Frequently Asked Questions

AQI stands for Air Quality Index. It's a 0-to-500 scale defined by the U.S. EPA that translates measured concentrations of pollutants — most often fine particulate matter (PM2.5) and ozone (O₃) — into a single number you can act on.

• 0–50 Good (green) — air quality is satisfactory; no health impact expected.
• 51–100 Moderate (yellow) — acceptable for most; unusually sensitive individuals may notice symptoms.
• 101–150 Unhealthy for Sensitive Groups (orange) — people with respiratory or heart conditions, kids, and older adults should reduce prolonged outdoor exertion.
• 151–200 Unhealthy (red) — everyone may begin to feel effects; sensitive groups should avoid outdoor exertion.
• 201–300 Very Unhealthy (purple) — health alert; everyone should avoid outdoor exertion.
• 301+ Hazardous (maroon) — emergency conditions; stay indoors with filtered air.

The reported AQI on any given hour is whichever pollutant produces the highest sub-index. See Understanding AQI for the full breakdown.

Most reference networks publish new observations roughly once per hour — AirNow in the US, and the OpenAQ and national-agency feeds Smog Report uses elsewhere. Smog Report fetches the latest reading whenever you open the app, refresh the screen, or whenever a widget or Live Activity refresh fires. Where forecasts are available (currently US locations), they're issued once or twice per day and stay valid for 24–48 hours. For the technical details, see How AirNow data works.

Two air-quality apps can disagree honestly. Differences usually come from one of three things:

1. Different data sources. Some apps blend in PurpleAir or other low-cost sensors. Those sensors are useful, but they read differently than the EPA's regulatory-grade monitors and require correction factors.
2. Different averaging. Raw hourly concentrations look very different from EPA NowCast (which weights recent hours) and from 24-hour means. Two correct numbers can both be "right".
3. Different distance thresholds. If the nearest station is 30 miles away, one app may use it while another shows "no data".

Smog Report uses official AirNow data and the EPA's published AQI methodology.

PM2.5 is fine particulate matter — particles 2.5 micrometers across or smaller, roughly 30 times thinner than a human hair. Because they're so small, they bypass the body's normal defenses, deposit deep in the lungs, and can cross into the bloodstream. The WHO and EPA link long-term PM2.5 exposure to heart disease, stroke, and lung cancer; short-term spikes (wildfire smoke, for example) trigger asthma attacks and emergency-room visits within hours. See Common air pollutants.

Ground-level ozone forms when sunlight reacts with nitrogen oxides and volatile organic compounds — pollutants from vehicles, industry, and even gasoline-powered yard tools. It peaks on hot, sunny afternoons, especially in summer. Ozone is the dominant AQI driver in most U.S. cities outside of fire season. It irritates the airways, reduces lung function during exercise, and worsens asthma.

Yes. Smog Report is free on the App Store with no subscriptions and no account required. There's an optional in-app Tip Jar (consumable in-app purchases from $0.99 to $9.99) for users who want to support development; tipping also removes ads. Starting with version 2.5.1, the app shows a small number of native ads in two places to keep it free for everyone.

Starting with version 2.5.1, Smog Report shows a small number of native ads via Google AdMob in two engaged-surface places: the bottom of the main AQI scroll and the bottom of the About screen. Ads never appear on watchOS, in widgets, or in Live Activities. They're suppressed when the AQI is Unhealthy or worse, during your first three launches, when accessibility features are on, when you're offline, and when you've tipped. You can remove ads permanently — and support development — by leaving a tip in the in-app Tip Jar. Tips stack: a $0.99 tip grants 30 ad-free days, $9.99 grants 450 days, and multiple tips add together.

Yes. Starting in version 2.5.3, harmful-AQI alerts fire for every location you've saved in Smog Report, not just your GPS-derived current location. If you save Orlando and the AQI there crosses your alert threshold, the Live Activity appears on your Lock Screen even if you're physically somewhere else or have Location Services off.

When more than one saved location qualifies at the same time, Smog Report shows the alert for the closest one to you by default — that's the air you're actually breathing. You can change this in Settings → Live Activity → Alerts for:

• Closest to me (default) — alerts follow you as you travel.
• My primary location — alerts stay anchored to the top location in your list, the same one your widget shows.

