Citizens for A Safe Learning Environment
www.chebucto.ns.ca/Education/CASLE

Portable Air Cleaning Devices                   February, 2002

Research has suggested that schools tend to have poor indoor air quality (IAQ) for several reasons, including a concentration of classroom materials (such as art and science supplies), tight budgets that reduce maintenance, and because of high occupant density (approximately four times as many people as office buildings). There is much pollution-generating activity amongst these people, and often there are non-existent, outdated or poorly maintained mechanical ventilation systems. Source control (avoiding the problem of dirty air) and ventilation (diluting pollution) help provide good air quality, but sometimes indoor air pollution needs to be reduced by the use of portable air cleaning devices.  The units may be used for temporary relief in a newly built or renovated school or in acute situations where lasting source improvements will take some time to achieve. Using portable air cleaners as a long term solution to a major source control issue is not likely the best solution. However, in some cases where staff or students suffer from severe sensitivity to otherwise normal levels of pollutants, a portable air cleaner may be of benefit.

Choosing the best unit:

Buyers tend to have four main concerns: noise, ease of filter replacement, replacement filter cost, and the machine's overall quality.  The task in choosing air cleaners is to find a good quality and effective air cleaner but also to make sure the device does not add to the pollution load.

Avoid machines with foam parts/foam filters, plastic parts, wood, glue, or glued wood, plastic housings.  Choose metal housings with baked on enamel.
Do the filters fit snugly?
Is there a HEPA filter, verified to HEPA standard?
Are activated charcoal or alternative VOC (volatile organic compounds) filters available if needed?
Is there an easy way to tell if filters are in need of changing?
How often do filters need changing in general?
Are filters easy to change, and maintenance easy?
Are motor fumes evident? There are two kinds of motors: open, where moving parts can be seen, and fully encased.  Some units filter the motor fumes.  Others use fully encased, low-odour  motors which have low power consumption and tend to be of better quality. A four to five year motor warranty indicates manufacturer's confidence in their product.

When choosing a portable system, is the machine built for safe, easy handling and moving? (wheels, handles, no sharp edges, not too heavy)  Is it CSA approved, properly grounded, etc.?  Is there a protective grill around any moving parts?
Is it as quiet as possible and not causing an uncomfortable level of air motion? Where is air directed?
What is the reputation of the manufacturer/supplier in regards to warranties and service?
What are indications this is a stable business so parts and replacement filters will always be available?
To function well, units need to be cleaned and serviced according to manufacturer's specifications and have the filter media replaced at the right interval.
The unit has to be cost effective, but well designed.  Well built machines tend to be worth their price.

Kinds of Air Cleaners:

1. Ionizers disperse negatively (or positively, depending on the design) charged ions into the air.  They attach themselves to particles and cause them to become negatively (or positively) charged so they will attach themselves to surfaces such as walls or furniture.  Research has shown ionizers to be less effective in removing particles of dust, tobacco, smoke, pollen and fungal spores than high efficiency particle (HEPA, see below) filters.

2. Turbulent Flow Precipitation (TFP) units target particulate, but they don't remove VOCs. Before HEPA filters became less expensive, TFP units were a good but somewhat less effective alternative.  TFP claim to remove about 95% of 1 micron size and 90% of .5-.9 micron, as compared to HEPA's 95% of 0.1 micron particles and 99.97% of 0.3 micron particles.  Some TFP manufacturers are augmenting their particulate removal efficiency by adding HEPA filters. One should watch for foam parts, as offgassing foam can be a problem for some people.

3. Electrostatic air purifiers can be up to 50% efficient (particle removal) on average (MERV 5-8),  but these can create finer particles when dirty and create ozone when clean.  They tend to be expensive to repair and require high maintenance.

