does API (air pollution index) has a unit?

The API has no units, being a ratio of pollutant concentrations to a standard.

"The EPA's AQI 100 corresponds to 0.08 ppm ozone, and to other levels for other pollutants. Source: EPA"

Presumably a combination of pollutants is reduced to an equivalent to the effects of an ozone concentration (in ppm). This resultant is then divided by 0.08 ppm and multiplied by 100 to give a sort of percentage.

Canada's index is approximately twice as restrictive as that used in the US.