AirLocal by Hyperstatic

Data & methodology

Know the number behind the number.

AirLocal keeps each provider, station, observation time, and display method explicit. This page explains what the app shows—and what it deliberately does not imply.

PurpleAir observations
AirNow official sources
US AQI presentation

The rules AirLocal follows

Name the sourceA reading is identified as Nearby PurpleAir, official AirNow, or your own sensor.
Show its ageObservation time remains visible because a fresh 52 and an old 52 are not the same answer.
Keep unlike data distinctAirNow never silently replaces the PurpleAir station behind Near you.
Prefer honest absenceUnavailable, delayed, and cached states are labeled instead of inventing a zero or another provider.

Three source classes

PA

Nearby PurpleAir

The default public reading is one named outdoor PurpleAir station chosen on the iPhone. Public map and Nearby views can use 10-minute or 60-minute averages and either no correction or PurpleAir's documented US EPA outdoor correction. The default is a 10-minute average with no correction.

AN

Official AirNow

AirLocal displays the nearest suitable named AirNow monitoring station as a separate official source. AirNow observations are preliminary and commonly update on an hourly cadence. When several pollutants are reported at one monitor, the displayed AQI follows the highest individual pollutant AQI at that monitor—not a maximum assembled from several monitors.

YOU

Your PurpleAir sensor

For supported PurpleAir Zen and Flex sensors, AirLocal first tries a direct reading over the same local Wi-Fi network. Current US AQI is calculated from the sensor's two-minute PM2.5 average. The iPhone keeps one-minute local history for the latest 24 hours and can use carefully bounded cloud fallback when configured.

How “Near you” is selected

Near you means a quality-aware nearby PurpleAir station. AirLocal considers distance, reading freshness, confidence, and source suitability. The closest dot is not automatically the best answer.

The selection happens on the iPhone using bounded candidate data. Exact phone coordinates are not sent to AirLocal's backend. When alternatives are available, the app can explain why the station was selected and show up to three nearby candidates.

AirNow stays separate.

If no usable nearby PurpleAir reading exists, Near you remains unavailable or shows an honestly aged PurpleAir snapshot. It does not quietly become an AirNow reading.

AQI, pollutants, and supporting measurements

AirLocal presents United States Air Quality Index categories and colors. AQI is a communication scale, not a direct concentration unit. A sensor's PM2.5 concentration and its AQI answer related but different questions.

US AQI
A category-oriented index used to communicate potential health concern. AirLocal preserves the source and method behind it.
PM2.5
Fine particulate matter concentration, shown in micrograms per cubic meter when the source reports it.
Ozone & PM10
Shown for AirNow monitors when published. Missing pollutants remain missing; they are not treated as zero.
Temperature & humidity
Shown for PurpleAir sources when available. Sensor placement and device characteristics can influence these readings.
History
Charts preserve real gaps and source cadence. AirLocal does not fabricate observations between missing samples.

Changing the public PurpleAir average or correction affects compatible public Nearby and Map readings. It does not rewrite owned-sensor history, AirNow readings, or alert interpretation.

Freshness, caching, and unavailable states

Each current reading includes its observation age. AirLocal can retain a last valid reading through a short provider or network interruption, but it advances the age and marks delayed data rather than presenting it as live.

Widgets follow the same rule. Because iOS decides when a widget receives execution time, every widget shows the observation age. A configured widget remains bound to its selected source between app synchronizations; it does not search for a convenient replacement in the background.

Public data is cached on the iPhone and backend to improve speed, preserve honest fallback, and avoid needless provider requests. Cache policy is source-specific, so one averaging or correction method is never served as another.

Forecasts and alerts are separate concepts

Official outlook

When available, AirLocal shows AirNow's official reporting-area outlook for today and tomorrow. It is forecast context, not a current station reading and not a replacement for Nearby PurpleAir or the named AirNow monitor.

Alerts

AirLocal can evaluate separate rising and falling AQI threshold crossings for supported PurpleAir sources, along with offline and back-online events. Alerts describe a crossing event; they do not promise continuous medical monitoring or instant delivery.

Limits, independence, and attribution

Low-cost optical sensors, official monitoring networks, and local conditions each have different strengths and limitations. Readings can vary with placement, humidity, smoke composition, maintenance, station cadence, and distance. AirLocal is an informational product, not a medical device or emergency-warning service.

AirLocal is an independent product of Hyperstatic LLC. It is not affiliated with or endorsed by PurpleAir or the U.S. Environmental Protection Agency. PurpleAir data originates from PurpleAir. Official station and forecast data originates from the AirNow program.

When health or safety is at stake, follow current guidance from public-health authorities and the source agency for your area.

Methodology last updated July 31, 2026