Choosing the Right Technology for Clean Indoor Air

Air cleaning technology is effective in solving a wide variety of IAQ concerns, including tobacco smoke.

Office, commercial and special purpose buildings (restaurants, daycare and wellness centers, etc.) have significant air pollution sources, other than tobacco smoke, that can cause or worsen existing health conditions. Sending smokers outside only deals with a small part of a dramatically bigger issue. It has been estimated that the concentrations of pollutants we face indoors are two to five times greater than outdoor concentrations. Sometimes it is as much as 100 times greater.

Since the typical American spends 90 percent or more of his or her time indoors, it makes sense that the indoor air quality of the office we work in, or the restaurant we go to, will affect us more than the quality of outdoor air.

Current air cleaning technology is more than sufficient to meet today's requirements for clean indoor air. Our purpose here is to take a comprehensive view of the problem, describing the most serious pollutants in both the office and public gathering place. Then, take a look at current air cleaning technologies and their effectiveness. Finally, we will provide considerations for selecting an air cleaner.

The Nature of Airborne Particulates

Particles are measured in microns. One micron is 1/25,400 of an inch. Or, better stated, there are 25,400 microns to an inch. Particles have diameters ranging from .001 to 100 microns. Those we see on furniture or floating in a ray of sunlight are approximately 50 microns or larger.

Individual particles 5 microns in size or smaller cannot be seen by the naked eye. These tend to be perpetually suspended in air and are generally referred to as "respirable." They are capable of being readily inhaled into the lungs with the tendency to stay there. Respirable particles make up 90+ percent of the total number of particles in the air that we breathe. Figure 1 compares the sizes of various types of particles and the types of air cleaning equipment that will effectively remove them.

Biological Organisms: All office environments are alive with biological organisms that the workers themselves introduce. Any group of people is accompanied by a fairly predictable array of bacteria and viruses. Besides causing the common cold, these organisms are the source of tuberculosis, staphylococcus infections, influenza and measles. Most of the diseases transmitted among co-workers can be classified as respiratory infections. It has been estimated that this category accounts for one-half of all acute health conditions.

Bacteria and viruses in office and public environments are transmitted widely by airborne dispersal. Infectious particles form free-floating "droplet nuclei" well within a respirable size range. Since people cannot be disinfected upon entering a building, there can be no effective elimination of infectious organisms at the source. The only way to do so is to treat the air to remove airborne particles.

Synthetic Materials: Buildings can create their own pollution because many building materials, textiles and furnishings create undesirable vapors, odors and fibers. Probably the biggest offender is formaldehyde, which enters the indoor atmosphere by outgassing, a process similar to evaporation except that outgassing emissions increase as indoor humidity rises. Formaldehyde is a component of foam insulation, plywood, particleboard, new furniture, ceiling and wall panels and carpets.

Airborne formaldehyde traces in offices have been found to range from 0.01 to 0.30 parts per million (ppm). Concentrations below 0.1 ppm do not cause worker problems. At 0.1 ppm, formaldehyde has a detectable pungent odor. At higher concentrations, it can cause eye burning and irritate mucous membranes.

Today, formaldehyde use in new buildings is closely monitored and the problem is declining. The best treatment for problems in existing buildings is ammonia fumigation, which specifically reduces formaldehyde emissions.

Dust: Unless there are special processes in place to trap and remove it, dust can come from a wide range of sources and circulate repeatedly and indefinitely throughout a building. Concentrations of total dust fall into the 20 to 40 micrograms/cubic meter range. When inhaled, respirable particles are deposited on the tracheal and bronchial surfaces. Larger particles (5 to 30 microgram size range) collect in the upper respiratory tract and are observed in the pharynx and trachea. It has been estimated that people typically inhale more than 2 heaping tablespoons of particulates a day.

Types of Air Cleaners

Along with replenishment of fresh air and correct sizing and specification of HVAC equipment, the best way to reduce particulate concentration to acceptable levels is by direct intervention with an air cleaner. There are three basic types: electrostatic precipitators, mechanical filters and negative ion generators.

Electrostatic precipitators are also known as electronic air cleaners. They have an ionizer that electrically charges the particles as dirty air passes through the unit, and a collecting cell where alternately charged parallel plates use electrostatic force to send the charged particles from the air stream to the plates as the air stream moves through the unit (Fig. 2). Both the collecting cell and ionizer are cleaned on a regular basis, depending on use and contaminant loading. In general, electrostatic air cleaners are effective in removing particulates in the 0.001 to 10-micron range. Typical particle ranges include:

Micron Range

  • Atmospheric dust 0.001 to 100
  • Bacteria 0.15 to 50
  • Tobacco smoke 0.01 to 1
  • Pollen 10 to 100

Because of their high collection capacity and "cleanability," electrostatic air cleaners are often specified for smoke removal, yet are also very effective in general IAQ applications. Electrostatic air cleaners are very effective in removing pollen, bacteria, viruses, fungi, pollen, hairs, aerosols and tobacco smoke. Electrostatic air cleaners have a noticeable, positive effect on work performance, productivity and reduction in illness-related absence.

