it's start morning because difference in pressure between 2 point. air move from and area are higher pressure , 2 an area are lower pressure the lower pressure in exhaust system is generator by a fan and it's call static pressure. the static pressure created by the fan. it's less than barometric pressure on the suction or inlet side on the fan.
and greater then barometric pressure on the discharge or outlet side. in act's equally and on direction and can be major.