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Indoor Air Quality – Introduction for HVAC Contractors

The field of Indoor Air Quality (IAQ) is complex and rapidly changing. Considerable research is underway as to the causes and correction of IAQ problems. This document serves as an introduction to the field so that contractors can expand their businesses to incorporate IAQ services. Contractors who decide to grow their business to include IAQ should seek additional training and information in the various areas outlined in this document.


You may think that poor indoor air quality is a new problem. This is because we have seen frequent articles and news reports of indoor air problems just during the past few years. IAQ was virtually unheard of before 1985.

However, IAQ is far from a new concept. The ancient Egyptians wrote that stone cutters working outside were far healthier than those working inside. In the Middle Ages, air quality in Gothic cathedrals with their high ceilings was better than small low ceiling buildings. We now know that this is because the overhead spaces held a reserve capacity of clean air for occupants. In 1600, King Charles I of England issued an order stating that all houses built must have a ceiling height of no less than 10 feet and windows to be higher than they were wide to remove smoke by natural ventilation.

In 1777, Lavoisier (the father of gaseous chemistry) conducted studies of the relationship between oxygen and carbon dioxide in crowded rooms. Formal ventilation standards were set as early as 1895 and included as part of building codes ever since. So, why has it only been recently that we have been hearing about the problems caused by poor indoor air quality? This question has two answers.

Fresh Air Makeup

Beginning with the first ventilation standards established in 1895, codes have called for twenty to thirty cubic feet per minute (CFM) of outside air for each occupant in a ventilated space. Actually, rates have probably been greater as many facilities lacked mechanical ventilation and depended on open doors and windows to permit large amounts of fresh air. Even mechanically ventilated buildings enjoyed high levels of outside air exchange due to the loose construction methods used before the 1970s. Under these earlier standards, it was not common practice to tightly seal the building perimeter.

Energy costs dramatically increased in 1972 due to the Arab oil embargo. Suddenly, it made sense to spend the extra money to prevent leakage of outside air into or conditioned air out of buildings. In 1979, ASHRAE revised ventilation standard 62 downward from 30 to 5 CFM per occupant. Many localities adopted this as part of the code.

The previous high rates of ventilation provided enough air-changes that contaminates indoors were largely diluted before reaching harmful levels. Where the outdoor air quality was very poor, of course, this dilution of the indoor air was of no value or worse — dangerous. In these cases, however, occupants tended to blame outside "air pollution" conditions for their respiratory and other problems. They did not recognize the separate and potentially hazardous problems caused by indoor air environments. Even so, reduction in ventilation was not the sole cause of indoor air problems.

Health Issues

Although observations throughout history have noted that poor indoor air can greatly affect health, it is not seen as serious compared to other deadly threats to humanity. Medical science throughout history has concentrated its research and treatment resources on such scourges as the plague, malaria, TB, heart disease, cancer, and even problems caused by outdoor air pollution. Professionals viewed discomforts caused by poor indoor air quality as trivial compared to other maladies.

That all changed in 1976 when 29 American Legionnaires died because of a mysterious illness acquired while attending a convention at the Bellevue Stratford Hotel in Philadelphia. The deadly bacterial infection, duly labeled Legionnaire's Disease, focused attention on indoor air quality as a major issue. Once people began to study the impact of poor indoor air quality, they noticed three facts: a) people were affected by the quality of indoor air; b) poor air quality often resulted in debilitating and/or life-threatening ailments; c) buildings with seemingly adequate supplies of outside air could have air quality problems. IAQ arrived as a separate and important science.

The Age of IAQ

It is not clear at what point we came to recognize that indoor air quality problems were not simply going to go away. From 1980 to 1990, extensive debate occurred on the subject. Many engineers and scientists believed that there was no problem. They maintained that, in a few cases, due to poor design and lack of maintenance, isolated buildings had problems that needed to be corrected. For the most part, however, they insisted that most buildings were perfectly ok. Meanwhile, news coverage increased of cases where numerous employees or other building occupants were facing significant health problems. The terms "building related illness" (a specific condition in one individual directly connected to a building centered cause) and "sick building syndrome" (generalized, non-specific symptoms experienced by one or more occupants of a given building – which often subside when the individual remains out of the building) became part of our language.

By 1990, there was broad acceptance that IAQ was a long-term problem that would require significant attention and resources to bring under control. Influential industry organizations and government agencies began to respond to this new challenge. ASHRAE revised ventilation standard 62 (published in 1989.) ASTM began developing test methods for indoor contamination. The EPA published a manual entitled Building Air Quality, to help building owners understand the problem and start to develop solutions. Both the Centers for Disease Control and the National Institute for Safety and Health began research projects. On the federal level, activity led to the decision by OSHA to propose a new standard on Indoor Air Quality for employers on April 5, 1994.