Yes, for your primary saved location. Starting in version 2.5.3, Smog Report uses iOS Background App Refresh to fetch fresh air-quality data for your primary location periodically, so the Live Activity on your Lock Screen can update with harmful-AQI alerts even while the app is suspended. iOS controls the exact timing — typically every few hours, with no guarantee on cadence. Non-primary saved locations refresh only when you open the app.

The last successful reading is cached and will continue to display when you open the app offline, but new readings and forecasts require an internet connection because the data is fetched live from the air-quality service each time.

It means people with asthma, COPD, heart disease, diabetes, or active respiratory infections, plus children, pregnant people, and adults 65+, may experience symptoms at this level even though the general public probably won't. The EPA's recommendation at AQI 101–150 is for those groups to limit prolonged or heavy outdoor exertion. The general public can usually go about their day normally. See AQI and health.

Reference-monitor networks are dense in metropolitan areas but sparse in some rural regions, and coverage varies by country. If no reference monitor reports within a reasonable distance of your selected location, the app falls back to a clear no-data state rather than guess. You can manually choose a nearby city that does have a monitor; the reading won't be perfect for your exact spot, but it's usually directionally correct for the same airshed.

NowCast is the EPA's algorithm for converting hourly raw measurements into an AQI value that reflects recent conditions without overreacting to a single noisy hour. It weights recent hours more heavily than older ones, with extra smoothing during stable conditions. The result feels stickier than raw hourly data — the AQI doesn't whipsaw on every gust of wind — but it's also a more reliable number to base health decisions on.

Yes. Comprehensive international coverage shipped in 2026 — Smog Report now covers about 100 countries. In the US it uses EPA AirNow, in Singapore it uses the National Environment Agency's PSI feed, and everywhere else it uses OpenAQ's network of government and research-grade reference monitors.

The app automatically shows the air-quality index your country actually uses — DAQI in the UK, EAQI across Europe, AQHI in Canada, PSI in Singapore, NAQI in India, CAI in South Korea, the AU AQI in Australia and New Zealand, or the US EPA AQI — and you can override the scale in Settings. Coverage depends on how many reference monitors a given area has, so sparse regions may still show a no-data state. See AQI around the world for how the indices differ.

Your location is sent only to the reference-grade air-quality service for that location — EPA AirNow for US locations, OpenAQ for ~100 other countries, or Singapore's NEA (data.gov.sg) for Singapore — purely to look up the nearest monitoring station. It's not stored on any server we operate and it's not sent to Google AdMob, the ad SDK, or any other advertising or analytics service. AdMob does collect its own data when ads are shown (advertising identifier, ad interaction events) but not your location. Full details are in the Privacy Policy.

When AQI is in the Unhealthy range (151+) and the dominant pollutant is PM2.5 — typical during wildfire smoke — a well-fitted N95 or KN95 respirator dramatically reduces the dose you breathe. Cloth and surgical masks are not effective against PM2.5. For ozone, masks don't help; the gas passes through filter material. The right response to high ozone is reduced outdoor exertion, particularly in the afternoon. See Wildfire smoke.

Micrograms per cubic meter (µg/m³) is the raw concentration of a pollutant in air; AQI is a translated 0–500 index that takes the raw concentration and maps it onto a unitless scale that's the same regardless of pollutant. You'll often see PM2.5 reported in µg/m³ on regulatory dashboards (the EPA's annual standard is 9 µg/m³). AQI is a more digestible version of the same information for the public.

Yes — and that's a major reason apps like this exist. Smoke from a fire 1,500 miles away can drive your local AQI to Unhealthy levels for days. Because the AQI you see is whatever the local AirNow monitor is measuring right now, far-field smoke shows up automatically. The app does not separately classify the source as smoke; it just reports what's in the air.

Treat the AQI as one input alongside your own symptoms and your healthcare provider's guidance. Many people with asthma find their personal threshold sits well below the EPA's general thresholds — symptoms can begin in the upper Moderate range (AQI 75–100) when PM2.5 dominates. Keeping rescue medication on hand on Moderate days, and avoiding outdoor exertion above 100, is a common pattern. Talk to your provider about an asthma action plan, and never substitute an app reading for medical advice.

Email hello@smogreport.com. Please include your iOS version, the city/ZIP you were checking, and a screenshot if relevant. For known issues and troubleshooting, start with the Support page.