4. Ozone Generating Air Purifiers. Ozone doesn't remove particles. It destroys odours. There is controversy over the safety of ozone-producing purifiers.  Ozone is a gas that reacts readily with other chemicals/substances to form many byproducts, from harmless water to harmful breakdown products such as aldehydes. Manufacturers insist purifiers that use low level ozone clean the air without causing harm. International health and safety regulators have strong doubts about this, and recommend avoiding ozone-producing units. Ozone reacts with and can damage lung tissue. NIOSH recommends the upper limit of .01 ppm not be exceeded at any time. The US FDA limits ozone to .05 ppm for medical devices.  The US EPA's standard is a maximum 8 hour exposure of .08 ppm.  No agency in the US or Canada has approved ozone air cleaning devices for use in occupied spaces.  There is also evidence that at concentrations that don't exceed public health levels, ozone doesn't effectively remove viruses, bacteria, mold, or many chemicals. Also, variations in outdoor ozone levels, and indoor conditions such as ventilation and proximity to the ozonator make it difficult to control the actual level of ozone in the breathing space.  When there are other more effective, no-risk air cleaning devices available, why choose ozone?

Note the difference between air "filters" and air "purifiers".  Electrostatic purifiers and ozone generating purifiers are under examination for other possible health impacts.  Manufacturers and salespeople with these products are not happy about the controversy, but international health and safety agencies recommend use of proven alternatives such as source control, ventilation and filtration.

5.  Particle Filters.  Particulate are minute droplets of liquids or physical solids suspended in the air.  Some are large enough to be seen and some are microscopic. (One source estimates about .01% of house particles can be seen. That 8% of those not visible are fungus and spores, dust mites and their feces.  Ninety percent of house dust is microscopic. Of course this depends on the house!)

Examples of particle types:
Biological: pollens, spores, molds, bacteria, viruses, hair, skin cells, insect byproducts, and food byproducts.
Mineral: asbestos, clays/silica, carbon, lead, man-made fibres, hydrocarbons.
Combustion products: tobacco or wood smokes, particles generated by cooking or heating appliances, and industrial processes.
Radioactive: radon-decay products

Particles are measured in microns. Particles between 5 and 10 microns tend to be caught in the nose and throat.  Particles smaller than this travel readily into the lungs.
Examples of particle sizes:

 About 2 years ago ASHRAE developed a performance rating based on knowing the composition of test dust and size, and also on particles that are respirable.  This is called a MERV rating (Minimum Efficiency Reporting Value).  MERV ratings are on a scale of 0 to 20, with 20 being the best rating. There are 3 or 4 labs in the US that test for MERV ratings.

This rating system removes the ambiguity of the "% efficiency" claims made by manufacturers. For example, 95% efficient (95% of what?  Is it 95% efficient at filtering out .3 micron particles? .1 micron particles?  Is the claim based on the number of particles removed or mass removed?) "Ninety-five percent efficient" referred to particle weight means that the filter will remove 90% of the weight of particles.  This is deceiving because only 10% of airborne particles contribute about 90% of the weight of all particles in air.  In other words, this 95% efficient filter can let 90% of the air's particles through.
An analogy would be:
It is like having a big dog that weighs 200 lbs and 99 one-pound pups.  When they come to a fence, the 99 pups easily run through but the heavy dog cannot.  It can be claimed the fence was 67% efficient because it stopped 67% of the dogs (particles).  Reality is that the fence is like a filter that let 99% of the particles through!  It is the little particles that can cause most of the problems.

Furnace dust filters (rock-catcher) 2-3% efficient at particle number removal. MERV 1-4
Media filters up to 8% efficient at particle removal,   MERV 1-4
electrostatic up to 8% efficient,   MERV 1-4
DC charged electrostatic (up to 12VDC)  up to 10% efficient,   MERV 5-8
All of these can be advertised as being up to 95% efficient if the claim refers to weight and not particle number removal.

The following have commercial applications:
High density media filters (used in hospitals in general surgery)  MERV 13-17
HEPA filters  99.97% efficient down to .3 microns and a MERV rating of 17-20

Typically, a MERV rating of 12 or higher represents a filter that can help sensitive students and staff.

