What AQI and PM2.5 Mean

The lesson

PM2.5 describes a particle size. AQI translates pollutant concentrations into an air-quality index. They are related, but they are not interchangeable: the particle concentration, its averaging period and the pollutant behind the index all matter.

How small is PM2.5?

PM2.5 means particles no more than 2.5 micrometres in diameter. An average human hair is about seventy micrometres wide, roughly thirty times the width of the largest fine particle. About 90% of the particle mass emitted by wildfires is in this fine-particle range.

Size affects where particles can travel in the body. Larger particles tend to deposit higher in the airway; fine particles can reach deep into the lungs, and some may enter the bloodstream. EPA identifies fine particles as the main health threat from wildfire smoke.

AQI is an uneven ladder

The US Air Quality Index covers several pollutants, including fine particles and ozone. For PM2.5, the EPA breakpoints effective from May 2024 put the top of Good, AQI 50, at a 24-hour average of 9.0 micrograms per cubic metre. AQI 100 corresponds to 35.4.

Doubling the index from 50 to 100 therefore corresponds to nearly four times the PM2.5 concentration. It does not mean four times the health risk. The index is a set of concentration bands, not a direct multiplier of individual risk.

AQI 101–150 is Unhealthy for Sensitive Groups. For particle pollution, those groups include children, older adults and people with heart or lung disease. At higher categories, effects extend more broadly across the population.

Keep the averaging period in view

The PM2.5 AQI breakpoints are defined for 24-hour concentrations. Hourly displays help describe changing conditions, but are not the same measurement as a completed daily average. AirNow uses NowCast for current reporting; Forecast Bar’s hourly forecast applies the breakpoints to hourly guidance as an approximation.

That distinction matters during a fast-changing smoke episode. An hourly spike, a daily index and tomorrow’s forecast answer different questions. Read the time attached to the number before comparing them.

Smoke overhead is not always smoke at street level

Strong fires can loft smoke several kilometres into the atmosphere, where winds carry it far from its source. An elevated plume can colour the sky while air near the surface remains relatively clear. On another day, smoke reaches the layer where people breathe.

A photograph of a red sun cannot settle which situation applies. Surface-smoke guidance and current air-quality readings provide different information from the appearance of the sky. Forecasts can miss sudden smoke spikes, so check the current reading before making an outdoor plan.

What to take away

Use these checks next time:

Identify the pollutant and averaging period behind an AQI value.
Treat a forecast as guidance about change, not a current measurement.
Check near-surface conditions rather than judging exposure by sky colour.

Transcript

Video transcript

Read the full transcript

June seventh, twenty twenty-three. The sky over New York turned orange, and one hourly air quality reading hit four hundred and eighty-four: hazardous. The fires were hundreds of miles away, in Quebec. So what was New York breathing, and what does that number mean?

Take a human hair: about seventy micrometers wide. P M two point five means particles two and a half micrometers across, or smaller. About thirty of the biggest span that hair. In wildfire smoke, about nine-tenths of the particle mass is this small.

Size decides where it goes. Bigger particles tend to settle higher, in your nose and throat. These slip deeper, into your lungs, and some may even reach your bloodstream. That's why health agencies call them smoke's biggest health threat.

The U.S. air quality index covers several pollutants. For P M two point five, it's a ladder with uneven rungs. Fifty, the top of "good", is a daily average of nine micrograms per cubic meter. One hundred is about thirty-five. So going from fifty to a hundred means nearly four times the particle concentration.

From one-oh-one to one-fifty, sensitive groups are warned: kids, older adults, people with heart or lung disease. Higher up, everyone.

Smoke rides the wind. Big fires can loft it miles up, and winds up there can carry it across a continent. That's how a sky can turn milky and the sun blood-red while the air at ground level stays fine. The smoke is passing overhead. Other days, it sinks to where you breathe. Looking up can't tell you which.

Remember the smoke you see and the smoke you breathe? Forecast Bar's Wildfire Smoke layer maps the federal forecast for smoke near the ground, hour by hour.

And Air Quality adds an hourly P M two point five forecast, so you can see when the air should clear. Here, it peaks at one sixty-two this afternoon, and drops to moderate by nine tomorrow morning. That could be a better window to run. Check the current reading before you go.

Forecast Bar. Know when the air clears.

Want more of the sky, explained? Pick your next video, and subscribe so you catch the next one.

Where Forecast Bar comes in

Put the lesson to work.

Air Quality

See the hourly PM2.5 outlook.

Forecast Bar shows current air quality from The Weather Company and, in the contiguous US, up to 48 hourly PM2.5 forecast values from NOAA. The chart converts the hourly guidance using the 2024 US EPA breakpoints.

Air Quality is a Gold feature on iPhone and iPad
Day tabs, a peak label and chart scrubbing help compare hours
Hourly forecast AQI is an approximation; sudden smoke spikes can be missed

Wildfire Smoke

Follow the plume near the surface.

The Wildfire Smoke layer animates NOAA HRRR Smoke forecasts hour by hour. It maps smoke near the ground, with Low, Moderate, Heavy and Dense legend bands.

Gold on iPhone and iPad
Smoke layer and NOAA PM2.5 forecast: contiguous US
AQI, PM2.5 and smoke threshold push alerts are not supported

FAQ

Common questions

What is the difference between AQI and PM2.5?

PM2.5 names particles 2.5 micrometres across or smaller. AQI is an index derived from pollutant concentrations; fine particles are one of several pollutants it covers.

Is AQI 100 twice the pollution of AQI 50?

For PM2.5 under the 2024 EPA breakpoints, 50 corresponds to 9.0 and 100 to 35.4 micrograms per cubic metre as 24-hour averages: nearly four times the concentration. This is not a health-risk multiplier.

Does a smoky sky prove the air at ground level is bad?

No. Smoke can pass overhead without reaching the surface. Check current air quality and near-surface smoke information.

Can Forecast Bar send an AQI threshold alert?

No. The app shows air-quality data and forecasts, but does not offer custom AQI, PM2.5 or smoke threshold notifications.

Put the forecast in context.

Explore the Forecast Bar features described above. Feature availability depends on your plan, device and location.

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