As others have mentioned, positive pressurization is a good idea. Helps with infiltration, VOCs and generally helps preventing dust/dirt accumulating in the space, which is bad for datacenter equipment.
One design I've seen done is to have each inline fan on a t-stat interlocked with a normally closed motorized damper. Each inline fan is ducted to a main exhaust duct/riser with the motorized damper in the branch duct near the main duct. Inline fan turns on and damper opens - fans turns off...