You opened your forecast this morning and found a number you weren't looking for — a small orange dot beside a two-digit figure, sitting about where the UV index usually lives. That dot is the Air Quality Index, and it is making a specific claim about your afternoon.
The claim has three parts: a pollutant, a threshold, and a population that should change plans. Most readers see only the color.
That gap costs real decisions. The same orange dot can mean a six a.m. run solves the problem on Tuesday, and that no hour of Wednesday will.
What The AQI Number Actually Measures
The Air Quality Index is a translation layer sitting on top of a measurement. The EPA takes a measured pollutant concentration, runs it through a piecewise linear formula, and returns a value on a 0 to 500 scale where 100 always lands on the national health standard for that pollutant.
Six pollutants each get their own sub-index: fine particulate matter (PM2.5), coarse particulate (PM10), ground-level ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The AQI you see is the highest of those six, and the pollutant that produced it is labeled the dominant pollutant.
The AQI converts pollutant concentrations into a 0–500 number where 100 marks the national health standard. It reports only the highest sub-index among six pollutants, so it describes the worst one rather than the mixture.
Therefore the number alone underdetermines your day. An AQI of 88 driven by ozone and an AQI of 88 driven by wildfire smoke share a color, a category, and very little else.
Keep in mind that the scale is deliberately nonlinear. Climbing from AQI 50 to AQI 100 on PM2.5 takes roughly 26 additional micrograms per cubic meter, while climbing from 150 to 200 takes about 70 — the index compresses the ugly end so one scale can hold both a hazy Tuesday and a plume event.
Accordingly, the categories are thresholds with health evidence behind them rather than evenly spaced rungs. Two-thirds of the way up the scale is considerably worse than two-thirds of the way up the concentration.
The Six Categories, And Where The Lines Came From
Each category corresponds to a documented change in health effects, which is why the names read like instructions instead of adjectives. Here's what each band covers for the two pollutants that drive nearly every consumer-facing reading:
| Category | AQI | PM2.5 (µg/m³, 24-hr) | Ozone (ppm, 8-hr) | What it changes |
|---|---|---|---|---|
| Good | 0–50 | 0.0–9.0 | 0.000–0.054 | Nothing. Plan freely. |
| Moderate | 51–100 | 9.1–35.4 | 0.055–0.070 | Unusually sensitive people ease off intensity. |
| Unhealthy for Sensitive Groups | 101–150 | 35.5–55.4 | 0.071–0.085 | Asthma, heart and lung conditions, children, older adults shorten outdoor exertion. |
| Unhealthy | 151–200 | 55.5–125.4 | 0.086–0.105 | Everyone moves hard efforts indoors. |
| Very Unhealthy | 201–300 | 125.5–225.4 | 0.106–0.200 | Outdoor exertion is off the table; filter indoor air. |
| Hazardous | 301+ | 225.5 and up | — | Emergency conditions; stay indoors with filtration. |
Note that eight-hour ozone above 0.200 ppm hands off to the one-hour table, which is why the top ozone cell is blank. Ozone at that level is effectively absent from modern U.S. monitoring, so in practice the red-and-above end of the scale belongs to particulate.
AQI 0–50 is Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, 151–200 Unhealthy, 201–300 Very Unhealthy, and 301 and above Hazardous. The 100 line always marks the national health standard.
The 2024 rescaling
In May 2024 the EPA tightened the PM2.5 breakpoints, moving the Good/Moderate line from 12.0 to 9.0 micrograms per cubic meter. A reading of 10 µg/m³ that once reported around 42 now reports in the low 50s.
The air did not change; the ruler did. If your neighborhood seemed to acquire more yellow days without acquiring more haze, this is the reason.
Two Days Can Both Read 88
Ozone and PM2.5 produce almost every AQI reading a general audience ever sees, and they behave like different weather phenomena entirely. Ozone is manufactured in the air above you; particulate is delivered to you.
Ground-level ozone is a secondary pollutant, which means nothing emits it directly. Nitrogen oxides and volatile organic compounds cook in sunlight, and the reaction rate climbs with temperature and stalls without wind.
