What is the difference between AQI and PM2.5?

PM2.5 is a pollutant — fine particles under 2.5 microns. AQI (Air Quality Index) is a summary score that converts several pollutants (PM2.5, PM10, ozone, NO2, SO2, CO) into one 0-500 scale. When news says 'AQI 150, unhealthy,' PM2.5 is usually the dominant pollutant driving that number. Think: PM2.5 is the ingredient; AQI is the recipe score.

Which is more important: AQI or PM2.5?

For most people, PM2.5 concentration (in µg/m³) is the more informative number — it tells you the actual particle load in the air. AQI is easier to act on (color-coded categories) but can be driven by ozone or other pollutants on some days. For air purifier decisions, PM2.5 matters most, because that's what HEPA filters remove.

What is a safe PM2.5 level?

The WHO recommends annual average PM2.5 below 5 µg/m³ and 24-hour average below 15 µg/m³. The EPA's 'good' AQI category corresponds to PM2.5 under 12 µg/m³ (24h). Above 35 µg/m³, the EPA rates air as 'unhealthy for sensitive groups.' Indoors, you can realistically keep PM2.5 below 10 µg/m³ with a good HEPA purifier.

How is PM2.5 measured?

By laser particle counters that sample air and count particles in the 2.5 micron-and-smaller range, expressed in micrograms per cubic meter (µg/m³). Government monitoring stations use federal reference methods; consumer air purifiers and indoor monitors use cheaper laser sensors that are accurate enough for home use (within 10-20% typically).

Do air purifiers reduce PM2.5?

Yes — this is exactly what HEPA filters do: they catch 99.97% of particles at 0.3 microns, which includes essentially all PM2.5. Indoor PM2.5 can drop by 70-90% with a properly sized purifier running continuously, even on bad outdoor days with windows closed.

AQI vs PM2.5: What's the Difference and Why It Matters

Every air quality story mentions AQI. Every air quality story mentions PM2.5. They sound interchangeable — and they’re not. One is an ingredient, the other is a recipe score.

Here’s the plain-English difference, why it matters for your air purifier decision, and how to read both like a pro.

🧰 Interactive Tools

🌡️AQI ExplainerDecode any AQI number→ 📏Room Size CalculatorMatch a purifier to your rooms→

The One-Paragraph Difference

Air quality monitor showing PM2.5 readings

PM2.5 is a pollutant. It’s fine particulate matter — dust, smoke, soot, and chemical particles under 2.5 microns — measured in micrograms per cubic meter (µg/m³). It’s the physical stuff in the air.

AQI is a score. It’s the EPA’s 0-500 index that converts multiple pollutants — PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide — into one color-coded number. It answers “how bad is the air overall?” by taking the worst pollutant’s number.

	PM2.5	AQI
What it is	A pollutant (particles)	A composite score
Unit	µg/m³ (micrograms/cubic meter)	0-500 index
What it tells you	Actual particle load	Overall air quality category
Who reports it	Monitors, apps, purifiers	EPA, weather services, news

Why the Confusion Happens

PM2.5 is the dominant pollutant on most bad-air days in the US — wildfire smoke, winter inversions, and urban smog are mostly PM2.5 stories. So when the news says “AQI 200,” nine times out of ten, PM2.5 is the driver. People start treating the two as synonyms.

But they’re not always: on hot summer days, ozone can drive the AQI while PM2.5 stays low. That’s a different problem with a different fix (no purifier removes ozone).

The Numbers to Know

PM2.5 (24h avg)	AQI equivalent	Category
0-12 µg/m³	0-50	Good
12-35 µg/m³	51-100	Moderate
35-55 µg/m³	101-150	Unhealthy (sensitive groups)
55-150 µg/m³	151-200	Unhealthy
150-250 µg/m³	201-300	Very unhealthy

The WHO’s health guideline: annual average under 5 µg/m³; 24-hour under 15 µg/m³. The EPA’s “good” is under 12 µg/m³.

Which One Should You Watch?

For your health and your purifier: watch PM2.5.

AQI tells you the category; PM2.5 tells you the particle load
Your purifier removes PM2.5 — that’s the number it affects
Indoor monitors and many purifiers display PM2.5 in µg/m³ directly

For quick decisions: watch AQI. The color categories are designed for fast action: Green = open windows, Red = close them and run the purifier. Both work; they just work at different depths.

What This Means for Your Purifier

The whole air purifier industry exists to reduce PM2.5. A True HEPA unit removes 99.97% of particles at 0.3 microns — which is the PM2.5 class. With windows closed and a properly sized purifier running, indoor PM2.5 can drop 70-90% even when outdoor AQI is 150+.

The practical check: if your indoor monitor or purifier shows PM2.5 under 10 µg/m³, you’re in great shape — better than the EPA’s “good.” Above 35 µg/m³ indoors, something’s wrong (open window, undersized unit, or a source like cooking).

The Bottom Line

PM2.5 is the ingredient; AQI is the score. For air purifier decisions, PM2.5 is the number that matters — it’s what HEPA removes and what your indoor monitor shows. For daily “is it safe outside” decisions, AQI’s color bands are the fast answer. Know both, and you’ll never be confused by an air quality report again — and you’ll know exactly when to close the windows and run the machine.