What is the Gera US Air Health Index (GUSAHI)?

The Gera US Air Health Index (GUSAHI/100) is Gera's transparent composite measure of air quality for US counties. It combines three real EPA AQS measurements from 2023: annual mean PM2.5 (weight 40%), annual mean 8-hour ozone (35%), and annual days with AQI above 100 (25%), each min-max normalised across the 458 included counties. A higher GUSAHI means cleaner air. The formula is fully published on the methodology page so any reader can reproduce every value from the public EPA data.

Which US county has the best air quality in 2023?

Based on EPA AQS 2023 data, Bayamon County, Puerto Rico has the highest Gera US Air Health Index (94.8 / 100, rank 1 of 458), with an annual PM2.5 mean of 4.0 µg/m³, ozone mean of 21.5 ppb, and 0 days with AQI above 100. Source: EPA AQS AirData, June 2026.

Which US county has the worst air quality in 2023?

Based on EPA AQS 2023 data, Riverside County, California has the lowest Gera US Air Health Index (20.5 / 100, rank 458 of 458), with an annual PM2.5 mean of 9.4 µg/m³, ozone mean of 50.4 ppb, and 116 days with AQI above 100. Source: EPA AQS AirData, June 2026.

What are PM2.5 and ozone, and why do they matter for health?

PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometres or less. These tiny particles can penetrate deep into the lungs and enter the bloodstream, contributing to heart disease, lung disease, and premature death. Ozone (O₃) at ground level — formed when pollutants react in sunlight — can irritate airways, worsen asthma, and reduce lung function. The US EPA sets annual standards for both: 9 µg/m³ annual PM2.5 (2024 NAAQS) and 70 ppb 8-hour ozone (2015 NAAQS).

How many US counties are included in the GUSAHI?

The June 2026 GUSAHI covers 458 US counties that have monitoring data for all three metrics (PM2.5 annual mean, ozone 8-hour mean, and AQI day counts) in the EPA AQS 2023 dataset. Counties without monitors or with incomplete data for any metric are excluded entirely — they are never estimated or imputed. The mean GUSAHI across these counties is 55.5 / 100.

Where does the underlying air quality data come from?

All data comes from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS) AirData pre-generated annual files, published without a key or registration at aqs.epa.gov/aqsweb/airdata/. Gera uses the annual_aqi_by_county_2023.zip (for day counts) and annual_conc_by_monitor_2023.zip (for PM2.5 and ozone concentration means). Both are US federal government works in the public domain (17 U.S.C. § 105).

How is the GUSAHI different from the CDC PLACES County Health Risk Index on GeraClinic?

The Gera County Health Risk Index (GCHRI) on the CDC PLACES pages measures chronic-disease burden (obesity, diabetes, heart disease, smoking prevalence). GUSAHI measures outdoor air quality (PM2.5, ozone, bad-air days). Both are county-level and fully reproducible, but from different data sources and different health dimensions. Together they give a more complete picture of local environmental and health conditions.

How often is the GUSAHI updated?

The EPA AQS publishes new annual summaries each year (typically available by mid-year for the prior year). Gera re-computes and re-dates the GUSAHI each time a new EPA annual dataset is published. The current version covers 2023 monitoring data and was last updated 2026-06-20.

US Air Quality by County

Gera US Air Health Index (GUSAHI/100) for 458 US counties, computed from real EPA AQS June 2026 data on annual PM2.5 concentrations, 8-hour ozone means, and days with AQI above 100. Higher score = cleaner air.

Which US counties have the best and worst air quality, and what is the national average?

As of June 2026, Bayamon County, Puerto Rico has the highest Gera US Air Health Index (94.8/100) among 458 monitored US counties, with annual PM2.5 4.0 µg/m³ and ozone mean 21.5 ppb. Riverside County, California scores lowest (20.5/100). National mean GUSAHI: 55.5/100. Source: EPA AQS AirData June 2026.

Source:EPA AQS AirData — Annual AQI by County + Annual Concentration by Monitor, 2023·as of June 2026updated annually (last: 20 June 2026)

Gera US Air Health IndexMean: 55.5 / 100A transparent 0–100 composite of three EPA AQS annual air quality measures: PM2.5 annual mean (weight 40%), 8-hour ozone mean (35%), and days with AQI > 100 (25%), each min-max normalised across 458 counties. Higher = cleaner air. Range: 20.5–94.8/100.How this index is calculated

Cleanest and worst air quality counties

Top 5 cleanest US counties by GUSAHI — June 2026
Rank	County	State	GUSAHI
1	Bayamon	PR	94.8 / 100
2	Honolulu	HI	87.9 / 100
3	Skagit	WA	84.2 / 100
4	San Mateo	CA	81.3 / 100
5	Sonoma	CA	80.1 / 100

