By Doug Hoffman

I am a great believer in the idea of bringing in air from outdoors to mix with the air indoors. This is known in the industry as “fresh air make-up,” and it is actually required by most jurisdictions in commercial construction to improve indoor air quality (IAQ). This strategy of diluting the air to reduce contaminants has been touted for years as the answer to all IAQ problems, but does it really work?

The concept of dilution sounds like a good idea, and it certainly can be argued that if you overwhelm an IAQ problem by mixing in cleaner air there will be a benefit.  In fact, we teach fresh air make-up at NORMI and recommend that it be used in every environment, including residential construction, because of benefits such as reduced carbon dioxide. However, using fresh air for dilution is not a fix-all for every IAQ problem, and it certainly doesn’t address the root cause of many IAQ issues.

Part of the problem with using fresh air make-up as a universal solution is actually found in the phrase, “fresh air.” Just because the air comes from outdoors does not mean that it is automatically “fresh” or desirable for IAQ. For example, when outdoor spore and pollen counts are elevated during certain times of the year, is it advisable to bring that air inside?  In the southern climate (where I live), when the mugginess outside prevails, is it helpful to bring in air that will need to be cooled and dried out? How might that affect the efficiency of my air conditioning system?  If I lived next door to a meat processing plant or paper mill, would I want to bring all those odors into my living environment?  Solving IAQ problems is not as simple as just diluting indoor air with outdoor air.

Fresh air make-up is one part of the solution to indoor air pollution, but care should be taken as to its design.  Consider whether the outdoor air could be brought into the air conditioning system ahead of the filtration system (upstream, before it gets to the air handler) so any particulates could be filtered out prior to entering the environment.  Some clients add a filtration device on the fresh air inlet itself to reduce contamination.

Where possible, any fresh air ducting should be placed inside of the conditioned space so the hotter air from outside can be cooled before entering the air handler.  This is a very successful way to reduce energy costs and increase air conditioning and heating efficiency.

Finally, in addition to dilution, address indoor air pollutants directly with good air filtration and proactive air purification. Utilizing these two components, even the outdoor air can be cleaned as it is brought into the indoor environment. When outdoor air is filtered, heated or cooled, and purified prior to entering the indoor space, it can have significant benefits for IAQ; however, dilution through fresh air make-up is just one part of a holistic solution to indoor air quality.

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Doug Hoffman is the CEO of NORMI, the National Organization of Remediators and Mold Inspectors. He can be reached at or at 877.251.2296 x 876.