An AQI of 150 does not identify one substance. It is a public-health index calculated separately for different pollutants, and the pollutant with the highest sub-index normally determines the reported U.S. AQI.
That distinction changes what the number means and which protective action is relevant. A particle respirator can help with PM2.5, for example, but it does not filter ozone or carbon monoxide. An AQI-to-cigarettes calculation requires a confirmed U.S. PM2.5 AQI; an ozone or PM10 AQI cannot be treated as PM2.5.
Six criteria pollutants, but five AQI pollutant groups
EPA regulates six common air pollutants under the National Ambient Air Quality Standards: particulate matter, ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. EPA lists them on its criteria air pollutants page.
The U.S. AQI reports five major pollutant groups:
- ground-level ozone;
- particle pollution, with separate calculations for PM2.5 and PM10;
- carbon monoxide;
- sulfur dioxide; and
- nitrogen dioxide.
Lead is a criteria pollutant, but it is not one of the pollutants used for the daily AQI. AirNow's AQI Basics explains the five groups and six color categories.
Calling PM2.5 and PM10 two different “criteria pollutants” while omitting lead mixes the regulatory list with the AQI calculation. They are two size fractions within the particulate-matter criterion.
PM2.5: fine particle pollution
PM2.5 means particles with aerodynamic diameters of 2.5 micrometers or smaller. It is a mixture rather than one chemical. Combustion, traffic, power generation, wildfire smoke, wood burning, cooking, and atmospheric reactions can all contribute.
Fine particles can travel deep into the lungs, and EPA states that some may enter the bloodstream. Particle-pollution exposure is associated with aggravated asthma, decreased lung function, cardiovascular effects, respiratory symptoms, and premature death among people with heart or lung disease. EPA summarizes the evidence and affected groups in its PM health-effects guidance.
PM2.5 is not necessarily invisible as a group. Individual particles cannot be seen without equipment, but accumulated fine particles are a major cause of haze and reduced visibility. A clear sky also does not prove that PM2.5 is low.
Common PM2.5 situations
- wildfire smoke;
- winter wood smoke and temperature inversions;
- regional secondary particles formed from precursor gases;
- traffic and industrial combustion; and
- indoor cooking or smoking.
For outdoor conditions in the United States, check the PM2.5 sub-index and timestamp through AirNow. During wildfire smoke, the Fire and Smoke Map adds permanent monitors, temporary monitors, and screened, corrected sensor data.
Controls that address PM2.5
Reducing time and exertion outdoors, moving to cleaner air, using appropriately sized particle filtration, and wearing a well-fitting NIOSH Approved N95 for unavoidable outdoor particle exposure can reduce exposure. None of these produces a guaranteed percentage reduction for an individual.
A HEPA purifier or N95 does not remove carbon monoxide, ozone, or every gas in smoke. During an active fire, evacuation, heat, power, and combustion safety take priority over particle filtration.
PM10: inhalable particles that include PM2.5
PM10 means particles with aerodynamic diameters of 10 micrometers or smaller. The measurement includes PM2.5; the “coarse” fraction is the portion between 2.5 and 10 micrometers.
Dust storms, road dust, construction, agriculture, crushing and grinding, and some biological material can contribute to coarse particles. Sources and composition vary by event and location.
It is inaccurate to call PM10 merely an annoyance or to assume it cannot cause significant health effects. EPA links inhalable particles with respiratory and cardiovascular effects and notes that smaller particles generally travel deeper into the respiratory tract. Risk depends on concentration, duration, composition, and the person exposed.
PM10 needs its own AQI conversion
EPA uses different concentration breakpoints for PM10 and PM2.5. A PM10 AQI of 150 therefore does not identify the PM2.5 concentration that would produce a PM2.5 AQI of 150.
EPA's revised U.S. PM2.5 AQI breakpoints took effect on May 6, 2024. A later publication or technical-document revision date does not mean the breakpoints first became effective in 2026; EPA's 2024 AQI fact sheet records the change.
Particle filters and suitable respirators can capture many PM10 particles, but real protection still depends on fit, airflow, operation, and the event. “Any mask helps” is too broad: a loose face covering should not be represented as equivalent to a certified, well-fitting respirator.
