The Air Diffusion Performance Index (ADPI) is a single number index for design and evaluation of air diffusion systems. ADPI is defined as the percentage of the occupied zone which is considered comfortable (not objectionable) by 80% or more of the occupants. The index has been validated by over 250 tests with five different types of diffuser systems. Use of ADPI allows the system designer to specify human comfort conditions within the space as they relate to the performance characteristics of the diffuser and the room dimensions. This paper expands the definition of the ADPI to more closely relate to human comfort conditions and provides additional design data for use with air jet terminal velocities of both 0.5 and 0.25 meters per second.