The Pollutant Standards Index (PSI) values are derived from five major pollutants – particulate matter of 10 micrometres or less (PM10), sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide. In Singapore, the pollutant with the highest index over a 24-hour period is used to describe the air quality for the particular day, and Singapore has a 3-hour period PSI index.
The US Environmental Protection Agency (EPA) was using the PSI Index until June 2000, where EPA started using the new Air Quality Index (AQI). The AQI includes a new health risk category (unhealthy for sensitive groups) and two additional pollutants: ozone averaged over 8 hours, and fine particulate matter (PM 2.5).
Yes, US has been reported PM2.5 since year 2000. Go figure.
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