PM2.5 is primary in the cases that matter most to readers — wildfire smoke, agricultural burning, wood stoves, diesel exhaust. Those particles are small enough to reach the deep lung and cross into the bloodstream, which is why the health thresholds sit so low.
The practical consequence is a scheduling one. An ozone day is a timing problem, and a smoke day is a location problem.
Check the dominant pollutant before you plan. Ozone peaks in mid-afternoon sun, so an early run works; wildfire PM2.5 can hold all night, so timing helps far less than filtration.
Why Ozone Peaks At Three And Smoke Peaks At Dawn
Ozone tracks the sun with a lag. Concentrations bottom out overnight, begin climbing an hour or two after sunrise, and typically peak between early afternoon and early evening before collapsing after dark.
The worst ozone days share a signature you can read off an ordinary forecast: hot, sunny, and stagnant, usually under a summer surface high with light winds. If you understand how high pressure suppresses vertical mixing, you already understand why ozone stacks up under a ridge.
Smoke often runs the opposite clock. Overnight cooling builds a shallow inversion that traps particulate near the ground, and in valleys, drainage winds pool it in the low spots until the sun breaks the lid.
Daytime mixing then cuts both ways. It can dilute the trapped surface layer, or it can drag down smoke that spent the night riding a few thousand feet overhead.
What's more, humidity changes what the particles themselves do. Many aerosols swell as relative humidity rises, which is one reason a smoky morning at a high dew point looks and reads worse than the same smoke on a dry afternoon.
How A Plume Finds You
Smoke does not spread evenly outward from a fire. It rises to whatever level its buoyancy and the surrounding stability allow, then travels with the wind at that level, which can point somewhere entirely different from the wind at your face.
At continental scale, the steering comes from the upper flow. A trough or ridge in the jet stream can route Canadian or Western smoke into the Midwest and Northeast for days at a stretch, with the arrival lagging the fire by hundreds of miles and dozens of hours.
NOAA's HRRR-Smoke model publishes two fields that answer two different questions. Near-surface smoke tells you what you will breathe, and vertically integrated smoke tells you what the sky will look like.
Smoke aloft can redden the sky while surface AQI stays green. Check the near-surface smoke field for what you'll breathe and the column field for what you'll photograph.
Close to an active fire, dense plumes return a radar signal, appearing as a plume-shaped echo with no precipitation behind it. If you've spent time reading a radar loop for structure rather than rainfall, a plume and its evening collapse are legible the same way.
Be aware that smoke forecasts carry wider error bars than temperature forecasts. Plume height, fire behavior, and mixing depth compound each other, so treat a 48-hour smoke outlook the way you would treat any forecast carrying low confidence.
The Number On Your Screen Is Already Behind
AirNow reports current particulate using NowCast, a weighted average of roughly the last 12 hours that leans hard on the most recent ones. The weighting exists to keep the number from thrashing, and the cost is lag.
During a fast-arriving plume, NowCast reads low relative to the air you are standing in, sometimes by a full category. During clearing, it reads high for exactly the same reason.
Regulatory monitors are also sparse. A metro area may have a handful of them, and the nearest one can sit twenty miles and one terrain feature away from your block.
Low-cost sensor networks fill those gaps with far denser coverage, at the cost of a known bias: uncorrected optical sensors read high in smoke. The EPA correction applied on the AirNow Fire and Smoke Map exists to reconcile the two, so prefer the corrected layer over a raw neighborhood sensor.
What Bad Air Does To Light
Aerosols scatter short wavelengths preferentially, so a smoke layer subtracts blue from transmitted sunlight and leaves the red end behind. The visible result is a sun that turns orange and then a flat red disk you can look at directly near midday.
That same scattering wrecks distance contrast. Aerial perspective compresses hard — ridgelines two miles out lose separation, autofocus hunts on low-contrast edges, and long lenses suffer more than wide ones.
The light itself gets warmer and softer, and the usable window stretches. A heavy smoke layer effectively extends the warm, low-angle light of golden hour deep into the afternoon, because the atmosphere is doing the same filtering the horizon normally performs.
