Demystifying Air Filtration and Purification

As one who spent many years in field research and development in indoor environmental remediation for those with severe chemical sensitivities, allergies, asthma etc. I believe this article is overdue. There are a bewildering variety of types of portable and central air cleaning systems each with their own strengths and limitations.

HEPA filtration
Benefits: True HEPA units are rated @ 99.97% of all particles 0.3 microns or larger going into the filter will be captured. This does not mean 99.97% of the particles will be removed from that space. In order to consistently see good results for those with severe allergies I recommend a minimum 1-1.5 air changes per hour for central systems. Portable systems are noisy and can only treat relatively small areas although they can be useful in a car or moldy hotel room.

Limitations: HEPA filters cannot capture gaseous contaminants or odors (Volatile Organic Compounds, VOCs). Many come equipped with activated charcoal and/or zeolite filters but these become loaded quite quickly and are then more of an odor flywheel; absorbing odors and other VOCs @ high concentrations then slowly releasing them again when the concentrations drop.
Class II registered medical device in Canada.

Turbulent Flow Precipitators 
Benefits: very quiet. Somewhat efficient; When Honeywell first released their TFT units they claimed 80% efficiency at particle sizes 0.5 microns or larger and a flow rate of 130 CFM.

Limitations: Pricey for the volume of air treated. In a discussion @ Honeywell’s TFP technology launch, the cost for this 130 cfm unit with efficiency of 80% @ 0.5 microns was virtually identical to that of a 675 CFM Amaircare true HEPA filter with 99.97% efficiency @ 0.3 micron particle sizes. Cannot capture household odors or other VOCs.

UV Photo Oxidation units
Benefits: If sized properly, they do a tremendous job of breaking down both gaseous and particulate organic compounds and odors in the air stream (VOCs) including those toxic VOS produced by some molds and mildew as well as killing viruses and bacteria. I’ve never seen a central Sanuvox system fail to control animal allergies for example (when sized properly). Simple to install in the return air drop to the furnace and the only regular servicing required is to change the UV photo oxidation bulb once every three years @ modest cost. Reported satisfaction has been very high in those who’ve used portable units for moldy hotel rooms when vacationing.

Limitations: Although very effective at breaking down the organic or carbon based gaseous and particulate matter in the air stream, they don’t actually capture the mineral dust left behind. Commercial and hospital units do integrate true HEPA filters as well, but are rather costly. Class II registered medical device in Canada. For those with extreme sensitivities I recommend burning the photo oxidation bulb in for 2-3 days before installing in the home. Although the manufacturers tests show that even when brand new, the bulb produces less than one per/million at the unit, some are off the scale sensitive and I’ve always been very conservative.

UV Photo Catalytic units Benefits: Similar to photo oxidation units above, but use different frequencies of UV to irradiate titanium dioxide coated panels to catalyze odors and other VOCs .
Limitations: Some may become sensitive to titanium dioxide dust from the photo catalytic panels, costs are high per unit of effectiveness for purchase and installation and annual maintenance are relatively high.

HRVs or Heat recovery Ventilators
Benefits: Exchanges indoor air for outdoor air. Can reduce household cooking and bathroom odors. Useful for reducing humidity in new homes from damp wood used in building.

Limitations: 1) This technology was primarily developed for residential use to reduce excess humidity; particularly in new homes. When sized properly and set at the highest flow rate these units will only exchange 1/3 of an air change per hour, while greatly increasing heating and cooling costs. When people see 80% efficiency advertised, they think they’re saving 80% of all the energy. This is not the case. They’re saving 80% of the theoretical amount of heat which could be exchanged from the ingoing air to the outgoing air; only a fraction of the total energy involved. 2) Allergens and contaminants from the outside are brought into the house often causing an increase in allergy or asthma symptoms. I do not recommend this technology for allergies, asthma or mold issues.

Electronic filters
Benefits: Relatively useful for capturing large and medium sized dust particles.

Limitations: Relatively expensive to install, produces small amounts of ozone which can be an issue for those who are sensitive, loses ability to capture very small particles quite quickly as the system accumulates dust, power packs are very expensive to replace, difficult to clean effectively and if put in dishwasher will permanently turn interior grey. Require monthly cleaning to maintain any degree of effectiveness. Quite effective for smaller particles when squeaky clean, small particle efficiency drops rapidly as system loads. Has no effect on household odors, contaminants or other VOCs including toxic VOCs produced by some molds and mildews.

Electrostatic filters.
Benefits: Quite low cost. Average efficiency on medium and large dust particles similar to electronic versions over a month. Produces no ozone, not quite as efficient when new as electronic but efficiency drops more slowly as it becomes loaded (conversation with National Research Council scientist).

Limitations: Requires monthly washing to maintain effect, not greatly effective against very small particles which cause much of the problems for those with airway issues. Has no effect on household odors, contaminants or other VOCs including toxic VOCs produced by some molds and mildews.

Special note re basements: Cold air drops and as it cools the relative humidity increases. This is why so many basements are musty or moldy. When installing any type of central air device it’s important to install a six inch return air pipe to within six inches of the basement floor. This allows the furnace blower to suck the cool damp air up while pulling warmer dry air down to the floor; de-stratifying the air. In most cases this will make the basement floor too dry for molds or mildew to thrive. In the summer time it will reduce cooling costs by bringing the pooled AC air back up to the living areas. In winter it will prevent warm head, cold feet syndrome. Although the difference between a five inch and a six inch pipe sounds small, the difference in air volume passing through is immense. Always use six inch pipe.

None of the above information is as medical advice. If vulnerable, as in all cases, consult with your trusted healthcare professional or other specialists and do your own home work.

The above is not intended as medical advice. As always, consult with your trusted healthcare professionals before making changes to your health regime.

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