In 1954 Alfred Hitchcock directed the movie “Dial M For Murder” starring Ray Miland, Grace Kelly and Robert Cummings.  The story is about a husband who carefully plans to have his wife murdered. If the plot goes as planned the wife is gone and the husband is never suspected. As with many ideas, controlling all aspects of the concept often is seldom accomplished.

In the world where air pollution and airborne contaminants are a common occurrence, it is interesting to see how many individuals, maintenance companies, contractors and IAQ professionals under estimate the ability to control the levels of these contaminants. The initial plan often include using the least expensive filters or filtration device as many mistakenly believe one type of air filter is as good as another. This is seldom true.

One of the common mistakes is to only look at the particles we can see with the naked eye as the harmful contaminants. The irony is that particles you can see with the naked eye are about 20 microns or larger. Most of the particles that are harmful or that are aspirated into the lungs are less than 10 microns in size. These tiny particles are not easily trapped in general duty filter media, which are typically the type used when attempting to provide a cleaner environment.

Most HVAC systems use filter racks located in return air grills. These are normally the place for standard pleated media or ring panel filters and typically have a low MERV rating. (usually  MERV 5 to 7) These air filters will capture almost all of the 20 micron particles but their efficiency drops off dramatically as the size of the particles gets smaller. The other variable to take into consideration is the resistance of the filters relative to their overall efficiency.

The standard air filters used in most HVAC systems have a lower efficiency but also a lower pressure drop or resistance. This is important in the scheme of things as putting in a more resistant filter would reduce the air flow. On the up side though, they would be more effective in removing the harmful airborne contaminants.

HEPA filters are extremely effective at removing very high percentages of extremely small particles, those particles that are truly harmful. Their standard efficiency is 99.97% at o.3 microns. For comparison, the standard air filters used in most HVAC systems are approximately 15% to 20% effective at removing these tiny particles and that only takes place close to the time they have to be replaced.

Because of the design of HEPA filters, they are normally part of a system or package that also includes another blower/motor assembly to assist in moving the air across the HEPA as well as picking up the additional resistance across the filter. For this reason most HEPA filter systems are attached in a partial bypass mode to the HVAC ducting systems so no additional resistance is added to the entire air handling unit.

Pure Air Systems has been making HEPA and Carbon based air filtration systems for over 26 years. They have a HEPA system that will work for any type of environment and any kind of HVAC system. Even for areas where no ventilation systems exist. For more information on their complete line of HEPA and Carbon based units please go to their website at: www.pureairsystems.com or on Twitter @pureairsystems. You can also call them on their toll free line. 800-869-8025.




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About the Author: Don Musilli

My name is Don Musilli. I started Pure Air Systems in 1985 and then sold it in 2006. I continue to consult for PAS and write their blog and control their Twitter account. I also, on occasion, make changes to the website. The company is now almost 26 years old and has been a major player in offering commercial grade, high performance HEPA and Carbon based filtration systems for the commercial, institutional, industrial and residential markets.

I currently reside in Englewood, Florida where I write blogs and do social media marketing for a number of clients.


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