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Building Ventilation – The Proper Air Changes Per Hour (ACH)

If you need ventilation consulting or testing discussed in this article, call us at 1-800-344-4414 or e-mail us at info@atlenv.com for details and a free estimate.

Written By:  Robert E. Sheriff, MS, CIH, CSP, President
April 5, 2020

The amount of fresh air—outside air—that needs to be introduced into a building varies widely depending upon the activity and level of occupancy.

Air Changes per Hour (ACH)

The most common term used to refer to the amount of outside air that needs to be introduced into a building is referred to as Air Changes per Hour (ACH).  The ACH can vary widely depending upon what is going on inside the building: for example – it is generally considered that 4 ACH’s is the minimum air change rate for any commercial or industrial building.  Other examples are Classrooms, 6 – 20 ACH (is it a lecture hall or a chemical laboratory?), a Machine Shop, 6 – 12 ACH, a warehouse, 6 – 30 ACH.

It is important to note that the application of the ACH concept is only intended as a general reference and does not include dealing with a specific situation where air contaminants are involved that require local exhaust ventilation to capture a chemical, dust, or gas before it gets into the building’s air stream.

The following is a good reference table for some of the more common building use and the ACH for those uses.  The following list comes from “the Engineering Toolbox website at www.engineeringtoolbox.com and is a reasonable reference for ACH’s.


Building/Room Air Change Rate in ACH
All Areas in General 4
Auditorium 8 – 15
Bakeries 20 – 30
Beauty Shops 6 – 10
Boiler Rooms 15 – 20
Classrooms 6 – 20
Computer Rooms 15 – 20
Dental Centers 8 – 12
Garages –Repair 20 – 30
Hospital Rooms 4 – 6
Kitchens 15 – 60
Machine Shops 6 – 12
Malls 6 – 10
Municipal Buildings 4 – 10
Police Stations 4 – 10
Precision Manufacturing 10 – 50
Shops, Paint 15 – 20
Wood Shops 5
Theatres 8 – 15
Warehouses 6 – 30
Waiting Rooms, Public 4


Please note that the above list identifies ACH rates without special consideration for industrial/commercial processes that may introduce air contaminants into the air that should be controlled through the use of local exhaust ventilation.

 MEASURING THE ACH 0 Two (2) Methods

There are two methods of determining the ACH rates in a building.  The first—and most obvious is—to actually measure the air flows.

The second method is to introduce a tracer gas into the air and measure it’s decline over time to determine the ACH.


Using a velometer, thermometer or vane anemometer, taking airflow measurements at all the exhaust and air supply points including all the local exhausts that may be used to control specific operations.  The calculation of ACH is simple.

ACH  = total air supply rate (feet/minute) x 60 minutes

Volume of space (ft3)

or ACH = total air exhaust rate (feet/minute) x 60 minutes

Volume of space (ft3) 

Which is better—using air supply totals or air exhaust rate totals?

Generally, using air exhaust rates is better only because most buildings exhaust more air than they supply.  A condition often referred to as “Negative Pressure.”  For example, it’s hard to open the Machine Shop’s door due to the lack of make-up air compared to exhaust volume.

Ideally, the best situation is to measure both exhaust air and supply air.  This will tell you what may have to be done to better balance the supply and exhaust air—usually to release the “negative pressure” condition in the building.

A negative pressure condition (too much exhaust—not enough supply) also creates temperature control problems such as too cold near the loading docks and too hot on the opposite side of the building.  The strong negative pressure also reduces the ACH because the fans must work harder using more electricity and developing less exhaust volume than desired.  Yes, Yes, I know it is very expensive to heat or A/C the additional supply air.

Start On The Roof

When actually taking the air-flow measurement, the roof is usually the best place to start.  It may require drilling a small hole into the ductwork to do a traverse of the duct to get an average air-flow rate.  (Don’t drill the hole too near the fan or a direct elbow if possible—too much turbulence—select a spot where laminar/linear flow is better.

For wall fans, do a traverse across the face of the floor as well.  The ideal traverse is at equal areas but measurement at ½ inch or 1-inch intervals is more practical.

There are other ways to measure the adequacy of air movement in a building.  ASHRAE (American Society of Heating, Refrigeration, Air Conditioning Engineers) has established, ‘Ventilation for Acceptable Air Quality” ASHRAE Standard 62.1-2016 which is primarily designed based upon human occupancy and recommends certain volume of air per occupant.  Example:  lecture classroom – 7.5 CFM/person, beauty and nail salons – 20 CFM/person.  This will be discussed in other articles—go to www.atlenv.com-newtecharticles-industrialhygienenews-ventilation.

Contaminant Control Ventilation

As mentioned earlier, local exhaust ventilation is the more appropriate method of capturing emissions from a particular process before it can get into the workroom’s air for example; Paint Spray, Booth, Welding, Chemical Blending, Grinder Dust.  Then best reference for this approach is the ACGIH Publication, “Industrial Ventilation—A Manual of Recommended Practice,” 29th edition, a two-volume set.

Using a Tracer Gas

The method of using a tracer gas to measure air changes per hour (ACH) is covered in other articles related to ventilation on our website.  Also under:  Ventilation.

For more information regarding building ventilation contact Atlantic Environmental. – Industrial Ventilation Specialists.

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