Notes on Air Filtration & Ventilation Systems
Overview
This section is meant as a general guide to those individuals who are looking at purchasing some form of air filtration system for their house, apartment, or bedroom. Before discussing specific applications several points should be addressed. The ideal living accommodation for an allergy prone individual should be dry (less than 40% humidity prevents dust mite breeding and inhibits mold spore production) and have no carpeting (carpets contain dust mites, molds, and usually pet allergen). Every single footstep on a carpet sends a cloud of allergy material (allergen) into the air. Carpets that are thicker can harbor more allergens but even the thinnest carpet contains unacceptable amounts of allergen. Air filtration and ventilation units will do their job most effectively when carpeting has been completely removed and encasements are put around pillows, mattress and box spring. Filtration units will not provide any meaningful reduction in allergen exposure when an allergy sufferer has their face on a pillow and mattress that are riddled with dust mites or when children are lying or playing on carpeting that is laden with dust mites, mold spores, and pet allergen. If you could see what was in a carpet with a microscope you would have great difficulty sleeping at night. I have had many allergy patients come back years later telling me that had they known what a difference removing carpeting would make they would have done this years earlier.
There are several types of frequently marketed air filtration units that should NOT be used including ionizers, ozone generators, and electrically charged electrostatic units. None of these come close to the efficiency of HEPA filters. Both ionizers and ozone generators produce ozone which is considered a ground level air pollutant and is a known respiratory irritant. UV (ultraviolet) filters are used to kill biologically active particles such as bacteria, viruses, mold spores, etc. UV units are totally unnecessary from an allergy perspective as particles are physically allergenic whether they are alive or dead. You need a filter that will physically remove actual allergen particles from the air. Activated carbon filters can remove odors/chemicals from the air but do not efficiently remove physical particles. Never use a humidifier in a central ventilation system as this increases the humidity in the house (which increases dust mite breeding and mold spore production) and frequently causes permanent mold problems in the ventilation ducting.
Reusable versus Disposable Air Filters
None of the washable-reusable air filters are efficient. We do now have available some disposable filters that are acceptable and quite efficient in removing most allergy particles. These filters are made of material that carries a permanent small electrostatic charge which is what attracts and holds the smaller allergenic particles. These filters get more efficient as they are loaded with particles and should be changed when you notice a reduction in airflow through them. I recommend “Filtrete” allergen filters made by 3 M (or similar filters) if you require a disposable filter in your ventilation system. These disposable filters are readily available at Canadian Tire, Home Depot, etc. There are different grades available and in general the more expensive the replaceable filter the better it is. Ideally the filter should have a MERV rating (explained below) of 12 or greater (never buy one with a MERV rating less than 10). Individuals living in apartments or in rented houses can go one step further if you have forced air heating, by buying an extra replaceable filter, and cutting out a piece of filter with a pair of heavy scissors to fit the incoming warm air vent into the bedroom (so that bedroom air passes through 2 filters).
HEPA filters
There is no question that HEPA filtration
units are efficient at removing the vast majority of indoor airborne
allergens. Many small portable units are
available for single rooms and there has been a significant increase in larger
units that can be placed on a central air ventilation system. Many of the systems are now approved as a
medical device and can be claimed as a tax deduction (as a medical expense). While the basic dust/mold free bedroom is by
far the most important avoidance measure, I recommend local HEPA filters in the
bedroom for those with significant mold allergies and pet allergy where the pet
remains in the house.
MERV rating (Minimum Efficiency Reporting Value)
Several years ago a new rating system was developed for air filters. This new rating system- MERV -is now used to assess air filter efficiently in a scientific and reliable way. The higher the MERV rating the better the air filter. In general, a MERV rating under 10 is not acceptable for allergies. A MERV rating between 10 and 12 would be considered in the lower end of acceptable for allergies. A MERV rating of 13 or greater is excellent and removes almost all allergen particles. There are now central air filtration units available with MERV rating of 16.
Central Air Conditioning
For those individuals who have forced air (gas, oil, electric) heating there is the option of installing central air conditioning. For an individual with significant outdoor allergies (molds, pollens) central air conditioning is recommended. It allows the windows to be closed and reduces the amount of mold spores and pollen particles entering the dwelling. Air conditioning reduces indoor humidity during the summer which reduces dust mite breeding and mold spore production inside the house (mold=mould). Another advantage of central air conditioning is that a HEPA filter can be installed with these units to provide constant cleansing of the indoor air (with or without the A/C or heating unit on).
Air Exchangers-HRVs (heat recovery ventilators)
These units are not complicated. They basically consist of a fan which blows fresh outside air into the house and allows stale indoor air to move outside. The HRV is designed so that the outgoing old house air heats the incoming fresh air to reduce heating costs. HRVs are extremely useful in houses that do not have central air conditioning but have a significant problem with indoor dampness. Running one of these units continuously can significantly reduce indoor humidity. Several considerations have to be kept in mind including the size of the dwelling, air movement within the dwelling, energy efficiency of the units, etc.
Tips
Some practical points regarding air filters are as follows:
-If you are looking at a portable HEPA filter, always run the unit prior to purchasing to ensure that it is quiet. These units will be running in the bedroom continuously and you want them to be as quiet as possible.
-Assess the energy efficiency of a portable HEPA filter. Most of the good units give you an idea of the electrical consumption of the unit when running continuously. You want to be as environmentally friendly as possible.
-If you are happy with a particular HEPA Filter unit that you bought, attempt to obtain replacement HEPA filters that you feel you will require for the lifespan of the machine. Once a specific model changes, you may not be able to obtain replacement HEPA filters anymore
-If you are thinking of installing central air conditioning with or without HEPA filtration or an air exchanger-HRV, you should definitely shop around. All heating and ventilation contractors will give you quotes along with details of specific units. The capital cost of the unit, filtration efficiency, energy consumption, filter replacement costs, and service availability are all factors that have to be considered. If you feel overwhelmed with this information just send me the information package and I will review the unit you are looking at and let you know if it is adequate from an allergy viewpoint. (It's quite easy for me to do with my engineering background.)
Dr. Bruce Sweet 2008