The following article I wrote was published in Indoor Environment Connections, the trade paper for IAQ professionals.
“Help! I’m looking for some guidance in getting the right air cleaner or air filtration system. I know the sharper image ozone producing ones are bad. can you direct me to something safe? Our building has no windows to open for air exchanges. I’m looking for something I can clean the air with on a daily basis.”
What do you recommend and why?
Applications
Before we talk about what types of filters are available and how they work, lets’ talk about why we need an air filter or cleaner in the first place. Do they have allergies? Are they sick and think having an air cleaner is going to cure them? What ever their intended needs, a filter or air cleaner should not be a substitute for source control and ventilation. If there’s a source of air pollution indoors, attempts should be made to remove it and not just filter the air.
Types of air filters
Most people think of an air filter as something you put in the furnace. That’s because air filters were traditionally used to protect the moving parts of the furnace from getting dirty.
Washable Electrostatic (passive electrostatic media)
These use electric charge to capture particles. They are made out of polyester or polypropylene. The advantage to using them is when they get dirty instead of replacing them you simply wash them. The MERV numbers on these probably do not exceed 5 (on a scale from 1-16 with 1 being the least efficient). They may load quickly and require periodic washing and maintenance, something homeowners tend not to do.
Disposable filters
The least expensive (and least effective) filters are made with fiberglass or paper held in a cardboard frame. They have low MERV ratings. The exception to this is media such as the 3M Filtrete™ which uses electrostatically charged fibers to help attract small particles. Electret media is synthetic fibers that are charged during manufacture. These filters require less media and hence add less pressure to the system, due to the fact that the fiber can collect particles completely around the fiber as opposed to the upstream side as in most mechanical filters.
If you want a disposable filter pick the one with the highest MERV number that your heating and air-conditioning system can handle. LEED and ASHRAE recommend a filter with a minimum of MERV 6. This is just going to protect the furnace; not clean the air. You can usually install a filter with a MERV number of up to 11 without restricting too much air flow and that’s pretty good. If you use one higher than MERV 11 the cooling coils may freeze up or the furnace may overheat.
Throw-away fiberglass or paper filters cost less than $1. Pleated throw-away filters (MERV 6 and higher) are between $6 and $12.The extra cost is worth it. Besides protecting moving parts and ductwork, they also better protect furniture, fixtures and people. The dirt has to go somewhere - one either captures it on the filter or it goes into the home on everything else.
HEPA (High Efficiency Particulate Air) Filters
HEPA filters are so designated because they remove 99.97% of particulates that are 0.3 microns in size. HEPA filters are manufactured using small pieces of microglass, dewatered on a moving screen that turns it into a paper filter media similar to what’s done to make cellulose paper. You can’t install a HEPA in a furnace or air cleaner without a fan to push the air through it because HEPA filters have a pressure drop in excess of 0.8 inches water gauge – way too much for the 0.1 allowed for filters in most residential systems.
How much do HEPA filters cost? Since a HEPA filter should remove just about everything in terms of particles, and is often used by people with allergies or some medical conditions, a HEPA filter is priceless.
Sorbent media (gas and odor) filters
Gaseous pollutants and odors come from sources including gas stoves, heaters, tobacco smoke, building materials, cleaning products, pesticides and paint.
The only effective gas absorption air filter is one with a high level of carbon – literally pounds. For all practical purposes, the slurry coated carbon pre-filters, the ones with carbon embedded in a thin polyester pre-filters, are not effective. In residential applications people might get two to three months of use out of a carbon impregnated media filter.
Some chemical compounds are more effectively absorbed with something other than carbon. Formaldehyde for example is best adsorbed with a blend of carbon and potassium permanganate impregnated aluminum oxide.
Types of air cleaners
Most people in the general public think of an “air cleaner” as something that cleans the air and takes out everything – dust, mold, allergens, smoke, dust mites, bacteria and gases. Basically, an air cleaner should make the air clean.
There are three types of air cleaners: those that remove particles and gases from the air; those that make them stick to other surfaces in the house; and those that claim to kill or neutralize contaminates but fail to remove them.
Ion Generators
Ionization has historically not been shown to be highly effective. Ionizers work by putting a charge on the dust particles. The charged particles then stick to walls, furniture and even people in the room. One of the problems is that the particles are not removed. When the charge wears off they fall back into the environment.
Ozone Generators
Ozone is not recommended. The IICRC S520 Standard Reference Guide for Professional Mold Remediation states that ozone can not be generated in sufficient quantities to kill or even suppress microbials (S520 pp.117) . An article in Indoor Environment Connections reports that there are intermediate by-products produced during the oxidation process including formaldehyde, VOCS and ultra fine particles.
Electronic and Electrostatic Precipitators (ESP) Air Cleaners
These work on the same principle as ionizers and ozone generators. They use charged particles to clean the air. However, before the air flows back into the room, an oppositely charged plate attracts and captures the particles.
These have been shown to be effective in removing small sized particles. The problem is that their efficiency goes down after a short period of time. They need to be constantly cleaned and most home owners don’t do this. Another problem is that all electronic air cleaners unintentional produce some level of ozone. The American Lung Association states that air cleaners should not produce any amount of ozone.
Antibacterial and Germicidal Air Cleaners
UV light is documented to kill microbial materials under static conditions. To be effective, however, a high intensity level of UV light and a long exposure time are required, something you can’t put in the hands of the consumers because of safety issues. For air moving in furnace or air-conditioning duct work the exposure time is only 1/10,000 of what if would take to really impact mold or bacteria. The only effective application is shinning UV light directly on the air-conditioning coils and drip pans.
DO AIR FILTERS WORK?