A New Opportunity

IAQ is a dynamic and continuously evolving discipline. We have much to learn about it. Many research projects are under way and the information we have now is very preliminary and narrow in scope. Unfortunately, the research needed to completely understand IAQ is complex and will take years to complete. Until then, we must do something to improve air quality to the point where people will not be at risk. Also, employers want to have the best working conditions possible so employee productivity will be high. We have learned that poor air quality in offices is responsible for many of the lost workdays that were previously thought to be normal.

Homeowners, building owners, and employers will look to HVAC contractors to meet their IAQ needs. For the contractor that is knowledgeable and ready, this new interest can lead to additional profits. Conversely, customers may perceive contractors that are not ready for this new field as behind the times, resulting in lost business. The next several pages will present the basic concepts that influence IAQ and what you can do to help your customers with their IAQ needs.

What Causes Poor IAQ?

Currently, the best definition we have of air quality is the beliefs and perceptions of those who live or work in the space being studied. If they think the air quality is good, then we conclude that it is. The question is, how many complaints need to be made before we decide that there is a problem? There is not a universally accepted standard on this matter. However, the most widely used rule-of-thumb is, if 20% or more of the individuals in a given space complain about the air quality, then there is a problem.

It is helpful to understand what we mean by poor air quality. Ask for an example of clean air and many people will mention the outdoor air on a spring morning following a rain shower. This example illustrates several facts we have learned about what makes air clean:

  • Free of particulate matter – Falling rain removes dust from the air, making it free of particulates. Water can be a very efficient filter. Unfortunately, water also can support the growth of bacteria and fungus. Consequently, we do not normally use water as a filter indoors. Not only would it be more complicated to handle than a panel filter, but it would also lead to serious microbial contamination.
  • Low microorganism count – Lightening generated by electrical storms produces large amounts of ozone. Ozone is a very powerful disinfectant. That is, it has the power to destroy bacteria, mold, and fungus. These microorganisms contribute significantly to poor indoor air quality. Microorganisms either grow in, or are carried by, HVAC systems where conditions are often ideal for rapid and uncontrolled growth. As a result, they become highly concentrated to a level where they overcome our natural resistance and cause illnesses ranging from mild discomfort to serious illnesses.
  • Rapid air changes – The breeze that accompanies a storm is like a mechanical ventilation system that constantly provides a supply of fresh air. This fresh air prevents the buildup of contamination to a point where it can cause harm and discomfort. Without that change of air, odors, gases, and chemical residues can build up to the point where they cause discomfort (or, in high concentration, harm.)

It seems logical, therefore, if we can control the level of particulates in the air and provide sufficient fresh air inside, we will have an acceptable indoor environment. The balance of this document will deal with each of these areas and provide a simple practical plan for a dramatic increase in indoor air quality in most situations.


Effective control of particulates is the first step in providing an acceptable level of indoor air quality. Air borne particles can be a source of problems. Many of the bits of dust and debris floating in the air directly cause illnesses or discomfort in humans. Most people have heard of and seen magnified photos of fearsome looking dust mites. These small insects cause allergic reactions in many and can harbor disease causing bacteria. But these are just the beginning. Bacterial and fungal spores, toxic or irritating substances, and chemical residues are just some of the components of the dust found in room air. These contaminates cause discomfort and may be potentially dangerous to building occupants. The danger becomes greater the higher the concentration and the longer the exposure. Therefore, sales of portable air cleaners have grown rapidly in recent years. Users hope that these devices will clean dangerous particles from the air.

Particles also have undesirable, indirect effects. Many particles are organic in makeup and thus serve as a food source for microorganisms. If these particles accumulate in the HVAC system, it may lead to a rapid growth of microorganisms. Consequently, when air moves over the growth sites, it dislodges particles and distributes them to the conditioned space and occupants. Buildup of microorganisms on heat exchange surfaces can also drastically reduce heat transfer efficiency. This will increase operating costs and may reduce system efficiency.

Buildings with properly designed and maintained mechanical ventilation systems have a much greater potential for effective control of particulate matter. This is because the design of the supply outlets and returns allows for more thorough and efficient circulation. High volume air exchange (four or more complete air changes per hour) has the potential of reducing airborne particles even further. Unfortunately, few systems are properly operated or maintained and do not realize this potential.


The vast majority of HVAC systems use spun-glass media or "throwaway" filters. The weave of these filters is often so loose that they trap only very large particles in the air stream. Many believe these filters are popular because they are so inexpensive. Also, they represent a safety compromise. The operating cost and service life of a blower motor (especially in residential and small commercial systems) is highly dependent on the air resistance the blower has to overcome. Low efficiency spun-glass filters provide little resistance to air flow, even when they have a significant dust buildup.

