Updated April 2026 · EPA Air Quality System
Air Quality Blog
AirHistory's research archive collects data-driven articles on U.S. air quality — covering 1,020 cities, 346 of them on improving 10-year trends and 244 worsening — using the EPA Air Quality System dataset as the single source of truth.
What This Blog Covers
Articles fall into four broad categories. Health and lifestyle pieces translate the EPA Air Quality Index into practical guidance for runners, asthma sufferers, parents of young children, and anyone whose daily routine puts them outside in the morning rush. Geography and ranking pieces surface the cleanest and most polluted U.S. metros and explain why specific cities cluster at the extremes. Pollutant deep dives walk through PM2.5, ozone, NO2, and PM10 separately because the source mix and health profile differ for each. Trend analysis pieces use the 10-year AQS history to show where air quality is genuinely improving versus where wildfire smoke is masking real progress.
Every article cites the underlying EPA data and links to the relevant city profiles, ranking pages, or pollutant glossary entries on AirHistory. For real-time alerts at your specific zip code, the EPA AirNow portal is the authoritative source — AirHistory focuses on the long-run pattern rather than today's reading. For raw underlying data, the EPA Air Quality System publishes the annual files this site builds on, and the World Health Organization air quality guidelines publish the health-protective thresholds many articles reference.
How These Articles Are Researched
Each article starts with a question that a real reader might Google — "is it safe to run outside today," "which cities have the cleanest air," "did COVID lockdowns help." We then pull the relevant slice of the EPA AQS dataset, compute the medians, trends, and counts that answer the question, and cross-check against the EPA's own annual Air Quality Trends report and the American Lung Association's State of the Air. Findings are written in plain English with the underlying numbers visible in tables and charts. Where the EPA's methodology has known limitations — for example, county-level monitoring gaps in rural areas — the article calls them out. Read the full AirHistory methodology for details on data cleaning, trend calculation, and the Air Quality Grade composite.
Articles
April 14, 2026
Is It Safe to Run Outside? AQI Guide for Runners
AQI thresholds for outdoor exercise, best time of day to run, and city rankings for athletes, backed by EPA monitoring data.
April 7, 2026
Best Cities for People with Asthma
The best US cities for asthma sufferers, ranked by unhealthy air days, average AQI, and trend direction using 10 years of EPA data.
March 30, 2026
How Wildfires Changed Air Quality in America
Wildfire smoke has reversed decades of air quality progress in western cities. Data reveals which areas are most affected and how far smoke travels.
March 24, 2026
Air Quality Before and After COVID Lockdowns
Did COVID lockdowns improve air quality? EPA data for 1,000+ cities reveals which areas saw the biggest drops, and whether improvements lasted.
March 17, 2026
Fastest-Improving Air Quality Cities in America
These US cities have the biggest AQI improvements over the past decade. EPA data reveals where air quality is getting better the fastest.
March 9, 2026
PM2.5 Trends: Which Cities Are Getting Worse?
PM2.5 fine particulate matter is rising in dozens of US cities. EPA data reveals where levels are increasing, driven by wildfires and industry.
March 3, 2026
Cities Where Air Quality Is Getting Worse
These US cities have seen the biggest increases in AQI over the past decade. Data shows which areas are declining, and why.
February 24, 2026
Best Cities for Air Quality in 2026
The top 20 US cities ranked by air quality using EPA data and Air Quality Grades. See which cities earned an A grade.
February 16, 2026
What Is a Good AQI Score? Ranges Explained
The Air Quality Index ranges from 0-500. A good AQI score is 0-50. Learn what each range means for your health with real city examples.
Frequently Asked Questions
Where does the data behind these articles come from?
Every chart and statistic in AirHistory's research articles comes from the U.S. Environmental Protection Agency Air Quality System (AQS), specifically the Annual AQI by County dataset published at aqs.epa.gov/aqsweb/airdata. The AQS aggregates daily readings from federally certified monitoring stations across all 50 states. AirHistory currently tracks 1,020 U.S. cities using ten years of AQS data. No estimates, no proxies, no synthetic numbers.
Do these articles offer medical advice for asthma or other conditions?
No. AirHistory articles surface the air quality data and explain what the numbers mean in general terms, but health decisions for asthma, COPD, cardiovascular disease, pregnancy, or pediatric conditions should be made with a qualified clinician. Where articles include health context, they cite the EPA, the American Lung Association, or the World Health Organization, and readers should treat that context as background rather than personalized medical guidance.
How often is this blog updated?
New articles publish on a roughly weekly cadence whenever a new analysis is ready. Underlying data is refreshed once per year when the EPA publishes the Annual AQI by County dataset for the prior calendar year — the current dataset was last refreshed April 2026. Older articles are updated in-place when underlying numbers shift after an EPA refresh, with revision dates noted on each piece.
Why do some western cities have worsening trends despite cleaner cars and power plants?
Of the 1,020 cities tracked here, 244 are on a worsening 10-year trend — and the overwhelming majority of those sit in California, Oregon, Washington, Nevada, Idaho, and Montana. The driver is wildfire smoke, not local emissions. Multi-day PM2.5 spikes from major fire complexes pull annual medians upward even in years where vehicle and power plant emissions continued to fall. Articles tagged "wildfire" or "PM2.5" go into more detail on the regional pattern.
Can I cite an AirHistory article in my own research or reporting?
Yes. AirHistory analyses build on U.S. government public-domain data and are free to cite. We ask that authors link back to the specific article and credit "AirHistory, with data from the U.S. EPA Air Quality System." For raw data citations, the EPA AQS bulk download portal at aqs.epa.gov/aqsweb/airdata is the authoritative upstream source.
AirHistory's research archive collects data-driven articles on U.S. air quality — covering 1,020 cities, 346 of them on improving 10-year trends and 244 worsening — using the EPA Air Quality System dataset as the single source of truth.