Mechanical filter air cleaners have a primary collection filter made in a bag or pleated design of woven fabric or paper. High-Efficiency Particulate (HEPA) mechanical filters are similar to, but longer than, a pleated filter. HEPA filters handle low dust loadings in areas where concentrations are already very low (hospital operating rooms, electronic manufacturing areas, etc.). They are 99.7 percent efficient on particles 0.3 microns and larger in diameter. Because airflow is quickly restricted as the primary filter gets dirty, mechanical filters are not practical for tobacco smoke and particle removal in ordinary commercial applications. Mechanical filters must be replaced on a regular basis to obtain the proper airflow needed to maintain the cleaner's effectiveness. Although mechanical air cleaning units are very efficient, these units cannot typically provide the same amount of air changes necessary to adequately attain an acceptable level of air cleanliness. Thus, these units are normally not specified for large rooms.

Some air cleaning systems new to the market offer the choice of using an electrostatic or mechanical filter. The filters are interchangeable and can be easily replaced, depending on the needs of the environment.

Negative ion generators (or ozone generators) feature an ionizer and use an electrical charge to generate ozone, but do not have a collection capability. Particles are charged as they pass through the ionizer. Most charged particles flow out of the unit and attach themselves to any oppositely charged surface (furniture, walls, carpets, etc.). These surfaces can then be vacuumed to assist in removing the particles. Because of the absence of a collecting component, negative ion generators typically have less than 1 percent of the collecting capability of a good electrostatic precipitator air cleaner.

Although ozone is a necessary part of the upper atmosphere (10-30 miles above us), in the part of the atmosphere we breathe, ozone is a potent lung irritant. It can have damaging health effects, especially for persons with asthma and other lung diseases, children and the elderly. It is produced directly by ozone generators. The American Lung Association suggests that ozone generators not be used.

An air cleaner is not a total air processing system. These units only remove particulate matter and some odors. They do not remove carbon dioxide and do not replenish oxygen. Thus, some fresh air must be supplied to the room. The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) recommends 40-60 CFM of fresh make-up air per person in gathering places. This minimum requirement for fresh make-up air should never be omitted.

Installing Air Cleaners

Generally, air cleaning units can be used as portable units, mounted systems or central systems.

  • Portable units are used when the user prefers to move a unit from room to room.
  • Mounted systems can be installed in or on a wall, or in, or flush mounted to, the ceiling. These units are usually specified when air in only a few rooms needs to be cleaned.
  • Central filtration units are ducted systems that are used when whole-facility air cleaning is needed.

Dependent upon the manufacturer, many advanced air cleaning systems can be outfitted with special options to improve the unit's efficiency and to enhance its effectiveness in removing odors or killing micro-organisms. A few of these features include:

  • UV Light Kits A UV Light option can be added to assist in killing micro-organisms, such as bacteria, viruses and fungi. This considerably reduces the number of indoor air problems. Areas of application might include: medical institutions, childcare facilities, schools and offices.
  • Odor Control When odors that are more pervasive need to be eliminated, units can be equipped with odor control filters. Examples might include crowded meeting rooms, nursing homes and other busy places.

Considerations for Specifying an Air Cleaner

In general, there are four important factors that should be considered to properly specify and select an air cleaning unit. These considerations include: efficiency, air changes, capacity and placement.

Efficiency: The efficiency of an air cleaner is measured in terms of the particle size that the device can capture. The efficiency of an air cleaner is typically measured in terms of the weight or volume of particles the device can remove from an air stream. Keeping in mind that the very small particles are the ones that penetrate deep into the lungs and can cause health problems, one should be especially concerned about an air cleaner's efficiency in removing these submicronic particles. While there are multiple methods used to evaluate efficiency, because submicronic particles are the most dangerous, only the test methods that measure an air cleaner's effectiveness on the submicronic particles are the most valuable.

The primary function of air cleaning equipment is to remove some large particles and a multitude of small particles that account for the soiling and dangerous health effects of air pollutants. By weight method testing, we can tell that most air cleaners remove large particles very well, with over 95 percent removal efficiency by weight. However, weight testing methods primarily identify larger particles (which most cleaners can capture), thus there is no distinguishable difference from air cleaner to air cleaner, regardless of make or model. But what about the far more numerous, injurious and detrimental small particles that can only be accounted for in the remaining 5 percent by weight? This final 5 percent typically consists of 99 percent of the total particles by volume. These include the submicronic portion of the particles in the air stream. Therefore, when evaluating air cleaner efficiency, one must consider the specific particle size efficiency of the air cleaner you select by anticipating the type of airborne particles that you wish to remove.