Where system maintenance is lax and there is a high likelihood that filters may not be changed for many months (or even years,) they may be the only choice. They filter out enough large particles that they provide some protection from damage for the equipment yet will not stop air flow if they are not changed at the end of their useful life. For this reason, it is important for you, as a contractor, to sell your customers on an ongoing service relationship where you can routinely service their systems, change filters, and spot potential problems.

The most universally accepted filter rating is the ASHRAE atmospheric dust spot test. This test attempts to measure the effectiveness of the filter to trap all sizes of particles. This test is often confused with the "arrestance" test, which measures the percentage of particles captured by weight. Therefore, it is biased to larger and heavier particles. Although these large particles can damage equipment, small particles are more important from a health standpoint as they evade normal body defenses and can end up in the lungs.

The ASHRAE dust spot test does not quantify ratings for filters that test below 20% efficient. Therefore, such a filter may be as low as 6% (and often are) in efficiency. You should have an objective when working with residential and small commercial (5,000 CFM and below) systems of providing a filter that will remove 30-45% of the particles from the air. A higher efficiency filter may be desirable in theory; however, systems of this size cannot usually overcome the air resistance of these higher efficiency filters. As a result, air flow will be reduced below the level where the system can operate properly, and equipment life may be drastically shortened.

To increase efficiency, contractors are often tempted to switch to a panel filter with a denser media and corresponding higher efficiency. This can be a poor idea. Usually, the air resistance goes up along with media density, so you are trading higher efficiency for risk of improper operation. Another option is to use an alternate air cleaning technology such as ionizers, electronic air cleaners, or the so-called electrostatic air filters. These devices are unlikely to cause high air resistance even if they are not well maintained and dust load builds up quite high, so they have the advantage of preserving equipment life.

They do, however, have two potentials for problems. The electronic devices utilize high voltages and can give off high levels of ozone if they are not operating properly. Ozone is a toxic substance that can offset any benefit provided by cleaner air. Also, they must be properly and routinely maintained. If all these devices are not regularly cleaned, dust and other contamination will build up to the point where it can no longer be held in the device. At that point it will be released in a cloud that can have an effect worse than the contamination would have been in small amounts over time.

A safer option for the average contractor (who does not have access to an expert in air filtration) is to utilize pleated panel filters. These provide a denser media that has removal efficiencies well above 20%, yet the pleats provide an extended surface area that allows the filter to trap a much greater load of particulates before air resistance builds up to a problem level. These filters are available in most common sizes and are competitive in cost with other filtration options. They are available in thicknesses of one inch and thicker versions. You should utilize the thickest version that can be accommodated in the filter holder of the units you are servicing. Often filter frames that previously held one-inch-thick filters can be modified simply by bending a tab so they will accommodate a two-inch-thick filter. This is highly desirable and should be considered even if the filter frame must be replaced. Filtration performance will be dramatically better with a pleated filter at least two inches thick.

Recently, very large accessory filter frames that can accommodate multiple filters and much thicker filters have come on the market. These are especially desirable in situations where specialized filters may be required to remove toxic chemicals or smoke from the air. Vendors of these products should be consulted for direction as to the proper media to utilize in different applications. Purchasing and installing these can be expensive but may be highly desirable in critical applications (healthcare facilities – homes of sensitive individuals.)


An often-overlooked fact is that all the air entering the HVAC system must be filtered. Where there is leakage around the filter, filter frame, or the air return ducts leak, contaminated air will reach the air handler and the benefits of improved filtration will not be realized. An important task in a filtration improvement program is inspecting and testing for duct leakage and repairing the leaks found. This is very important when the leak is in a duct that passes through a non-conditioned space. The quality of air in such a space is often very poor. Thus, such a leak can introduce significant contamination into the system.

In a similar manner, systems that include outside air inlets for fresh air makeup must bring that air in before the filters. Outside air must be filtered before being conditioned and circulated. Many IAQ problems have been traced to contaminated outside air.

Microbial Growth

The uncontrolled growth of microorganisms has come to be identified as a major source of indoor air problems. Early studies of air quality tended to dismiss the impact of such growths. This is because these small organisms are very common in the environment. Growing organisms can be detected in some quantity on almost any surface indoors and are common in soil and even can occasionally be found in food. In small concentrations, most microorganisms are seen as relatively harmless. Unfortunately, organism populations in HVAC systems are seldom small. The temperatures and moisture levels in systems support very rapid growth. Trapped dust and other particles contain a high level of organic material, which serves as food and sustains that high level of growth.

In order to effectively control growth, it is necessary to understand a little about growth and how it occurs. Microorganisms are not isolated in nature. In fact, they are extremely common. They are, however, few in number in any given location. Microorganisms consist of bacteria that are actually very small animals, algae, and fungi, which are considered plants. The difference between algae and fungi is that algae need light to grow and fungi grow in the dark.

