Excellent 

Median and maximum pollutant levels in all parameters show little effect from human disturbances

Good

One or more parameters show minor or only occasional increases in pollutant levels from human disturbances

Average

Exhibits constant low levels of one or more pollutants or sudden significant, but short term increases

Below Avg.

Median pollutant levels are abnormally high in one or more parameters, or exhibits very high pollutant levels during certain weather conditions

Poor

Pollutant levels are consistently higher than average in several parameters and/or show extreme levels during certain weather conditions