Ozone: a reactive gas
Ground-level ozone is not emitted directly. It forms when nitrogen oxides and volatile organic compounds react in sunlight. Hot, sunny conditions often favor formation, but local and regional chemistry, transport, elevation, and weather determine the actual pattern.
EPA describes ozone as a gas that can cause coughing, throat irritation, chest discomfort, airway inflammation, and reduced lung function. It can aggravate asthma and other lung disease. Children, people with asthma, older adults, and people active outdoors may be at greater risk. See EPA's ozone health-effects summary.
Avoid universal rules such as “ozone is always near zero in winter” or “the cleanest time is always early morning.” Ozone can remain important outside a stereotypical summer afternoon, and the best time varies with the local forecast.
Particle controls do not control ozone
An N95 filters particles, not ozone gas. A standard HEPA purifier is also a particle cleaner and should not be presented as an ozone-removal device. Some air-cleaning devices intentionally generate ozone; EPA warns that ozone is a lung irritant and recommends avoiding ozone-producing devices in occupied spaces.
For an ozone-driven AQI, follow ozone-specific activity guidance. AirNow publishes separate activity guides for ozone and particle pollution.
Carbon monoxide, nitrogen dioxide, and sulfur dioxide
These pollutants receive less attention in consumer particle-sensor apps, but each has its own AQI calculation and health guidance.
Carbon monoxide
Carbon monoxide is a colorless, odorless gas produced by incomplete combustion. High concentrations can prevent the body from using oxygen effectively and can be fatal. A PM2.5 sensor, N95, or HEPA filter does not detect or remove CO.
Use working CO alarms and never run a generator, grill, vehicle, or other fuel-burning equipment in an enclosed or partly enclosed space. If a CO alarm sounds, leave for fresh air and contact emergency services. Do not wait for an AQI app.
Nitrogen dioxide
Nitrogen dioxide is associated with combustion sources, including road traffic and fuel-burning appliances. It can irritate airways and aggravate respiratory disease. Indoor gas combustion can matter even when an outdoor particle AQI appears low.
Sulfur dioxide
Sulfur dioxide primarily comes from sulfur-containing fuel combustion and some industrial processes. Short-term exposure can affect the respiratory system, particularly among people with asthma. It is a gas, so particle-only controls are not the appropriate response.
EPA provides pollutant-specific technical definitions and averaging periods in the current AQI Technical Assistance Document.
Why two apps can show different AQI values
One source may report overall AQI while another displays PM2.5 only. They may also use different stations, averaging periods, model inputs, sensor corrections, timestamps, or national index systems.
Before comparing the final numbers, match:
- AQI system;
- pollutant;
- location;
- observation or forecast status;
- averaging period;
- timestamp; and
- monitor, sensor, or model method.
Use our guide to conflicting app readings for the complete workflow.
Can this AQI be converted to cigarette-equivalents?
Only when the input is explicitly identified as a U.S. PM2.5 AQI.
Do not convert:
- ozone AQI;
- PM10 AQI;
- CO, NO2, or SO2 AQI;
- an overall AQI with an unknown dominant pollutant;
- a foreign or proprietary index unless it explicitly uses the U.S. PM2.5 method; or
- a fire icon, smoke plume, pollen index, or indoor comfort score.
If a reliable source supplies PM2.5 directly in µg/m³, use the direct PM2.5 calculator. Otherwise, follow the AQI conversion eligibility checklist.
The cigarette-equivalent output uses Berkeley Earth's rough population-level risk-communication framework. It is not particle mass, inhaled dose, cigarettes actually smoked, or a personal health prediction.
A practical decision workflow
- Open the pollutant details rather than relying on the overall color.
- Confirm the AQI system and timestamp.
- Follow the activity guide for the pollutant and population group.
- Choose controls that match particles, gases, or an emergency source.
- Recheck conditions when weather, smoke, location, or activity changes.
Bottom line
PM2.5, PM10, and ozone can produce the same AQI number through different concentration scales. They also require different guidance and controls. PM2.5 is fine particle pollution; PM10 includes a broader inhalable particle range; ozone is a reactive gas. Carbon monoxide and the other gaseous AQI pollutants create additional hazards that particle-only devices cannot solve.
Identify the pollutant first. The number becomes useful only after you know what it represents.