You will come across two types of fans during your search for the best exhaust fan. It is important to understand your fans capabilities and limitations since proper ventilation is needed for heat control, fresh air, and plant transpiration. The first is a propeller or axial flow fan. This is the computer case fan or oscillating fan you are likely familiar with. This type of fan moves air perpendicular to the axis of rotation and is best suited for low-resistance, high-flow applications. The second fan type is a centrifugal flow fan. These types of fans are suited for high pressure-resistance, low-flow applications. These fans are usually seen inside of your homes furnace or inside of squirrel cage fans. Both fans are rated using Cubic Feet per Minute (CFM), and this number should be close to accurate when used for its specified application. An axial/propeller fan running inside a closed room will move the amount of air according to its CFM rating. Inside of this closed room, the air pressure or resistance is effectively zero. If you use this fan to try to extract air from the room, and in turn pull air in through an intake opening, the fan will encounter resistance. The fans CFM rating will drastically drop. Attach a carbon filter to the ducting, and it drops even more. Thought exercise: If a computer case fan rated at 120CFM is installed on a computer case that is 2 cubic feet in size, why does it still seem to blow warm air from the computer? If the 120CFM rating was accurate, it would be clearing the air in the case once every second. Your computer is not a nuclear reactor. The fans effective CFM drastically drops because it is put in a high-resistance application. In a centrifugal (or radial) fan the rotation of the impeller causes the air to travel through it in a radial direction, developing pressure as it does so. The enclosing structure determines the exhausting direction. This type of fan is uniquely suited for pulling air through high resistance obstructions, such as carbon filters, and pushing/pulling air through duct work.