Top 5 worst air quality US counties by GUSAHI — June 2026
Rank	County	State	GUSAHI
1	Riverside	CA	20.5 / 100
2	San Bernardino	CA	21.1 / 100
3	Tulare	CA	22.6 / 100
4	Los Angeles	CA	30.5 / 100
5	Summit	OH	34.2 / 100

Air quality in the worst-rated county

Riverside County, California — EPA AQS June 2026
Metric	Value	Weight in GUSAHI
Annual mean PM2.5	9.4 µg/m³	40%
Mean 8-hour ozone	50.4 ppb	35%
Days AQI > 100	116 days	25%
GUSAHI composite	20.5 / 100	—

Find your county's air quality score

Search any of the 458 included counties to see its GUSAHI, PM2.5, ozone, and AQI day count.

Search for your county

Search for your county above, or browse the counties with the worst air quality (lowest GUSAHI):

Counties with the cleanest air (highest GUSAHI):

Browse all 458 counties

Frequently asked questions

What is the Gera US Air Health Index (GUSAHI)?: The Gera US Air Health Index (GUSAHI/100) is Gera's transparent composite measure of air quality for US counties. It combines three real EPA AQS measurements from 2023: annual mean PM2.5 (weight 40%), annual mean 8-hour ozone (35%), and annual days with AQI above 100 (25%), each min-max normalised across the 458 included counties. A higher GUSAHI means cleaner air. The formula is fully published on the methodology page so any reader can reproduce every value from the public EPA data.
Which US county has the best air quality in 2023?: Based on EPA AQS 2023 data, Bayamon County, Puerto Rico has the highest Gera US Air Health Index (94.8 / 100, rank 1 of 458), with an annual PM2.5 mean of 4.0 µg/m³, ozone mean of 21.5 ppb, and 0 days with AQI above 100. Source: EPA AQS AirData, June 2026.
Which US county has the worst air quality in 2023?: Based on EPA AQS 2023 data, Riverside County, California has the lowest Gera US Air Health Index (20.5 / 100, rank 458 of 458), with an annual PM2.5 mean of 9.4 µg/m³, ozone mean of 50.4 ppb, and 116 days with AQI above 100. Source: EPA AQS AirData, June 2026.
What are PM2.5 and ozone, and why do they matter for health?: PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometres or less. These tiny particles can penetrate deep into the lungs and enter the bloodstream, contributing to heart disease, lung disease, and premature death. Ozone (O₃) at ground level — formed when pollutants react in sunlight — can irritate airways, worsen asthma, and reduce lung function. The US EPA sets annual standards for both: 9 µg/m³ annual PM2.5 (2024 NAAQS) and 70 ppb 8-hour ozone (2015 NAAQS).
How many US counties are included in the GUSAHI?: The June 2026 GUSAHI covers 458 US counties that have monitoring data for all three metrics (PM2.5 annual mean, ozone 8-hour mean, and AQI day counts) in the EPA AQS 2023 dataset. Counties without monitors or with incomplete data for any metric are excluded entirely — they are never estimated or imputed. The mean GUSAHI across these counties is 55.5 / 100.
Where does the underlying air quality data come from?: All data comes from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS) AirData pre-generated annual files, published without a key or registration at aqs.epa.gov/aqsweb/airdata/. Gera uses the annual_aqi_by_county_2023.zip (for day counts) and annual_conc_by_monitor_2023.zip (for PM2.5 and ozone concentration means). Both are US federal government works in the public domain (17 U.S.C. § 105).
How is the GUSAHI different from the CDC PLACES County Health Risk Index on GeraClinic?: The Gera County Health Risk Index (GCHRI) on the CDC PLACES pages measures chronic-disease burden (obesity, diabetes, heart disease, smoking prevalence). GUSAHI measures outdoor air quality (PM2.5, ozone, bad-air days). Both are county-level and fully reproducible, but from different data sources and different health dimensions. Together they give a more complete picture of local environmental and health conditions.
How often is the GUSAHI updated?: The EPA AQS publishes new annual summaries each year (typically available by mid-year for the prior year). Gera re-computes and re-dates the GUSAHI each time a new EPA annual dataset is published. The current version covers 2023 monitoring data and was last updated 2026-06-20.

Concerned about air quality where you live?

Poor air quality is linked to respiratory and cardiovascular conditions. A GeraClinic care navigator can help you understand your county's EPA data, assess your personal risk, and connect you with specialist care — online, available now.

Talk to a GeraClinic care navigator How the Gera US Air Health Index works

Contains public sector information published by U.S. Environmental Protection Agency (EPA) and licensed under the U.S. Public Domain (federal government work, 17 U.S.C. § 105). Source: EPA AQS AirData — Annual AQI by County + Annual Concentration by Monitor, 2023 (June 2026, published 2023 (released 2024)).

Informational/educational only — not a substitute for professional medical advice; a clinician interprets results.