients | | Poor | 91–120 | Prolonged exposure causes discomfort | | Very Poor | 121–250 | Respiratory illness on extended exposure | | Severe | 250+ | Affects even healthy people; serious health impacts |

For comparison, the WHO guideline recommends PM2.5 should not exceed 15 µg/m³ as a 24-hour average — far stricter than India's "Good" category upper limit of 30 µg/m³.

🏭 Where Does PM2.5 Come From?

PM2.5 has both primary sources (emitted directly) and secondary sources (formed by chemical reactions in the atmosphere):

Primary Sources

• Vehicles: Diesel engines are the worst offenders — exhaust contains soot and ultrafine particles
• Industry: Power plants, brick kilns, steel mills, and refineries
• Construction: Demolition, concrete mixing, and unpaved road dust
• Biomass burning: Household cooking with wood/dung, crop stubble burning, waste incineration
• Natural: Wildfires, dust storms, volcanic activity

Secondary Formation

• Gaseous pollutants (SO₂, NOₓ, ammonia, VOCs) react in the atmosphere to form fine particles
• This is why PM2.5 can remain elevated even far from emission sources

India-Specific Concern: Stubble Burning

Every October–November, farmers in Punjab and Haryana burn millions of tonnes of crop residue. The smoke drifts southeast, blanketing Delhi-NCR and the entire Indo-Gangetic Plain, spiking PM2.5 levels to 10–20 times safe limits.

🔄 PM2.5 vs PM10: What's the Difference?

| Feature | PM2.5 | PM10 | |---------|-------|------| | Size | < 2.5 µm | < 10 µm | | Visibility | Invisible | Sometimes visible as haze | | Penetration | Reaches alveoli and bloodstream | Reaches upper airways and bronchi | | Main sources | Combustion, chemical reactions | Dust, construction, road abrasion | | Health risk | Higher — systemic effects (heart, brain, lungs) | Lower — primarily respiratory irritation | | WHO 24h guideline | 15 µg/m³ | 45 µg/m³ |

Key takeaway: PM2.5 is a *subset* of PM10 — all PM2.5 is also PM10, but not vice versa. When checking air quality, PM2.5 is the number that matters most for your health.

🛡️ How to Protect Yourself from PM2.5

Outdoors

• Check your city's AQI before going out — especially the PM2.5 reading
• Avoid outdoor exercise when PM2.5 exceeds 60 µg/m³ (Moderate and above)
• Wear an N95 mask during Very Poor/Severe days — surgical masks do NOT filter PM2.5
• Avoid high-traffic roads — pollution is highest within 150 metres of busy roads

Indoors

• Use an air purifier with a HEPA filter — this is the single most effective indoor intervention
• Keep windows closed during high-pollution hours (typically early morning and evening)
• Avoid indoor sources: incense, mosquito coils, smoking, and gas stoves without ventilation
• Indoor plants have minimal effect on PM2.5 despite popular claims — an air purifier is far more effective

📈 Track PM2.5 in Your City

On AQI Today, every city page shows live PM2.5 concentration alongside the composite AQI. You can also view the dedicated PM2.5 breakdown page for any city to see:

• Current PM2.5 level in µg/m³
• NAQI category and health advice
• How your city compares to WHO guidelines
• 7-day PM2.5 trend

Check your city's PM2.5 now → Search any city on AQI Today