Microorganisms are also classified as pathogenic (disease causing) or non-pathogenic. Pathogenic types are, fortunately, less common than others. However, organisms that are not pathogenic can cause irritation or allergies in sensitive individuals, especially when they are present in large quantities. Normally, microorganisms grow at a fairly slow rate. They have relatively short life spans and do not become highly concentrated in any given location. This is good because most organisms are not a problem in small quantities. We call concentrations of microorganisms "colonies" and single organisms "colony forming units" (or CFU for short.) This is because, with the right conditions, a single CFU will rapidly divide and soon form an active colony. To grow rapidly, microorganisms need moisture, food, and the right temperature.

Some organisms can grow by pulling water out of the air. It is safe to assume that growth of these will speed up as relative humidity rises above 60%. This is why mold and fungi are greater problems in high humidity areas and in the summer when the average humidity is higher. When standing water is available, additional organisms will grow, and growth will be much more rapid. The longer water stands without being diluted by fresh water, the larger and more concentrated a colony of organisms will become. Standing water doesn't always have to be a visible puddle. Porous materials such as fabrics (carpets), insulation, and wallboards can hold enough water after being flooded because of a leak or spill that the effect on microorganism growth is the same as standing water.

An important part of a growth control strategy is to minimize the presence of water. This starts with the control of indoor relative humidity to below 60%. Often building managers will turn off HVAC systems when the facility is not being used in order to save energy. This can allow the humidity level to rise significantly and lead to uncontrolled growth. A better strategy is to shut down the supply of moist outside air during periods of low use and run the air conditioning system enough to maintain humidity at below 60%. Special control system modifications may be needed to accomplish this. Designing and installing these can provide additional income and greatly benefit your customers. A major location for standing water that you can help control is the drain pan under the cooling coil. Level the unit and locate the drain, so the pan drains completely if possible. Advise your customers to drain other locations where water may accumulate, and have leaks repaired quickly before growth takes off. Wet building materials must be quickly dried or removed if growth is to be minimized.

Most organic matter is an excellent source of food for microorganisms. Since organic matter is so common, it is impossible to eliminate all sources of food. For example, building materials, fabrics, and carpet usually contain a high percentage of organic matter. For this reason, it is often necessary to remove these materials when they become wet in order to avoid harmful levels of microbial growth. Much of the soil and dirt that is present in homes and offices is also organic in nature. Therefore, good housekeeping is an important part of a clean indoor air strategy.

Good housekeeping inside of the HVAC system is even more important. Moisture and temperature conditions are normally ideal for rapid microbial growth. If a large amount of soil is present, extensive colonies will form. These growths become thick enough on heat exchanger surfaces that they significantly reduce heat transfer efficiency. It is not uncommon for built up growth to reduce efficiency as much as 29%. The bacteria that grow on coils and in drain pans are of a category known as "sulfite fixing bacteria." As part of their growth cycle, they remove sulfur from the air and give of sulfur dioxide, which forms sulfuric acid when introduced to water. This acid causes much of the corrosion in air handlers and other system components and dramatically shortens component life.

Most importantly, parts of colonies that are in the HVAC system are constantly being broken off and blown by the air stream into the conditioned space. Thus, contaminated HVAC systems are a major source of the biological contamination that causes illness and discomfort in indoor environments. It is vital to keep the interior of the entire system clean and free from growth. Most contractors clean accumulated growth from cooling coils because of the obvious negative effect on efficiency. It is equally important to clean the drain pan, blower housing, blower wheel, and other parts of the interior of the air handler. Most system growth originates in the air handler, so control must start there.

The need to clean the air ducts is less agreed on. This may be because the air duct cleaning industry has had many less than totally ethical members in the past. This has left many HVAC contractors with aa belief that duct cleaning is not a legitimate activity. Also, studies are not available that quantify the benefits gained by cleaning ducts as opposed to leaving them dirty. Some of that research has been started. We will soon have more facts available with which to judge the need for having a high level of cleanliness in air ducts.

In the interim there are good arguments both for and against cleaning ducts. On the plus side, a heavy buildup of contamination in a duct can easily be dislodged by air currents and blown into the conditioned space. That fact argues for cleaning of any duct that is highly contaminated. On the other hand, air ducts are normally very dry. It is doubtful that much active microbial growth takes place in air ducts. The organisms that are in ducts have probably been blown there from other locations (most likely the air handler). Because of this, they will probably stay where they are and will not increase in concentration. There are exceptions to this that those in the duct cleaning industry will point out. There are often active growths on turning vanes, dampers, and the supply grills associated with ducts. This growth must be removed and regrowth prevented.

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