Particulate Filters
The MERV number is probably the best way to assess the effectiveness of a filter at removing particles. The MERV number (Minimum Efficiency Reporting Value) is based on a standard from the American Society of Heating Refrigerating and Air Conditioning Engineers, ANSI/ASHRAE 52.2 Method of Testing General Ventilation Air-Cleaning Devices. The standard uses laser particle counters. It’s called minimum because a filter gets the minimum reporting value based on its performance for twelve different particle sizes. Filters may have a MERV number from 1-16 with 1 being the least efficient; 16 being the most effective.
Air cleaners such as ionizers and electronic do not utilize the ASHRAE 52.2 because they are not filters. Stock fiberglass or paper filter are approximately MERV 1-4; Electrostatic filters MERV4; electret media MERV 11-12. While HEPA filters can be tested to ANSI/ASHRAE 52.2 and would have a MERV of 17-20, other methods of test such as the Institute of Environmental Sciences and Technology (IEST) RP-CC-1.4 and MIL Std 282 are traditionally used to test HEPA filters.
Filters with an efficiency of less that 20% (MERV 1 through MERV 4) must be tested per the arrestance test of ASHRAE 52.1. This might cause some confusion because a MERV4 filter, for example, might rate 75% on the arrestance scale but not remove small particles very well. Always check the MERV rating. For more information see the National Air Filtration Association’s Guide to Air Filtration available at www.nafahq.org.
Gas and Odor Filters
Activated carbon specially treated with a chemical for specific contaminants, potassium permanganate, and zeolite are the types of molecular filters used to remove gases and odors. There is currently no ASHRAE standard for testing the effectiveness of molecular filters. ASHRAE is putting the finishing touches on a standard for testing carbon and other molecular phase filters. ASHRAE 145P is currently out for public review.
Activated carbon lasts for about seven days and then you can throw them [filters] in the garbage. For that to be effective, an extensive steam regeneration system would be required, something which is not available for the residential or commercial markets. If it were it might cost upward of $50,000.
Do Air Cleaners Work?
Many air cleaners do not work as they claim. Remember, an air cleaner is supposed to clear the air of everything, not just particles. Many don’t remove particles as well as they claim and since carbon has a short life span, it is likely that none of them remove gases as effectively as promised.
Why do air cleaners, even HEPA air cleaners, not always perform as expected? It’s because of filter bypass. Lots of products have a HEPA filter but it is not installed in a proper manner to prevent air from leaking and going around the filter instead of through it. Having a good filter is one thing, Making sure it’s installed to prevent by-pass is equal important.
How about air cleaners without filters? Sharper Image is one example of a manufacturer who got into trouble with claims to clean the air that were unjustified. Air cleaners using the TIO2 (titanium oxide coated filters) technology is another thar doesn’t work.
TI02 works by shining UV light on a titanium oxide coated filter to create ions. The ions are supposed to react with pollutants and oxidize (burn) them into harmless carbon dioxide and water molecules. Oxidation is the same process by which ozone and ion generating devices claim to clean the air. Whether you believe in the process or not, their effectiveness is short lived at best. They may work under extremely controlled conditions for about thirty minutes in a test chamber such as that used to determine the Clean Air Deliver Rate (CADR). It would nice to see a test for thirty days or longer in a residential environment.
The Association of Home Appliance Manufacturers utilizes the Clean Air Delivery Rate (CADR) to compare air cleaners to each other. The CADR rates removal of particles – not gasses or odors – and for brand new units. Veeck says this is a good test when used properly. Improvements are being made to take into consideration how well the air cleaners perform when filters get loaded. There have been complaints from the manufacturer of at least one brand of HEPA air cleaner that the CADR test does not evaluate the effectiveness of an air cleaner to remove particles. One can get a CADR rate for anything put inside the test chamber. If particles stick to the walls in the chamber, as they do when ionizers or electronic air cleaners are used, a CADR will be reported even though the particles have not been removed.
On other fronts, an IESO committee is working on a portable air filter testing standard that would combine several methods of test including ASHRAE 52.2 and European testing standards to test the effectiveness of air cleaning devices.
The best way for to assess the effectiveness of an air cleaner in real life scenarios is to place a laser particle counter up to the exhaust of an air cleaner when it’s running. For a HEPA air cleaner, the number of dust particles exiting a unit should be 99.97% less than the reading obtained when measuring the room air. This means that in the average residential environment the exhaust should read close to zero.
What should people do?
First consider what the health effects are from breathing dust particles and chemicals. More research needs to be done in this area. Brandys says that as homes tighten up we have seen an increase in indoor air quality problems. Why is this? Have the levels of particulates gone up or have VOCs gone up? Is off-gassing inside green buildings more significant than we realize?
It’s likely that a combination of both particles and gases are affecting indoor air quality and our health. And consider that as chemicals off-gas from building materials, they attach to dust particles. When you breathe particles you are being exposed to both particulate matter and the chemical compounds attached to the dust.
What is needed is a combination of both particulate and gas removal, what most air cleaners claim to provide, yet as we have seen, many can not.
In the absence of effective air cleaners on the market to remove gases and odors, Brandys recommends ventilation using air exchangers. It may be possible to filter particulates using an air filter, but an outdoor air-exchanger is required to reduce the gases indoors.
As the cost of energy goes up it seems to have become attractive to make our buildings airtight and install ventilation systems. How much energy do these air exchangers cost to operate vs. having a leakier building? Has anyone studied the cost of health effects from living in a poorly ventilated building vs. the extra cost of heating a leaky building?
As more research is done on the health effects of green buildings we may find this has been poorly thought out. Just because a green building saves energy does not mean it produces a healthy environments and that needs to be dealt with.