E-E-A-T · Transparent by design
Methodology &
Data Sources
Every AQI reading on WildfireWindow carries full provenance — source, station, and age. This page explains exactly how we compute values and where our data comes from.
The 2024 EPA AQI Standard
In February 2024, EPA finalized a revision to the National Ambient Air Quality Standard (NAAQS) for PM2.5 — lowering the annual average standard from 12 µg/m³ to 9 µg/m³. This changes where the "Good" / "Moderate" breakpoint falls, effectively meaning more readings will display as "Moderate" than under the old standard.
WildfireWindow adopted the 2024 breakpoints as soon as they were published. All AQI values displayed use the updated table below.
| AQI Band | AQI Range | PM2.5 (Pre-2024) | PM2.5 (2024 ✓) |
|---|---|---|---|
| Good | 0–50 | 0.0–12.0 µg/m³ | 0.0–9.0 µg/m³ |
| Moderate | 51–100 | 12.1–35.4 µg/m³ | 9.1–35.4 µg/m³ |
| Unhealthy for Sensitive Groups | 101–150 | 35.5–55.4 µg/m³ | 35.5–55.4 µg/m³ |
| Unhealthy | 151–200 | 55.5–150.4 µg/m³ | 55.5–125.4 µg/m³ |
| Very Unhealthy | 201–300 | 150.5–250.4 µg/m³ | 125.5–225.4 µg/m³ |
| Hazardous | 301–500 | 250.5–500.4 µg/m³ | 225.5–325.4 µg/m³ |
AirNow NowCast Algorithm
Standard AQI is computed from 24-hour average concentrations — too slow for rapidly-changing smoke events. NowCast uses a weighted average of the past 12 hourly measurements, giving more weight to recent hours when conditions are changing quickly.
// NowCast weight calculation
w = max((Cmax - Cmin) / Cmax, 0.5)
NowCast = Σ(w^i × C[i]) / Σ(w^i)
// where i = hours ago (0 = current), C[i] = PM2.5 at hour i
A weight of 0.5 means only 50% weighting decay per hour (conditions stable); a weight near 1.0 means recent hours dominate (rapid change). This makes NowCast especially useful during fast-moving smoke fronts.
How We Validate
- 1
We ingest raw AirNow hourly PM2.5 concentration data from the AQS API.
- 2
NowCast weighting is applied: hours within the 12-hour window are weighted by the ratio (w) where w = max((Cmax - Cmin) / Cmax, 0.5), then each hourly concentration is multiplied by its weight. Weighted average is computed.
- 3
The NowCast concentration is mapped to AQI using the 2024 EPA linear interpolation formula.
- 4
Readings more than 3 hours stale are flagged and rendered in a "last known" degraded state.
- 5
AQI values are cross-checked against EPA's published NowCast reference values daily.