Home Safety Score Methodology
Version 1.2 — Last updated: March 18, 2026
What Is the Home Safety Score?
The Home Safety Score is a composite rating from 0 to 100 that summarizes measurable risk factors for any U.S. ZIP code: water quality, lead contamination, radon exposure, and flood risk. Higher scores indicate safer conditions.
Every score is derived entirely from federal government data. We do not accept payment to influence scores or rankings.
ZIP-to-Water System Mapping
A single ZIP code may be served by multiple public water systems — and a single water system may serve multiple ZIP codes. This many-to-many relationship is common, especially in suburban and rural areas where small community water systems, mobile home parks, and schools each operate independently.
When a ZIP code is served by multiple systems, we aggregate violations across all systems that serve that ZIP. The water quality sub-score reflects the combined compliance picture rather than cherry-picking one system. Lead level data uses the primary (largest population served) system for that ZIP, since 90th percentile lead levels are system-specific and cannot be meaningfully averaged.
Resolved vs. Unresolved Violations
EPA SDWIS tracks both active (open) violations and resolved (returned to compliance) violations. We include both in our scoring, but apply different weights:
| Status | Weight | Rationale |
|---|---|---|
| Active / unresolved | 1.0x | Indicates an ongoing compliance failure |
| Resolved / returned to compliance | 0.25x | Historical context preserved, but communities that fixed problems are not unfairly penalized |
This approach balances two goals: (1) giving credit to utilities that address problems, and (2) preserving the historical record so users can see whether a system has a pattern of violations, even if each individual violation was eventually resolved.
Score Weights
The Home Safety Score (0–100) is the sum of equally weighted components. The number of components depends on data availability for the ZIP code:
When FEMA flood data is available (4 components × 25 points each):
| Component | Max Points | Weight | Data Source |
|---|---|---|---|
| Water Quality | 25 | 25% | EPA SDWIS |
| Lead Level | 25 | 25% | EPA LCR samples |
| Radon Risk | 25 | 25% | EPA Radon Zones |
| Flood Risk | 25 | 25% | FEMA OpenFEMA NFIP Redacted Claims v2 |
Fallback — when no FEMA flood data exists for a ZIP (3 components × 33 points each):
| Component | Max Points | Weight | Data Source |
|---|---|---|---|
| Water Quality | 33 | 33% | EPA SDWIS |
| Lead Level | 33 | 33% | EPA LCR samples |
| Radon Risk | 33 | 33% | EPA Radon Zones |
Equal weighting was chosen because each component represents a distinct, independent health risk. Future components (e.g., PFAS, air quality) may adjust the weighting model — any changes will be documented here with a version bump.
Score Components
The score is the sum of equally weighted components. When FEMA flood data is available, there are four components worth 0–25 points each. When FEMA data is unavailable for a ZIP, the score falls back to three components worth 0–33 points each.
1. Water Quality (0–25 points / 0–33 points fallback)
Source: EPA Safe Drinking Water Information System (SDWIS)
Measures compliance of public water systems serving the ZIP code.
| Condition | Points (4-component) | Points (3-component fallback) |
|---|---|---|
| Zero health-based violations | 25 | 33 |
| Monitoring/reporting violations only | 10–25 (−2 per violation, minimum 10) | 15–33 (−2 per violation, minimum 15) |
| Health-based violations | 0–25 (−5 per violation) | 0–33 (−5 per violation) |
Health-based violations (Maximum Contaminant Level exceedances, treatment technique failures) are weighted more heavily than monitoring or reporting violations because they indicate actual contamination rather than paperwork gaps.
2. Lead Level (0–25 points / 0–33 points fallback)
Source: EPA Lead and Copper Rule (LCR) Sample Results
Uses the 90th percentile lead level (PB90) from the most recent sampling round at the primary water system.
| Lead Level (mg/L) | Points (4-component) | Points (3-component fallback) |
|---|---|---|
| Below 0.005 | 25 | 33 |
| 0.005–0.015 | Linear scale (25 → 0) | Linear scale (33 → 0) |
| Above 0.015 (EPA Action Level) | 0 | 0 |
| No data available | 17 (neutral) | 22 (neutral) |
The EPA Action Level for lead is 0.015 mg/L (15 parts per billion). Levels above this trigger required remediation. When no sampling data exists for a ZIP code's water system, we assign a neutral score (17 in 4-component mode, 22 in 3-component fallback) rather than penalizing or rewarding the absence of data.
3. Radon Risk (0–25 points / 0–33 points fallback)
Source: EPA Map of Radon Zones
Uses the EPA's county-level radon zone classification (3,138 counties mapped).
| EPA Zone | Risk Level | Points (4-component) | Points (3-component fallback) |
|---|---|---|---|
| Zone 3 | Low (< 2 pCi/L) | 25 | 33 |
| Zone 2 | Moderate (2–4 pCi/L) | 13 | 17 |
| Zone 1 | High (> 4 pCi/L) | 0 | 0 |
| No data | — | 17 (neutral) | 22 (neutral) |
Radon is the second leading cause of lung cancer in the U.S. Zone 1 counties have predicted average indoor radon levels above the EPA's recommended action level of 4 pCi/L.
