Gera Air Health Index — Methodology

1. Purpose

The Gera Air Health Index (GAH/10) is a single per-local-authority score that combines population-weighted PM2.5 air pollution exposure with the published health burden from that pollution. It is designed to help renters and buyers compare areas on a metric that is more actionable than raw concentration figures alone: a low PM2.5 in a rural area with healthy demographics is meaningfully different from the same concentration in an urban area with higher vulnerability.

Important: this index is a location comparison tool, not a clinical health assessment. The underlying data is real official government data (OGL v3.0); the composite formula is GeraRent's own and is not endorsed by DEFRA or UKHSA.

2. Data Sources

3. Formula

Step 1 — PM2.5 sub-score (0–10)

Min-max normalisation across all 296 LAs. Lower PM2.5 = higher score (better). min = 4.20 µg/m3, max = 10.96 µg/m3.

Step 2 — Health context sub-score (0–10)

Min-max normalisation across 132 upper-tier LAs. Lower mortality fraction = higher score. min = 3.179%, max = 8.091%. Only available for upper-tier / unitary LAs.

Step 3 — Composite GAH (0–10)

Weight rationale: PM2.5 exposure is the primary pollution metric (0.6) because it directly measures the environmental burden. Health context (0.4) adjusts for the published burden-of-disease evidence, reflecting that the same concentration can have different population-level impacts depending on local demographics and co-morbidities. Pages for lower-tier LAs clearly label the GAH as "PM2.5 component only".

4. Reference Benchmarks

5. Limitations & Honest Caveats

• NO2 not included: The original RNT-A6 brief called for a NO2 component from DEFRA AURN monitoring stations. AURN data provides site-level readings; no pre-aggregated NO2 by-LA file is available from DEFRA without a geospatial join of the 1km grid. We chose honest omission over proxy substitution. The PM2.5 component captures the majority of health-relevant exposure.
• Lower-tier LAs (164 of 296): UKHSA does not separately publish mortality-fraction data for lower-tier districts (E07 codes). Their GAH equals the PM2.5 score alone and is labeled "PM2.5 component only" on every page.
• Modelled, not measured: DEFRA PCM values are model output, not direct measurements. They represent the best available estimate of population exposure but may differ from site-level AURN readings for specific locations.
• Annual mean only: Peak pollution episodes (high-DAQI days) are not captured. Annual means are the relevant metric for long-term health risk.
• 2024 data, updated annually: Gera updates GAH when DEFRA publishes new PCM data (typically October each year).

6. Data Licence and Reuse

Source data is published under the Open Government Licence v3.0 by DEFRA and UKHSA. The licence permits free reuse with attribution. The Gera Air Health Index — the composite formula and resulting scores — is GeraRent's own work and is not endorsed by DEFRA, UKHSA, or any government body. We publish the full methodology here so the index is independently reproducible.

Attribution required: "DEFRA Pollution Climate Mapping 2024 / UKHSA Fingertips indicator 93861 / Gera Air Health Index, GeraRent (gerarent.com/air-health-index), Open Government Licence v3.0."