Your weather app says 118, and the sky outside looks ordinary. That number doesn’t mean the air turned dangerous overnight. It means at least one pollutant has climbed past the level the national standard allows, and the people most likely to feel it went outside an hour ago.
AQI 100 is a regulatory boundary. It sits where the Clean Air Act standard sits, not where your body starts reacting. Effects don’t wait for a round number, which is why a yellow day can matter if you have asthma.
TL;DR
An AQI above 100 means at least one pollutant has crossed the national standard. Between 101 and 150, sensitive groups feel it first. Check which pollutant is driving the number, shorten hard outdoor efforts, and clean the air indoors. For background on the scale, the Air Quality Index entry on Wikipedia is a solid overview. Filterbuy guide sits first in the Essential Resources below.
Top Takeaways
AQI 100 is a regulatory line, not a safety line. The WHO guideline for fine particles converts to a US AQI in the low 60s.
Check the dominant pollutant before the number. Ozone and particles peak at different hours and need opposite timing.
Two apps can disagree and both be right. They average different windows, and the shorter one tracks a moving plume better.
Sensitive groups are broad. Children, adults over 65, pregnancy, outdoor workers, and hard exercisers all qualify.
Indoors only helps on smoke days if you filter. MERV 13 in central air, or a portable cleaner rated two-thirds your floor area.
What the Number on Your Phone Actually Measures
The US Air Quality Index runs from 0 to 500 and covers five regulated pollutants: ground level ozone, particle pollution, carbon monoxide, nitrogen dioxide, and sulfur dioxide. Each gets scored separately, and the highest score becomes the AQI you see. That pollutant is called the dominant pollutant.
The number also isn’t a live reading. EPA estimates the current hour from a weighted average of the past 12 hourly measurements, leaning harder on recent hours when readings move fast. A fast moving smoke plume shows up late. If it smells like a campfire and your app says 96, believe your nose.
When Two Apps Show Two Different Numbers
Your phone says 96, a neighbor’s app says 130, and neither is broken. They average different amounts of time. AirNow uses that weighted 12 hour calculation. IQAir labels its readings US AQI+ because it uses hourly averages instead.
The official map also uses permanent regulatory monitors, while smoke maps and consumer apps often add low cost sensors closer to where people live. During a moving plume the shorter average is usually closer to what you are breathing.
Reading the 101 to 150 Band
Orange is the first category above the standard, and EPA calls it Unhealthy for Sensitive Groups. Here are the concentrations underneath those four words.
| AQI | Category | Color | PM2.5, 24 hours | Ozone, 8 hours |
|---|---|---|---|---|
| 101 to 150 | Unhealthy for Sensitive Groups | Orange | 35.5 to 55.4 µg/m³ | 71 to 85 ppb |
| 151 to 200 | Unhealthy | Red | 55.5 to 125.4 µg/m³ | 86 to 105 ppb |
| 201 to 300 | Very Unhealthy | Purple | 125.5 to 225.4 µg/m³ | 106 to 200 ppb |
| 301 and up | Hazardous | Maroon | 225.5 µg/m³ and up | above 200 ppb |
The ozone band is tight, 71 to 85 parts per billion, so one hot afternoon carries you through the whole orange range. And AQI 100 marks where the national standard sits, not where risk begins. The World Health Organization’s 24 hour guideline for fine particles is 15 micrograms per cubic meter, which converts to a US AQI in the low 60s.
An Ozone 110 and a Particle 110 Are Different Days
Ground level ozone forms when sunlight reacts with emissions, so it peaks in the afternoon and early evening. National Weather Service guidance for orange ozone days is blunt: move outdoor activity to the morning.
Particle pollution doesn’t follow the sun. PM2.5 comes from combustion, so it climbs with rush hour traffic, settles in on still evenings when wood stoves run, and arrives with wildfire smoke at any hour. EPA’s air quality guides put it plainly: particle levels can run high any time of year.
