Portable Air Cleaners

It would be a good idea to check with your building manager before purchasing or using PACs.

• By D. Jeff Burton, PE, CIH
• Jan 01, 2006

WHAT are portable air cleaners (PACs) and how are they used? Portable air cleaners (PACs) are small hand transportable air cleaning units used in occupied spaces (classrooms, offices) to reduce the concentration of airborne particles and sometimes vapors and gases. They typically weigh 10-20 lbs, are freestanding, use local electrical current, and can be placed anywhere in a room.

How do PACs work to clean the air? Commonly available PACs use several different methods to separate air contaminants from the air.

Aerosol filtration. The most common approach uses a highly efficient filter to separate aerosols (or, particles) from the air. Filter-based PACs are able to efficiently remove dust, asbestos fibers, smoke particles, pollen, mold spores, dander, shed-skin particles, and some bacteria from the air. Particle filters do not remove vapors or gases.

Electronic cleaning or electrostatic precipitation (ESP). These particle separators use electrical energy to charge airborne particles, which are then attracted to oppositely charged plates or fibers within the equipment. Some are equal to filtration in particle-cleaning effectiveness.

Open ion generators or negative ion generators. These devices place a charge on airborne particles, which then are attracted to walls and surfaces in the space.

Unfortunately, some electronic and ion-generator types may produce and emit ozone, an irritating and radiomimetic gas that should be avoided. Some also may produce airborne charged particles, which might be retained more easily in the pulmonary tract of occupants.

Less commonly used are vapor and gas separators or "sorbtion" equipment, which uses activated charcoal or other media to adsorb or absorb gas or vapor contaminants (e.g., formaldehyde, ozone, sulfur dioxide, and oxides of nitrogen, but not carbon monoxide). Some aerosol filtration units come with a carbon-based prefilter to reduce odors, but these are not considered very effective over long periods of time.

When should PACs be used? PACs may be useful when:

• The filter system in the central HVAC system is not capable of filtering air contaminants of concern.
• Pollen and mold spores are known to be excessive (i.e., during the summer blooming season).
• The space is dusty or smoky.
• Occupants are complaining or are known to be allergic to dust, pollen, or mold spores.
• Health-compromised individuals are present in the occupancy of concern (e.g, nursing homes).

It would be a good idea to check with your building manager before purchasing or using PACs.

What are some of the advantages and disadvantages of the use of PACs? Advantages include: portability, inexpensiveness to operate, ease of use, occupant satisfaction, and their ability to be used only when and where they are needed.

Disadvantages include: periodic maintenance is required, filter replacement is expensive, air cleaning is not as consistent or thorough as equivalent filtration located at the central air handling equipment, noise can be disturbing to occupants, it is drafty for those sitting or standing nearby, and a trip hazard exists for occupants moving around the space.

What do portable air cleaners cost to own and operate? Initial costs range from $100 to $500. Operating costs are reported to range from $35 to $60 per year, depending on the size, motor amps, etc. Replacement filters range from $60 to $170, and filters typically are replaced once or twice per year, depending on usage.

Okay, we might actually have a need for PACs. Which types should we use? Which should we avoid? For particle separation, the widely available HEPA filter PACs are both effective and reasonably priced. These do not produce ozone or charged particles. ESP-type air cleaners can be used when the manufacturer clearly states that ozone generation is not a problem and that charged particles will not be emitted into the occupant environment.

Occasionally, we find environments in buildings that might benefit from gas or vapor separation. Where vapor or gas odor is strong enough to elicit complaints from occupants (e.g., gas smells, formaldehyde odor, diesel smoke odor), the OS&H professional should seek additional information and guidance.

Filter efficiency. There are several recognized tests for reporting filtration efficiency. The most common are the "Arrestance test," the "Dust Spot test," the "Minimum Efficiency Reporting Value (MERV) test," and the High Efficiency Particulate Air (HEPA) test. Most good filter-based PACs use HEPA-grade filters. The table shows the relationship between these tests. If the PAC product literature reports filter efficiency, look for HEPA filters, MERV Ratings of 13 or above, or Dust Spot ratings of 95 percent or more.

Another PAC filter performance and efficiency reporting method, called the "clean air delivery rate" (CADR), compares the amount of air required for dilution of an air contaminant to the concentration that would exist after passing the air through a PAC. For example, if a CADR is reported as "250 for pollen," it means it provides the same pollen reduction as 250 cfm of dilution air.

Sizing. In addition to filter efficiency, we must also consider sizing, i.e., the air flowrate of the PAC and the room space volume (or square footage). A PAC that takes two hours to clean the air in the space would not be very useful.

Many PACS are rated for a specific floor area and assume a standard 7-8 foot ceiling. Where such sizing is not provided, a PAC might be sized to provide about six air changes per hour in the space. Use the following equation:

Air flowrate (in cubic feet per minute, cfm) = volume of room (in cubic feet) x 0.1

For example, if a room were 1,440 cubic feet in size, air flowrate = 144 cfm (for six air changes per hour).

Placement of the PAC also affects performance. If you have a specific known source of air contaminant, place the PAC near the source. Also, place PACs so they are not obstructed by walls, furniture, and people. Placement also affects the mixing of PAC clean air with room air. Filtered air should mix readily with room air and not simply be re-entrained into the PAC. Usually, running the building's heating and air conditioning system will provide the mixing necessary.

How should PACs be maintained? Prefilters should be cleaned or replaced on the schedule suggested by the PAC manufacturer, or more often if they become blinded with dust. Typically, prefilters are cleaned or replaced monthly or as required, depending on usage. Main filters should also be replaced as recommended by the manufacturer, or about once every 6-12 months.

What might be used instead of PACs? Most central HVAC systems in modern office buildings have filters already built into the air handling equipment. Ask your building manager for an assessment of the filter efficiencies already installed. If existing filters are rated at MERV 10 or higher, they are maintained properly, and air circulates freely through the target occupied space, then PACs are usually unnecessary.

Other optional controls include source removal (i.e., remove aerosol sources) and increased dilution ventilation (more outside air dilutes air contaminants). Generally, these two control approaches, when available, are preferred to air cleaning using PACs.

This column appeared in the January 2006 issue of Occupational Health & Safety.

This article originally appeared in the January 2006 issue of Occupational Health & Safety.