Negative air machines, or air scrubbers, circulate air and remove contaminants. They remove fumes, dust and drywall from other machinery and absorb other matter, such as mold and spores. The scrubbers create an area of negative pressure, which sucks air into the machine. The air passes through a filter, and the cleaned air then passes out. This maintains conditions in factories, hospitals and other buildings. The machines should change a building's air six times every hour. Use that figure and the building's size to find how many machines you need.

Multiply the building's square footage by its interior height. If the building contains 30,000 square feet of area, and its ceilings are 20 feet high: 30,000 x 20 = 600,000 cubic feet.

Divide the building's volume by your negative air machines' volumetric rate, measured in cubic feet per minute. If each machine moves 3,500 cubic feet per minute: 600,000 / 3,500 = 171.4. This answer is the number of minutes a machine takes to process the entire building's air.

Divide 60, the number of minutes in an hour, by how many minutes it takes a machine to process the air: 60 / 171.4 = 0.35. This answer is the number of air changes one machine produces per hour.

Divide 6, the industry standard for air changes per hour, by the number of air changes one machine would produce: 6 / 0.35 = 17 machines.

Divide the number of machines by the building's area: 17 / 30,000 = 0.00056 machines per square foot, or 5 to 6 machines per 10,000 square feet.
References
 Photo Credit rusted vent image by Karin Lau from Fotolia.com