Home Safety Score Methodology
Model version 4.0.0 - Effective August 4, 2026
What the score represents
The Home Safety Score is a 0-100 screening score for evidence available at ZIP or mapped county level. Higher values mean lower measured concern within the current model. The score is not a property inspection, health assessment, insurance rating, or statement that every home in the ZIP is safe.
ZipCheckup's Home Safety Profile is broader than the score. The product registry has 17 baseline verticals and can add future verticals through a versioned admission process. A vertical can appear as context without changing the score. It enters a score model only after its source, units, vintage, geographic mapping, coverage, and independent blocking validation are accepted.
Current model
Model 4.0.0 has three equally weighted domains. Every listed component is required.
| Domain | Component | Source |
|---|---|---|
| Drinking water | Water compliance | EPA SDWIS system identity plus accepted EPA ECHO SDWA violation lifecycle evidence |
| Drinking water | PFAS MCL screen | EPA UCMR5 |
| Indoor environment | Radon zone | EPA Map of Radon Zones |
| Outdoor environment | Annual AQI | EPA AirData annual county AQI |
The two drinking-water components are averaged into one domain score. The three domain scores are then averaged. This prevents two water inputs from giving drinking water twice the weight of radon or outdoor air.
Component normalization
Water compliance
The water component starts at 100. Current unaddressed and current addressed health-based violations subtract 16 points each. Other current violations subtract 8 points each. Resolved violations retain one quarter of their original penalty. Archived records receive no penalty. The component cannot fall below 0.
Only a current, typed public-water-system match is eligible. A nearest-system suggestion, quarantined match, hollow record, private-well area, or missing system identity is not direct water evidence.
PFAS MCL screen
A tested UCMR5 record receives 100 when the accepted current-MCL screen has no exceedance and 0 when it has an exceedance. A ZIP without qualifying UCMR5 testing is unavailable. It is not assigned 100.
UCMR5 coverage is partial by federal sampling design. This component is a screen of published monitoring evidence, not proof that an untested home or water source is PFAS-free.
Radon zone
| EPA radon zone | Component score |
|---|---|
| Zone 3 | 100 |
| Zone 2 | 50 |
| Zone 1 | 0 |
The EPA zone is county-level potential, not a measurement inside a specific home. EPA recommends testing regardless of zone.
Annual AQI
The AQI component requires at least 90 days with reported AQI in the accepted annual county file.
| Median annual AQI | Component score |
|---|---|
| 0-50 | 100 |
| 51-100 | 75 |
| 101-150 | 50 |
| 151-200 | 25 |
| Above 200 | 0 |
Counties without qualifying annual monitoring coverage are unavailable.
Coverage and missing evidence
There is no neutral imputation and no silent reweighting:
- Missing component evidence is
null, not zero and not safe. - A domain is unavailable if any component required by that domain is unavailable.
- The overall score and grade are unavailable unless all three domains are available.
- Coverage is published with the component and domain results.
This policy makes scores less widely available than a fallback model, but keeps two ZIPs comparable under the same formula.
Grade scale
| Grade | Score |
|---|---|
| A | 85-100 |
| B | 70-84 |
| C | 55-69 |
| D | 40-54 |
| F | 0-39 |
Grades summarize the current model only. They do not certify a property or make a claim about verticals outside the model.
Registry and future verticals
The original 17 vertical IDs are an immutable baseline, not a permanent maximum. A new vertical:
- is appended under a new registry version;
- begins as candidate, context, or absent;
- records known source evidence or debt while it remains outside the score;
- can enter a later score only after an accepted canonical source, exact blocking gates, compiled adapter, independent adapter test, and non-overlapping lineage are added; a source may supersede an earlier source in the same vertical only through one explicit linear chain whose terminal source becomes active;
- requires a model-version change and full consumer parity before publication.
Lead remains a candidate because the current independent cross-source comparison does not yet support score admission. Historical NFIP claim records remain flood context and do not represent current property-level flood hazard. Derived composites such as mold, school health, or environmental justice are not treated as independent score inputs when they reuse evidence already present elsewhere.
Data freshness
Each source follows its own publication schedule. A page publication date does not mean every source changed that day. ZipCheckup records source receipts and only rebuilds this derived score after the registered source and value gates pass.
Limitations
- ZIP and county evidence is not property-specific.
- Public water data does not measure a home's private plumbing or private well.
- UCMR5 PFAS monitoring does not cover every system or ZIP.
- County radon zones cannot replace an in-home radon test.
- Annual AQI covers monitored counties and does not reproduce indoor exposure.
- A high score does not mean other hazards are low.
- Context verticals, including flood history, wildfire, earthquake, Superfund, housing, infrastructure, and school health, do not change model 4.0.0.
Version history
| Version | Date | Change |
|---|---|---|
| 4.0.0 | August 4, 2026 | Replaced the water component's SDWIS-only source contract with accepted SDWIS system identity plus ECHO SDWA lifecycle evidence; current addressed and unaddressed records receive full weight, resolved records retain 0.25 weight, and archived records receive zero weight |
| 3.0.0 | July 30, 2026 | Replaced the water-centered fallback formula with required cross-vertical accepted evidence, explicit coverage, an append-only vertical registry, and source-lineage protection |
| 2.1.0 | July 29, 2026 | Retired from current use; retained only as historical methodology |
| 1.x | March 2026 | Earlier water, lead, radon, and claim-history formulas |
Questions or corrections can be sent to [email protected].