Air Changes: An air cleaner is required to filter the air in a given space a prescribed number of times each hour to maintain a satisfactory level of cleanliness. The number of air changes required to attain a level of cleanliness has a limit. To be collected at the air cleaner, particles cannot be removed from the air in a room any faster than they mix with the air. To go beyond 15 air changes per hour may not increase the effectiveness of the air cleaning system. In addition, a higher number of air changes create drafts, causing discomfort to the occupants of the room.

The number of air changes needed depends on the generation of contaminants in an application (see chart above). A designated smoking area/room is the only situation where more than 15 air changes is necessary. In this case, 20-30 air changes are required due to the high concentration of smoke.

Capacity or "Clean Air Delivery Rate": The overall effectiveness of an air cleaning unit depends on the efficiency of the unit and on the amount of air drawn through it.

Air cleaning systems have a capacity rating based on the amount of air the blower can deliver inside the cabinet. This volume of air through the air cleaner is expressed as cubic feet (of air) per minute (CFM). The CFM delivery rate can vary dramatically by unit and manufacturer and can be adjusted on some advanced units by either a selection switch to change the motor speed or even by an air monitor feature that monitors the level of pollutants in a room and adjusts the speed based on the level of concentration.

To properly size the application, three factors must be considered: room size, required number of air changes and load factor.

Air cleaners also have a capacity rating based on a load factor. This factor is used in applications where an unusually large number of people gather or where the contaminants generated by a work process are unusually heavy. In tobacco smoke applications, the load factor is based on the number of smokers who occupy the area's rated capacity. Determining the load factor is much more subjective for general IAQ applications such as biological organisms, chemicals or synthetic materials. (Some criteria that may be used in determining workplace load factor are visual observation, density of work stations, anticipated workload, pace productivity or concern for keeping the room as clean as possible).

Placement: Once the equipment requirements have been determined, the next task is to strategically place the equipment to develop an air pattern around the perimeter of the room. Unless this is accomplished, the air will not be properly cleaned. Many applications are simple the room is a basic square or rectangle and there are few, if any, obstacles to impede the air pattern. However, there are rooms with odd shapes, ones that have alcoves and ones that are decorated with items that impede airflow (canopies hanging from the ceiling, large ceiling beams and others). All must be considered when choosing the best locations to place the equipment. Other considerations for proper placement include:

  • Units should not be placed near HVAC system intakes or outlet grills where conflicting airflows could inhibit the performance of both systems.
  • Multiple units should be placed so that they are "balanced" in the room.
  • Equipment in the room must be located so that fixtures do not block or divert the air pattern.
  • Clearance for doors must always be provided so equipment can be serviced.
  • The objective is to create a continuous air pattern throughout the room so that air can be properly re-circulated.

For assistance in evaluating indoor air quality needs and equipment, contact a certified distributor and retailer of commercial air filtration equipment in your area.

Sidebar: Causes and Sources of Indoor Air Pollution

Indoor air quality problems can be traced to a variety of causes. Some buildings may be inadequately ventilated. The mechanical system, for example, may not be designed or operated to provide adequate amounts of outdoor air. Nor do we generally have control over the building's indoor environment. There are many pollutant sources even if the building is smoke-free that can cause a number of illnesses. These include asthma, hypersensitivity, pneumonitis and humidifier fever. While most of these can be treated, some pose serious risk. More than 50 million Americans live with allergic disease, and nearly 18 million have asthma. Indoor air pollution, even without tobacco smoke, is cited as one of the top five human health hazards in our country. For those who suffer with allergies and asthma, exposure to triggers can cause adverse responses such as sneezing, coughing, breathing difficulties, nausea, sinus, dizziness, eye irritation, sore throat, headaches and even life-threatening anaphylaxis reactions. Doctors have identified a constellation of these complaints as the so-called "sick building syndrome."

Approximately 15 percent of the population is affected by multiple chemical sensitivity (MCS), an adverse reaction to fragrances and other toxic chemicals in the environment at levels that have been generally accepted as non-toxic. In addition to the above symptoms, people with MCS may experience short- or long-term memory loss, exhaustion, depression, skin irritations and a host of other reactions.

There is no single manner in which these health problems appear. In some instances, problems begin as people enter their offices and diminish when they leave. Other times, the symptoms continue until the illness is treated. In the opinion of some World Health Organization experts, up to 30 percent of new or remodeled buildings may have unusually high rates of health and comfort complaints from occupants that may potentially be related to indoor air quality.

The most common triggers in addition to tobacco smoke odors include:

  • Formaldehyde from pressed-wood products
  • Organics from building materials, carpets and office furnishings
  • Chemicals found in materials, fragrances and disinfectants
  • Wall coverings and adhesives
  • Paints
  • Copying machines and office supplies
  • Dirty ventilation systems
  • Water-damaged walls, ceilings and carpets
  • Pesticides from pest management practices

Matt Hoffman is an air quality advisor with United Air Specialists Inc. (UAS) in Cincinnati, Ohio. UAS specializes in providing technologically innovative, high-performance air filtration systems for a wide range of commercial and industrial environments. He can be contacted at (513) 354-8746.

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