4. Flood Risk (0–25 points, when FEMA data available)
Source: FEMA OpenFEMA NFIP Redacted Claims v2
Uses the count of National Flood Insurance Program (NFIP) claims filed for properties within the ZIP code. Higher claim counts indicate greater historical flood exposure.
| NFIP Claims (ZIP code total) | Points |
|---|---|
| 0 claims | 25 |
| 1–10 claims | 20 |
| 11–50 claims | 15 |
| 51–200 claims | 10 |
| 201–1,000 claims | 5 |
| > 1,000 claims | 0 |
When FEMA NFIP data is not available for a ZIP code, the Flood Risk component is omitted and the score reverts to the 3-component fallback (water quality + lead + radon, each worth up to 33 points).
Grading Scale
| Grade | Score Range |
|---|---|
| A | 85–100 |
| B | 70–84 |
| C | 55–69 |
| D | 40–54 |
| F | 0–39 |
Calculation Example
ZIP 60172 (Roselle, IL):
- Water Quality: 1 health violation → 33 − 5 = 28 points
- Lead Level: PB90 = 0.010 mg/L → 33 × (1 − (0.010 − 0.005) / 0.010) = 17 points
- Radon: DuPage County, Zone 1 (High) → 0 points
- Total: 45 → Grade D
Limitations
- Data freshness: EPA SDWIS and LCR data may lag 30–90 days behind real-time conditions. Our system updates daily, but upstream data depends on utility reporting schedules.
- Geographic resolution: Water quality and lead data are linked to public water systems, not individual homes. Your home's plumbing age, condition, and private well status are not reflected.
- Radon zones are county-level averages. Actual radon levels can vary significantly between neighboring homes. The EPA recommends testing every home regardless of zone.
- Four factors (or three in fallback mode). The current score covers water quality, lead, radon, and flood risk (where FEMA data is available). It does not include air quality, soil contamination, or other environmental hazards. We plan to add components as reliable data sources become available.
- No data ≠ safe. When lead or radon data is unavailable for a ZIP code, we assign a neutral score rather than assuming safety (17/25 in 4-component mode, 22/33 in fallback mode). This is clearly marked on each page.
- Not a health assessment. The Home Safety Score is an informational tool based on public data. It is not a substitute for professional water testing, home inspection, or medical advice.
PFAS Data
PFAS (per- and polyfluoroalkyl substances) data displayed on ZipCheckup comes from the EPA's Fifth Unregulated Contaminant Monitoring Rule (UCMR5), which required sampling during the 2023 monitoring cycle. This data represents a snapshot from that period and may not reflect current PFAS levels in your water supply.
PFAS contamination can change over time due to treatment upgrades, source changes, or new contamination events. If PFAS is a concern in your area, we recommend contacting your water utility directly for the most recent test results, or commissioning independent testing through a certified laboratory.
The EPA finalized enforceable PFAS Maximum Contaminant Levels (MCLs) in 2024 (4 ng/L for PFOA and PFOS individually). Compliance monitoring under these new MCLs is ongoing and will be incorporated into our scoring as data becomes available.
Data Update Schedule
| Data Source | Update Frequency |
|---|---|
| EPA SDWIS (water violations) | Daily |
| EPA LCR (lead/copper samples) | Monthly |
| EPA Radon Zones | Static (updated when EPA revises) |
| Utility rate data (EIA) | Quarterly |
| Census / ACS demographics | Annual (with monthly BLS supplements) |
| PFAS data (UCMR5) | Static (2023 sampling cycle; updated when new monitoring data is published) |
| FEMA NFIP Claims (flood risk) | Periodic (updated when FEMA publishes new OpenFEMA data releases) |
Score History
Starting March 2026, we log daily snapshots of every ZIP code's score. Over time, this allows tracking whether water quality in your area is improving or declining. Score History data cannot be recreated retroactively — it accumulates from the day we begin recording.
Corrections and Feedback
If you believe a score is inaccurate, or if you have questions about the methodology, contact us at [email protected]. We investigate all reports and publish corrections when warranted.
Version History
| Version | Date | Changes |
|---|---|---|
| 1.2 | March 2026 | Added: 4th component — Flood Risk (FEMA NFIP Claims); updated score weights to 25/25/25/25 with 3-component fallback (33/33/33) when FEMA data unavailable |
| 1.1 | March 2026 | Added: ZIP-to-system mapping, resolved vs. unresolved violations, score weights, PFAS data vintage, expanded data update schedule |
| 1.0 | March 2026 | Initial methodology: water quality + lead + radon |