AQI vs. PSI: What Is the Pollutant Standards Index, and Why Do Countries Use Different Air Quality Scales?

If you've come across the term Pollutant Standards Index, or PSI, you've found a fossil of air-quality history that is still alive in parts of the world. In the United States, PSI was the original name of what is now the Air Quality Index; elsewhere — most prominently Singapore — PSI remains the name of the current official index. Knowing the relationship clears up a common confusion for travelers and for anyone reading older documents.

PSI was the AQI's earlier name in the U.S.

The United States began publishing a standardized national air quality index in the 1970s under the name Pollutant Standards Index. The name was apt: the index anchored its scale to the national ambient air quality standards, so that a score of 100 corresponded to the level of the relevant standard. Over time the index was revised — pollutants were added, breakpoints updated — and it was renamed the Air Quality Index (AQI), the term used today by EPA and AirNow. The core design survived the renaming: a 0–500 scale, color-coded categories, an anchor near 100 at the level of the short-term standard, and a rule that the day's reported value comes from whichever pollutant scores worst. The full modern specification lives in EPA's technical assistance document for reporting the daily AQI.

So when an older American report, textbook, or dataset says "PSI," it is describing the ancestor of today's AQI — same family, earlier revision. The mechanics of how such an index converts concentrations to a score are covered in how the AQI is calculated.

PSI today: Singapore's index

Singapore adopted an index of the same design and kept the PSI name, and it remains the country's headline air quality number — familiar worldwide from news coverage of Southeast Asian haze episodes. Like the U.S. AQI, it runs on a 0–500-style scale with named categories and reports the worst-scoring pollutant. But it is maintained by Singapore's authorities against Singapore's chosen benchmarks, so a PSI value is not interchangeable with a U.S. AQI value.

Why the same air can get different scores

This is the practical point for anyone comparing numbers across borders. National air quality indexes generally share a philosophy — convert measured concentrations into one simple public number — but differ in the details:

• Different anchor standards. Each index pegs its scale to its own country's air quality standards, and those standards differ. The U.S. levels are set out in the NAAQS table; other countries set their own. Move the anchor, and every score above and below it shifts.
• Different breakpoints and categories. Category names, colors, cutoff values, and even the number of categories vary between national indexes.
• Different pollutant sets and averaging times. Indexes differ in which pollutants they include and whether they use 1-hour, 3-hour, 8-hour, or 24-hour averages for each one.
• Different smoothing. The U.S. AQI uses the NowCast method to approximate current conditions from recent hours; other indexes handle real-time reporting their own way.

The result: identical air, measured identically, can yield noticeably different index values under two countries' systems — and neither is "wrong." Each is internally consistent with its own standards. This is the same lesson, internationally, as the domestic distinction between the AQI and the standards behind it: an index is a communication layer built on top of standards, and it inherits the choices of whoever set them. The American Lung Association's AQI guide is a useful plain-language reference for how the U.S. version communicates its categories.

The practical takeaway

• Reading a pre-renaming U.S. document? PSI ≈ early AQI; treat category descriptions as historical, since breakpoints have been revised.
• Traveling or reading foreign news? Check which index a number belongs to before mapping it onto the U.S. color categories. A "100" is only meaningful relative to the scale it came from.
• Comparing cities across countries? Compare the underlying pollutant concentrations (like µg/m³ of PM2.5) rather than index values — concentrations are the common currency; index numbers are national dialects.