The 3-hour Pollutant Standards Index (PSI) is based on PM2.5 concentration levels averaged over three hours. PM2.5 refers to fine particles in the air.
The 3-hour PSI is NOT an indication of how clear the sky is, and doesn't always correspond with how hazy it looks. Other factors such as weather and humidity also affect visibility.
The closest indication of air quality is the 1-hour PM2.5 concentration levels. This reading can change quickly when there is haze.
This article is accurate as of Sep 2015.