Is Air Quality Getting Better or Worse Where You Live? Reading a Multi-Year Trend

A different question than the rest of the site

Every other tool on this site answers a snapshot question: what does this ZIP, county, or state look like right now. The air quality trend tool answers a different one — is a county’s air getting better, getting worse, or staying about the same over time. That’s a genuinely useful thing to know if you’re weighing a long-term decision like where to buy a house, but it’s also the easiest of this site’s tools to misread from a single number.

Why one year alone isn’t a trend

A single year of EPA air data captures whatever actually happened that year — a smoke-heavy wildfire season upwind, an unusually calm or unusually stormy year, a temporary industrial event. Looking at just one year and calling it representative of “how this county is doing” risks mistaking weather for a pattern. A real trend needs more than one data point, which is the entire reason this tool exists separately from the ZIP score tool’s single-year air figure.

How the trend is actually measured

The tool compares the 90th-percentile AQI in the earliest complete year on file against the latest complete year on file — nothing fitted, forecast, or smoothed. That’s a deliberate choice: a fitted trend line is already a modelled estimate, and this site’s standard is deterministic computation over models wherever possible. Comparing two real, complete years keeps every number on the page traceable to an actual EPA record rather than a statistical construction layered on top of one.

Why a small move counts as “flat”

Year-to-year AQI naturally jitters even where nothing meaningful has changed — a slightly different mix of weather, a handful of measurement days one way or the other. To avoid reading that ordinary noise as a real trend, a move smaller than 6 AQI points between the first and last complete year is labeled “hasn’t moved much” rather than a small improvement or decline. Only a move of 6 points or more in either direction is called an actual trend.

What the headline can hide

Comparing only the first and last complete year means whatever happened in between doesn’t show up in the headline direction. A county can spike badly during one bad wildfire-smoke year in the middle of its record and still show up as “flat” or even “improved,” if its first and last years happen to land close together. That’s not a bug — it’s exactly what “first year vs. last year” means, stated honestly rather than smoothed away. Always read the full year-by-year table printed below the headline before drawing a conclusion, not just the two-word direction at the top.

When there’s no trend to report

A county shows no trend at all when it has no EPA air monitor — the same coverage gap that affects every air-based tool on this site — or when its monitor hasn’t yet reported at least two complete years (years with enough measured days to count as reliable). In both cases, the tool says so directly rather than guessing at a direction from incomplete data.

This guide is for informational purposes only. It explains how to read public EPA records and what this site's tools compute from them — it is not a health assessment, a water test, or a substitute for medical or professional advice about a specific address or a specific person's health.

Last reviewed: September 2026 · Against the trend tool's own methodology (lib/zip-score.js → trendDirection()).