Dartmouth Atlas of Health Care

The Dartmouth Atlas of Health Care maps how medical resources are distributed and used across the United States. Its lasting contribution is two geographic units built from Medicare patient flows: the Hospital Service Area (HSA), a local market of ZIP codes whose residents mostly use a given hospital, and the Hospital Referral Region (HRR), a larger region organized around tertiary referral care. These are the standard way to define a hospital’s local market. The Atlas also publishes regional utilization, spending, and workforce measures. The data is free and public. Used in, for example, Lewellen, where Dartmouth Atlas HSAs supply the geographic market definitions for hospitals (with demographics drawn from the 2010 Census).

• Cost: free, public.
• API key: none required.
• Coverage: national; ZIP-to-HSA-to-HRR crosswalk plus regional utilization and spending measures, derived from Medicare claims.
• Home: https://www.dartmouthatlas.org/ (data at https://data.dartmouthatlas.org/)

Access

The geographic crosswalk and the measure files are plain downloads, no authentication:

# ZIP -> HSA -> HRR crosswalk (the workhorse file; vintage in the name)
curl -sL -o ZipHsaHrr19.csv.zip \
  "https://data.dartmouthatlas.org/downloads/geography/ZipHsaHrr19.csv.zip"
unzip ZipHsaHrr19.csv.zip   # -> ZipHsaHrr19.csv

The crosswalk columns are zipcode19, hsanum, hsacity, hsastate, hrrnum, hrrcity, hrrstate: every ZIP maps to exactly one HSA and one HRR, labelled by the dominant city and state. Utilization, spending, and post-discharge measure files (by HSA, HRR, hospital, or state) are downloadable from the same data portal under their topic sections.

Load in Python

import pandas as pd

xw = pd.read_csv("ZipHsaHrr19.csv", dtype={"zipcode19": str})
# Map a hospital's ZIP to its market:
zip_to_hrr = xw.set_index("zipcode19")["hrrnum"].to_dict()
# ~3,400 HSAs nest into 306 HRRs nationally.

Gotchas (the ones that bite pipelines)

• HSAs and HRRs are care markets, not Census or postal geographies. They are built from where Medicare patients actually go for care, so they cross county and state lines and do not nest inside CBSAs, counties, or ZIP-code tabulation areas. Do not treat an HRR as a metro area or merge it to county-level Census data without an explicit, area-weighted crosswalk.
• The crosswalk is versioned; match the vintage to your sample. The file is named for its ZIP vintage (ZipHsaHrr19 = 2019 ZIPs). ZIP codes are added, retired, and reassigned over time, and the HSA/HRR boundaries were defined from a specific Medicare era (the canonical 1990s definitions, periodically refreshed). Using a 2019 crosswalk on 2005 ZIPs (or vice versa) silently drops or misassigns codes. Pick the crosswalk vintage closest to your data.
• It is built from Medicare (elderly) flows. The market definitions and most utilization measures derive from fee-for-service Medicare claims, so they describe the over-65 population’s care-seeking. Applying them to commercially insured or pediatric populations imports an age-skewed market definition; note the assumption.
• Utilization and spending measures are age-sex-(race-)adjusted. The Atlas reports rates standardized for demographic mix (and price-adjusted for spending), not raw counts. Two regions’ “spending” are comparable by design but are not the dollars actually spent; read each measure’s adjustment notes before using it as a level.
• One ZIP, one HSA, one HRR; population is not split. The crosswalk assigns each ZIP wholly to a single HSA/HRR by dominant flow, with no fractional allocation. A ZIP that straddles two markets is assigned to one, so aggregating ZIP-level counts to HRRs is exact but the boundary is approximate.
• hrrnum/hsanum are the join keys, not the city labels. Match on the numeric hrrnum (306 HRRs) and hsanum codes; the hrrcity/hsacity labels are not unique and change formatting across files.

Geographic units

Unit	Count (national)	Definition
ZIP code	~40,000	Postal ZIP (vintage-dated in the file name)
HSA (Hospital Service Area)	~3,400	Local hospital market: ZIPs whose residents use a given hospital
HRR (Hospital Referral Region)	306	Tertiary-care referral region (nests HSAs)

Citation

Cite the Dartmouth Atlas, the file and its vintage, the URL, and the access date, for example: The Dartmouth Atlas of Health Care, ZIP-to-HSA-to-HRR crosswalk (ZipHsaHrr19), retrieved from https://data.dartmouthatlas.org/, accessed YYYY-MM-DD. Record the crosswalk vintage and, for measures, the reporting year and adjustment, so the geography and rates are reproducible.