So “go early” is right on an ozone day and can be exactly wrong on a smoky morning. Most AQI explainers hand you one timing rule for a scale measuring two very different pollutants. Check which one is driving your number first.
What an Orange Day Does to Your Lungs and Heart
Ozone irritates the lining of your airways. The American Lung Association describes it as something close to a sunburn inside your lungs, which is why a hard effort on an ozone day can leave you coughing that evening.
Particles work differently. PM2.5 slips past the body’s defenses and settles deep in the lungs, and some crosses into the bloodstream. NIEHS reports that long term exposure has been associated with cardiovascular disease even at moderate concentrations, and that traffic related pollution can trigger heart attacks within hours.
One orange afternoon won’t harm you. The risk accumulates. If you manage a lung or heart condition, your clinician’s guidance beats any chart.
Who Counts as a Sensitive Group
- Children, teenagers, and adults over 65. Developing or declining lung function, and more time outdoors.
- People with asthma, COPD, heart disease, diabetes, or chronic kidney disease. CDC names all of these in its smoke guidance.
- Pregnant people. Exposure has been linked to preterm birth and lower birth weight.
- Outdoor workers and anyone exercising hard outside. Heavy breathing pulls more air deeper into the lungs.
What to Do at Each Level
AQI 101 to 150, Orange
- Sensitive groups: cut back on long or heavy exertion, and take more breaks.
- Follow your asthma action plan and keep quick relief medication with you.
- Everyone else: outdoor time is fine. Move hard efforts to the morning if ozone is dominant.
AQI 151 to 200, Red
- Sensitive groups: avoid long or heavy exertion, and move workouts inside.
- New York City’s health department puts a number on the word prolonged, treating more than an hour outdoors as the threshold.
- California requires employers to act at 151 for outdoor work, including checking each shift and providing respirators.
AQI 201 and Above, Purple to Maroon
- Stay inside, keep windows closed, and run filtration.
- If you go out, wear a NIOSH approved N95 or P100 with a tight seal. Clothes and surgical masks do close to nothing against PM2.5.
Keeping Indoor Air Cleaner When the Number Climbs
On an ozone day, going inside helps even without filtration, because ozone breaks down on contact with surfaces. On a smoke day, inside is only better if something is cleaning the air.
Outdoor air leaks in through gaps you can’t see, and particles stay in circulation until something removes them. CDC’s smoke guidance is worth following point by point.
- Pick one room you can close off and keep the air in it clean.
- Use a high efficiency filter in central air. MERV 13 or higher if your system handles that resistance.
- Set the system to recirculate or close the outdoor intake damper.
- Size a portable air cleaner to the room. Aim for a smoke CADR of at least two thirds the floor area. A 12 by 10 bedroom is 120 square feet, so look for 80 or higher.
- Stop adding particles indoors. No candles, incense, wood burning, or smoking until the air clears.
- Replace a loaded filter. Months of packed dust works against your blower.
Filterbuy, our partner on this page, makes the pleated filters described here. We kept the guidance source based so you can weigh it independently.
One caution. With no air conditioning in hot weather, sealing yourself in can be more dangerous than the smoke. Find a cooling center instead.
“I look at the dominant pollutant before I look at the number, because the number alone never tells me whether to go out early or stay shut in. An orange ozone day and an orange smoke day ask for opposite decisions.”
“On smoke days I close the house before the plume arrives instead of after. Keeping particles out is far easier than pulling them back out.”
Essential Resources on “AQI above 100”: 7 Places to Look Next
Each answers a different question, in the order they come up.
1. Turn the Number Into Something You Can Do Today
Filterbuy: AQI Above 100, What It Means and How to Protect Yourself walks through each band and the matching response, then covers the indoor filtration side. Start here if you want an answer before dinner rather than a chemistry lesson.