Health effects from inhaling various particles:
irritation of eyes, throat, nose, lungs
impaired respiratory mechanics such as coughing, wheezing, shortness of breath
aggravating existing respiratory conditions such as asthma, allergies, or bronchitis
impacts on the immune system or central nervous system
cancer

How many particles per square foot is the safety target?
It is a moving target because different people have differing needs. Some individuals may have an asthma attack triggered at 500,000 particles per cubic foot while others are much more tolerant.  At 300,000 particles per cubic foot, one dealer I consulted reported his worst client sleeps well through the night, feels well rested, is less congested and uses less medication.

If a company states its filter will remove 90% of allergens such as pollen, they are probably referring to 2 microns in size.   Good filtration usually means that the target area will have only 1/3 or less of the contaminant level outside. Depending on outside levels, that may or may not be adequate for the occupants, however.
 

6. Chemical Filters. Volatile Organic Compounds (VOCs) such as those from furnishings, personal care products, combustion, and mold mycotoxins are missed by particle catchers, but can be absorbed by carbon.  Some VOCs are removed by HEPA filters, but mostly because a few chemicals attach themselves to particles that are caught in the filter.  Cutter defines VOCs as "one of a class of chemical compounds that contain one or more carbon atoms and tend to evaporate at room temperature and normal atmospheric pressure."

There are naturally occurring organic compounds too, such as mold mycotoxins and aldehydes and keytones which are breakdown products from mold mycotoxins, that won't be removed by particulate filters. Activated carbon or other media such as zeolite are needed to remove VOCs from the air.

Activated carbon filters: Tiny cavities trap gasses.  Natural ozone which is trapped actually helps to break down chemicals within the "pockets".  Several kinds of media filters are available for use in removing different target chemicals.

Also important is the possibility that building occupants may be sensitive to materials in the filters or filter machines (such as ozone or oils coming off a lower-quality, unfiltered fan motor). Most people do well with activated carbon filters, but sometimes highly sensitive individuals need alternatives such as VOC cartridges or filter beds of tiny glass beads.

Further to this, some carbon filters have a microbial pesticide in the form of a silane-modified quaternary amine that is bonded to the carbon.  While it does kill molds and bacteria/viruses, it may create unexpected and subtle adverse health effects in sensitive individuals.  Basic, unmodified, activated carbon tends to be preferable.
 

Filters Need Maintenance:

One drawback of filtration is that filters need replacing.  Manufacturers can only give estimates of the life of filters.  It all depends on how polluted the area being cleaned is and how long the filters have been in.

For media filters there comes a point where they begin to shed the pollution they have collected.  One house I was helping with had a strong odour of fish throughout the house and no one knew where it was coming from, as no fish had been cooked in months.  It was the air filter in an upstairs bedroom in need of clean filters.  In a tour of the old Halifax West High School we noted that rooms with HEPA and carbon portable filter units smelled of sweat and stinky sneakers! The filters were over-loaded.

HEPA filters become more efficient as they fill up - until they reach a point where they are clogged.  The rate of air flow slows down and consequently the machine loses the desired rate of air cleaning ability. As well, the filter may collapse and therefore lose its filtering capabilities.  Reinforced media filters are recommended.  Typically, HEPAs can last up to five years and high density media up to one, depending on the pollution load.

So, maintenance is important to effective filtration.  A service schedule (or a professional service package) that will maintain filter units is important.

The cost of replacement HEPA filters has gone down by more than half ($300.00 vs $125.00) in the past 3 years.

It is extremely important to view a particle count demonstration before purchasing.  A filter is less effective when it does not have a 100% air seal.
 