Under heavy smoke, shoot RAW and lock white balance near 5,200–5,600K. Auto white balance neutralizes the warmth back toward gray, which is the one thing the scene has going for it.
Compose for the layers instead of the details. The technique is the one that works when fog flattens a scene into planes — find overlapping ridges, backlight the subject, and let the haze do the tonal separation.
What Actually Changes At Each Category
Below 50, nothing changes. Between 51 and 100, the only people who need to adjust are unusually sensitive individuals, and the adjustment is usually intensity rather than cancellation.
At 101 to 150, the sensitive-groups line does real work: children, older adults, pregnant people, and anyone with asthma, COPD, or heart disease. Everyone else can proceed, with the caveat that dose scales with breathing.
Dose equals concentration times the air you move. Heavy exertion can raise breathing volume five to ten times over rest, so an hour of hard effort at AQI 120 beats a quiet day at the same reading.
At 151 and above, the general population is included, and the honest advice is to move hard efforts indoors. Between 201 and 300, outdoor exertion stops being a judgment call.
Masks help in one direction only. A well-fitted N95 removes the large majority of fine particulate, which makes it a genuine tool on a smoke day and an empty gesture on an ozone day.
An N95 filters the fine particulate in wildfire smoke and cuts exposure meaningfully. It does nothing for ozone, which is a gas — against ozone, timing and filtered indoor air are the only levers.
Indoors, the levers are filtration and infiltration. A MERV-13 filter on the HVAC or a box-fan filter unit will pull down indoor PM2.5, though neither touches the ozone that a summer afternoon pushes through an open window.
One interaction deserves naming: bad ozone days are hot days, and heat and ozone stress the same people. When you're already weighing what the apparent temperature does to exertion, an ozone alert should push that decision further in the same direction.
The Decision Rule
Read the AQI in four moves, in order: dominant pollutant, category, trend direction, then your own exertion level. Three of those four are invisible in the orange dot.
Ozone at 110 means run at six in the morning and skip the afternoon ride. Smoke at 110 means the morning will not save you, so move the effort indoors or shorten it.
A rising trend deserves more caution than the same number falling, because NowCast understates the leading edge of a plume. Falling numbers, by the same mechanism, are usually a little better than they read.
If you're building a day around the dot, the same packing logic that turns a forecast into a decision applies here. One layer of judgment, applied before you walk out.
We publish the dominant pollutant beside the number for exactly this reason, because a category without a cause is a color without an instruction. That principle — translate the measurement into the decision — is how we write every brief.
Common Questions
They share a category and nothing else useful. Ozone is manufactured by sunlight and peaks in mid-afternoon, so an early-morning run avoids most of it. Wildfire particulate can sit over a city through a full 24-hour cycle and often peaks near dawn, so timing helps far less than filtration and time spent indoors.Is an AQI of 88 from ozone the same as an AQI of 88 from wildfire smoke?
No. An N95 filters particles, and ozone is a gas that passes through the filter media untouched. On a smoke day a well-fitted N95 meaningfully cuts your exposure and is worth wearing. On an ozone day your only real levers are timing outdoor hours away from the mid-afternoon peak, lowering intensity, and filtered indoor air.Does an N95 mask protect me on a high-ozone day?
The EPA tightened the PM2.5 breakpoints in May 2024. The line between Good and Moderate moved from 12.0 to 9.0 micrograms per cubic meter, so concentrations that used to report in the low 40s now report just above 50. The air did not change and the ruler did, which is worth knowing before reading a year-over-year trend.Why does my area suddenly have more yellow days than it used to?
Frequently. Smoke traveling a few thousand feet overhead reddens sunlight and flattens distant contrast without ever reaching breathing level, and surface monitors only measure breathing level. Check the near-surface smoke field for what your lungs will get and the vertically integrated field for what your camera will get; the two disagree often.Can the sky look smoky while the AQI still says Good?
A great deal, because dose is concentration multiplied by the volume of air you move. Heavy exertion can raise your breathing rate five to ten times over resting, so an hour of hard intervals at AQI 120 can deliver more particulate than an entire quiet day at the same reading. Lower the intensity before you cancel the outing.How much does exercising outdoors change my exposure?