For all spaces that may contain hazardous gas (such as garages, janitors' closets, and labs), calculate exhaust rates to confirm adequate negative pressurization. The pressurization requirements are: 

an exhaust rate of 0.5 CFM/SF with a pressure differential in relation to surrounding spaces of at least 5 Pascals (Pa)—.02 inches on water gauge—on average;
and when doors are closed, 1 Pa—.004 inches on water gauge—at a minimum.