2. Find Out Which Pollutant Is Driving Your Number
AirNow: Using the Air Quality Index is the official EPA source every weather app pulls from. It gives the current reading, tomorrow’s forecast, and the dominant pollutant, the detail that changes what you should do.
3. Decide Whether the Run Still Happens
The National Weather Service air quality action tables give band by band activity guidance, split between ozone and particle days. Useful when you want a straight answer on exertion.
4. Keep Smoke From Following You Indoors
CDC: Safety Guidelines for Wildfires and Wildfire Smoke lists the indoor steps in order: the filter grade to aim for, how to set a central system, and which respirators work. The shortest complete checklist available.
5. Buy an Air Cleaner That Fits Your Room
Health Canada: Using a Portable Air Cleaner to Improve Indoor Air explains clean air delivery rate without jargon, including the rule that smoke CADR should be at least two thirds of your room area in square feet. That line prevents most bad purifier purchases.
6. Understand What Repeated Exposure Costs You
NIEHS: Air Pollution and Your Health summarizes the research on heart and lung disease, and on outcomes in pregnancy. Read it to gauge whether orange days matter over years, not just an afternoon.
7. Know What Your Employer Owes You Outdoors
Cal/OSHA: Protection From Wildfire Smoke, Appendix B sets out what California employers must do once the PM2.5 AQI hits 151. Other states differ, but this is the obligation written down with numbers attached.
Supporting Statistics
Three numbers behind the guidance above, all from US agencies or research organizations.
44 percent of Americans breathe air that earns a failing grade.
- 152.3 million people live in counties graded F for ozone or particle pollution.
- 33.5 million are children, nearly half the kids in the country.
- Source: American Lung Association, State of the Air 2026
AQI 101 starts at exactly 35.5 micrograms of PM2.5 per cubic meter.
- The orange band runs 35.5 to 55.4 over 24 hours.
- In May 2024, EPA lowered the ceiling for Good air from 12.0 to 9.0. Same air, higher score.
- Source: EPA, official AQI breakpoint table
Heart deaths and disability tied to particle pollution rose 31 percent worldwide in three decades.
- The annual global total climbed from 6.8 million in 1990 to 8.9 million in 2019.
- Source: American Heart Association, on research in the Journal of the American Heart Association
Final Thoughts
An AQI above 100 is information, not an alarm. It can’t tell you which pollutant, when it peaks, or what the air is doing inside your house. We pulled every concentration straight from the EPA breakpoint table and separated ozone days from particle days, because the right response flips between them.
Frequently Asked Questions
Is an AQI Above 100 Dangerous?
It’s unhealthy for sensitive groups, which differs from dangerous for everyone. Between 101 and 150, children, older adults, and people with heart or lung conditions may notice coughing or breathlessness during activity. Healthy adults usually feel little.
Can You Exercise When the AQI Is 120?
Usually yes, with adjustments. Shorten the session and lower the intensity. On an ozone day, running in the morning cuts exposure meaningfully. On a smoke day, morning may be no better, so check the pollutant first.
Does Indoor Air Stay Clean When Outdoor AQI Is High?
Not by default. Outdoor particles work their way in through gaps, doorways, vents, and intakes, then stay suspended until filtration removes them. Closing windows helps. Closing windows plus a good filter helps far more.
Does the AQI Include Pollen or Indoor Air?
Neither. The AQI scores five outdoor pollutants regulated under the Clean Air Act. Pollen counts are reported separately, and nothing in the number reflects cooking, candles, or anything else happening inside.
Do Masks Help When the AQI Is High?
A NIOSH approved N95 or P100 filters fine particles well if it seals against your face. Cloth and surgical masks don’t. Facial hair breaks the seal, and a respirator does nothing against ozone, which is a gas.
Check the Pollutant, Then Check Your Filter
Look up your local number today and note which pollutant is driving it. Then check the filter in your system. If you can’t remember the last time you changed it, that’s the easiest improvement here.