Air changes and cfm (Cubic Feet per Minute)

Reported cfm can be misleading.  The cfm of a motor in a filter unit may be 400 cfm, but the cfm of the same motor within the machine, but with filters slowing down the air flow, could be half that.  Unless you know the cfm is a measurement of the functioning unit and not just of the fan capability, air changes per hour might be more useful to know. Four air changes per hour is approaching "hospital" standard and isn't usually necessary.  A machine with 250 cfm can change the air approximately once an hour in a room up to 1500 square feet with an 8 foot ceiling.  Depending on the air circulation in the room, there may be stagnant areas that are not being changed in the "once an hour".

Integrated Systems:
Some supplementary units can be integrated into new building systems or can be added to existing buildings, usually above the T-bar. They can serve single or multiple rooms, but should be sized accordingly.  The systems can have zero pressure drop, and should be sized for close to 1 air filtration per hour.  If ducted to the outside, they can be set with either a positive or negative pressure.  Systems can have flexible, semi-rigid ducting to help improve air circulation.  Supply and exhaust point away from each other and as far apart as possible.  Polypropylene soft plastic is more stable than some flexible ducts, and some manufacturers are colouring their polypropylene ducts to differentiate them from the cheap dryer ducts. Rust-proof metal ducts may be preferable, and should be washed to remove any surface oil film.  Be sure installation is done by a professional.

Some Practical Points about Portable Air Cleaners in Schools:
Does the capacity of the machine fit the area of the space to be cleaned?  If doors to halls or windows are open the machine may be trying to clean an area too big, but unless there is a lot of air flow to or from the hall or the outdoors, this may not be a problem.  Even if fresh air is introduced through open windows, the unit should stay on constantly.  A good system will be tested to run continuously 24 hours a day.

A larger machine being run on "low" may lengthen the time between filter changes and also be less noisy than a smaller machine roaring away on "high".

It may be best to fasten them to the wall or at ceiling level, depending on the model and the security issues. A general rule if ducts are added, is to "supply high and exhaust low".

Some manufacturers have models that diffuse air out the top of the machine but more commonly it is from the bottom or the side.  Top and side diffusers put cleaned air into the breathing space immediately. There is value in bottom diffusers, though. Air coming out of the bottom tends to stir up floor dust and helps carry it into the machine for removal.  This is perhaps more useful in homes where dust can stagnate, but not so important in schools where the movement of students tends to keep air moving.

Mr. Simon Labrecque, respirologist with Microaire suggests more filtration for smaller children, elementary schools, and less for older children, high schools. Also, farming or industrial communities may need more filtration, not only against polluted outdoor air, but also because farm children tend to bring more pollution into the classroom on their clothing.

If filters are being used in a newly built school it may be recommended to replace the filters after the first six months and then less frequently after that.

Particle count demonstration should be given before purchase.  Simple, less expensive models often perform better than expensive filters that have added features that may do little to improve air quality.  And, it is always wise to ask for testimonials and references regarding quality and service.
 

- K. Robinson

References:

Austin Air Systems, Buffalo, NY, promotional materials.

Carloyn Gorman, Environmental Health Centre, Dallas, Texas, personal communication.

CMHC Research Division.

Indoor Air Quality in Schools. Cutter Information Corporation, 1996.

Is This Your Child's World? Doris J. Rapp, M.D., Bantam,1996.

Mr Jim Woods, Amaircare, Toronto, Ontario. personal communication.

Mr. Mario Di Franco, Healthy Home and Lifestyle, St. Catharines, Ontario, 905-934-3168. personal communication.

Mr. Simon Labrecque,  RRT, Micron Air, Laurier, Quebec. personal communication.

Mr. Art McLaughlin, P.Eng. Environmental Specialist, OCL Group-Environmental Management Consultants, Halifax, Nova Scotia. personal communication.

Ozone Generators that are sold as Air Cleaners.  The US EPA  http://www.epa.gov/iedweb00/pubs/Ozonegen.html

Ozone Air Pollution. American Lung Association.

Ozone Generators. CBC Marketplace Files. January, 1998.

Volatile  Organic Compounds in Indoor Environments  Cutter Information Corporation, 1996.

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