PM2.5 vs AQI: What's the Difference? The Complete Guide
You open an air quality app and see two numbers: AQI 85 and PM2.5 22 μg/m³. What do they mean? Are they measuring the same thing? Why do different apps show different values for the same location? This guide explains everything.
What Is PM2.5?
PM2.5 stands for Particulate Matter 2.5 micrometers or smaller in diameter. To visualize the scale: a human hair is roughly 70 μm in diameter. A PM2.5 particle is about 30× smaller than that — invisible to the naked eye.
PM2.5 concentration is measured in micrograms per cubic meter (μg/m³) — the mass of particles in one cubic meter of air. A reading of 10 μg/m³ means there are 10 millionths of a gram of fine particles in each cubic meter of air you breathe.
Why PM2.5 specifically? Particles smaller than 2.5 μm are particularly dangerous because they can penetrate deep into the lungs (beyond the bronchial branches), reach the alveoli (air sacs where gas exchange occurs), and some ultrafine fraction can cross into the bloodstream. Larger particles (PM10, PM100) are filtered by the nose and upper airways.
PM2.5 sources
- • Vehicle exhaust (diesel > gasoline)
- • Coal and biomass combustion (power plants, household heating)
- • Wildfire smoke
- • Industrial emissions (steel, cement, petrochemicals)
- • Secondary formation: sulfate and nitrate particles formed from SO2 and NOx gases
- • Cooking, especially open fire
What Is AQI?
The Air Quality Index (AQI) is a standardized 0–500 scale designed to communicate air quality health risk to the general public in a simple, consistent format. The US EPA developed the AQI framework; China, India, and other countries have adapted it with different breakpoints.
The AQI is calculated from six pollutants:
- PM2.5 (fine particles)
- PM10 (coarse particles)
- Ozone (O3)
- Nitrogen dioxide (NO2)
- Sulfur dioxide (SO2)
- Carbon monoxide (CO)
The reported AQI is the maximum of the six individual pollutant sub-indices. This is important: if ozone is elevated (often a summer afternoon problem) but PM2.5 is low, the AQI is driven by ozone, not particles.
The Key Difference
PM2.5 (μg/m³)
- • Physical measurement of particle mass per volume
- • Universal unit — same meaning in any country
- • What air quality monitors actually measure
- • What epidemiological studies use for health research
- • Direct: 35 μg/m³ means a specific concentration
AQI (0–500 scale)
- • Standardized risk communication scale
- • Varies by country (US AQI ≠ India NAQI ≠ China AQI)
- • Calculated from PM2.5 and 5 other pollutants
- • Designed for public communication with color codes
- • Reports the worst pollutant, not just particles
Practical implication: PM2.5 = 35 μg/m³ is always the same physical reality. But AQI 100 means "Moderate" in the US system, while AQI 100 means "Good" in the Chinese system. When comparing cities across countries, always compare PM2.5 in μg/m³ — not AQI.
PM2.5 to AQI Conversion — US EPA Breakpoints
| PM2.5 (μg/m³) | AQI Range | Category |
|---|---|---|
| 0–12 | 0–50 | Good |
| 12.1–35.4 | 51–100 | Moderate |
| 35.5–55.4 | 101–150 | Unhealthy for Sensitive Groups |
| 55.5–150.4 | 151–200 | Unhealthy |
| 150.5–250.4 | 201–300 | Very Unhealthy |
| 250.5–350.4 | 301–400 | Hazardous |
| 350.5–500.4 | 401–500 | Hazardous (extreme) |
The interpolation formula
Within each range, AQI is interpolated linearly:
AQI = ((AQI_high - AQI_low) / (PM_high - PM_low)) × (PM_actual - PM_low) + AQI_lowUse our AQI Calculator to convert any PM2.5 reading instantly.
WHO vs EPA vs EU PM2.5 Standards
| Standard | PM2.5 Limit | Notes |
|---|---|---|
| WHO 2021 Annual | 5 μg/m³ | Most protective; very few cities worldwide meet this |
| WHO 2021 24-hour | 15 μg/m³ | Not to be exceeded more than 3–4 days/year |
| US EPA Annual (2024) | 9 μg/m³ | Tightened from 12 μg/m³ in February 2024 |
| EU 2030 Annual | 10 μg/m³ | Binding from 2030; down from 25 μg/m³ |
| India NAQI Annual | 40 μg/m³ | Far looser than WHO; most Indian cities technically 'comply' |
| China GB3095-2012 | 35 μg/m³ (class 2) | Urban standard; met by few Chinese cities |
Key insight: many cities in Asia and Africa technically "comply" with national standards while far exceeding WHO guidelines. This is why PM2.5 in μg/m³ is more useful for health comparisons than country-specific AQI numbers.
Why Different Apps Show Different AQI
If you check AQI for Delhi on IQAir, AirVisual, SAFAR, and the India government's Sameer app simultaneously, you may see four different numbers. Why?
Different AQI scales
India uses NAQI (National AQI) with different breakpoints than US AQI. A PM2.5 of 120 μg/m³ = US AQI 181 (Unhealthy) but NAQI ~220 (Very Poor). Always check which scale an app uses.
Different monitoring stations
Apps pull data from different sensors. Government CPCB monitors vs. low-cost sensor networks (IQAir AirVisual) vs. PurpleAir sensors can show very different readings, especially during localized events.
Averaging period
Standard AQI uses 24-hour PM2.5 averages. Some apps display hourly readings or NowCast (a weighted average that responds faster to changes). During rapidly changing conditions, these diverge significantly.
Sensor calibration
Low-cost optical sensors (used by most commercial networks) over-report PM2.5 in high-humidity conditions and during smoke events with unusual particle size distributions. Government regulatory monitors use gravimetric methods that are more accurate.
Which Number Should You Use?
Use PM2.5 (μg/m³) when:
- • Comparing air quality between countries
- • Assessing against WHO health guidelines
- • Evaluating long-term health exposure
- • Interpreting scientific research
- • Your city has PM2.5 as the dominant pollutant
Use AQI when:
- • Making daily activity decisions
- • Your city has multiple active pollutants
- • Communicating risk to children or non-experts
- • Using color-coded health recommendations
- • High ozone risk (summer afternoons in US cities)
Frequently Asked Questions
Is AQI 100 safe?
Under the US AQI scale, AQI 100 (the top of the 'Moderate' range) corresponds to PM2.5 of 35.4 μg/m³ — 7× the WHO annual guideline. For healthy adults, short-term exposure at this level poses minimal acute risk. But regular daily exposure at AQI 100 represents significant long-term health impact. 'Safe' is relative — for the general public, AQI below 50 (PM2.5 < 12 μg/m³) is the target.
Why does AQI matter if I feel fine?
Most PM2.5 health effects are long-term and cumulative. You don't feel fine particles entering your bloodstream — the damage accumulates over years. A person living in a city with chronic PM2.5 of 25 μg/m³ has statistically shorter lifespan and higher cardiovascular and cancer risk than an identical person in a city with 5 μg/m³, even if neither notices any day-to-day health difference.
What's a good AQI to exercise outdoors?
For most healthy adults, AQI below 100 (PM2.5 < 35.4 μg/m³) is acceptable for normal exercise. For sensitive groups (asthma, COPD, heart disease), keep outdoor exercise to AQI below 50. During intense exercise, you breathe 5–8× more air — so PM2.5 intake is proportionally higher. If AQI exceeds 150, consider moving exercise indoors regardless of health status.