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Layman’s Overview of Indoor Air Quality (IAQ)                    
By Denny Walker
Indoor Air Quality covers a very broad subject matter, for this article, we are going to breakdown the major problem areas into four categories. They are Microbials, Particles, Ventilation and Source Removal. Think of an old double hung window at your grandma’s house. You know the one, it is one window made up of four smaller panes of glass with a “t” of wood in the middle. The analogy is that it doesn’t matter which pane you hit with the baseball, once it’s broke you need to fix it. In our example the four windowpanes are:
1. Microbials - fondly referred to in the industry as “They’re back” (We’ll explain later!)
2. Particles - as in the removal there of, by using increased filtration. The little saying for filtration is “More is better” (within the limits of your forced air system)
3. Ventilation – often referred to by the saying “Dilution is the Solution”
4. Source Removal – Let’s put it this way, “Honey I still smell the baby’s diaper, will you take it outside to the garbage can.”
There is one interesting fact about IAQ in that a problem is not considered a problem until someone complains. That is to say the indoor air quality expectations are different for different situations. Let’s compare two businesses in a strip mall, say a print shop and an accountant’s office. The person working in an accountant’s office would complain about the IAQ if he had to work in the environment a printer works in, but the printer usually does not. 
With these facts in mind we are going to address potential problems encountered in the normal house hold or office environment. Remembering our windowpane analogy we will address each of the 4 panes and mention products and actions that will provide solutions for each.
Broken Pane #1 Microbials
Microbials, which include the vast families of bioaerosols, are the first broken window we will address. A bioaerosol is a biological aerosol. These particles are very small and range in size from less than one micrometer (0.00004") to one hundred micrometers (0.004"). Bioaerosols react to air currents and move quickly or slowly depending on the environment. Bioaerosols are impacted by gravity but due to their size and density, air currents play a large role in their movement. Air often also contains tiny organisms such as fungi, bacteria, mycotoxins and viruses. None of these organisms live in the air but they are often attached to other small particles such as dried residues from water droplets, dust, soil or skin flakes. Groups of the small organisms clump up and enhance survival while airborne. Fungal cells such as spores, molds and yeast can be active at low humidity levels and high or low temperatures.  
 Fungi, various molds, mildews, spores, bioaerosols, and bacteria, are by far the most difficult problem to address in relation to any forced air heating or cooling system. Their nickname saying, “They’re back” is one of the worst aspects of an IAQ problem. You can do all that I am going to suggest: clean, filter, sanitize, dilute, treat with UV light and still not win the battle every time. Our suggestions and solutions will go a long way toward improving your situations.
 So let’s look at the three most prone areas of potential microbial growth in your average forced air heating and cooling system and address solutions for each of them. The EPA (Environmental Protection Agency) has a position paper on mold (document # EPA 402-K-02-003) and clearly states, “The key to mold is moisture control”. With that in mind, the three most prone areas in your system are where there is a potential for moisture.
The number one source of microbial growth in the A/C system is the condensate drain pan for the evaporator coil of the air handler and the front surface area of the evap coil. This is the area where the water from the air condensates on the cold surface of the evaporator coil and by gravity runs down the coil surface into the drain pan. The water in turn, again via gravity, runs outside your home usually by PVC pipe and drains onto the ground. This pan almost always holds some of the condensate water. 
            The three necessary items for microbial growth are moisture, food and the microbial spores/reproductive agents themselves, are all present here. The food is the dust and particles that make it thru your filtration system, moisture we have addressed, and the active spores come from a multitude of sources outside your home and some inside. You can treat this with various tablets or liquids all of which gives various short-term results. 
The best products I have found are the family of products including: TR BioKlean, Trane Drain Pan Treatment, Hydro-Balance Pan Treatment, and Ace Pan Treatment. They are a Quaternary Ammonium (“quat” for short) based, water soluble, time released, micro-biocide product that usually last the whole cooling season. The time release nature of these products release a small amount of the anti-microbial into the water that collects in the drain pan and like chlorine in your pool keeps the water in the pan and connecting piping clean and microbial free for the season. (As a footnote, do not use chlorine products as they release chlorine gas into your air stream and become another IAQ problem in of it.)
The second most prone area for Microbial growth is the actual coil surface, blower wheel, blower housing, insulated and non-insulated surfaces inside you air handler and cased coil. The normal way to handle these areas is to vacuum the solids out with a HEPA Rated vacuum cleaner (more on this later), clean them with a high PH Alkaline Coil Cleaner, rinse and then treat them with an EPA registered disinfectant. The high PH aids in the process, while the sprayed on disinfectant “sterilizes” the area. I suppose you could use a product like “Lysol” which is readily available, but it labeling does not state that it is acceptable for HVAC (Heating, Ventilation and Air Conditioning) forced air systems. 
The top product for post cleaning sterilization is Nu-Calgon’s Bio Fresh CD and its sister product Enviro-Con. They are an EPA registered product formulated, tested to their standard, and labeled by the EPA specifically for forced air A/C system disinfecting. The second is a product called Oxine and needs to be mixed to a specific dilution and fogged. Again, this product is EPA registered specifically for cleaning and sterilizing Air Conditioning systems surfaces.  I normally use the Bio Fresh CD since it can be sprayed directly into the forced air systems airflow also.
The third most prone area for growth is the first 10’ or so of the supply ducts itself. This is because there is often a moisture blow by effect from the blower area or a surface condensation issue that provides the moisture necessary for microbial proliferation. The normal way to handle this area is to install an insulated access door in the first 4 feet of the supply duct. You would regularly inspect this area with a flashlight either seasonally or bi-annually and treat any growth areas with the same cleaners mentioned above. An extreme case would possibly entail sealing this area with an anti-microbial coating like Portercept, or Fosters 40-50 mold resistant coating. These provide a dual purpose in that they will not promote growth and they will make the visual inspection easier in that you can see inside your duct better.  If this area persists in being a area of microbial/mold/mildew growth we recommend putting a "second" UV-C lamp in the supply duct (see next paragraph).  Of all the choices for best place to put a UV-C lamp this is normally forth best on the list.
Now that your potential problem areas are cleaned and sanitized, you have one more important step and that is to prevent them from “growing” colonies again. That is where the latest and greatest item to hit the HVAC industry recently has become popular. This product is the UV-C Light. I could write a complete essay on how and why it works but I give you the short version.  
 UV is one of the wavelengths that come from the Sun that is just below the frequency of visible light.  In other words you cannot see it. The frequency used is called UV-C, which can be emitted from specially made bulbs for use inside your AC system. Our Sun controls populations of microorganism’s outdoors and UV-C lights bring this disinfecting energy into our duct systems.
UV-C works by physically penetrating microorganisms, much like x-rays. The energy promotes a chemical reaction in the DNA and permanently alters the structure and molecular bonds of the DNA. This causes the bioaerosol to no longer proliferate by either cell division or reproduction. It is now harmless and dies off shortly leaving no offspring and the population of the microorganisms diminishes rapidly.
There are two things we what this UV-C light to do in your system. The first and most important is to keep the potential growth area surfaces sterile, this is easy. The second is to “kill” everything in the air passing through your forced air system, which is more difficult. The “strength” of the UV and the “duration of exposure” have much to do with what microbial/bioaerosol you can and can’t “kill” right off or first pass.
Let’s look at the easy one first. A single bulb of medium strength will do a great job keeping the coil surface free of growth. This is because the light is on 24 hours a day, seven days a week and 365 days a year. (Here is a great time to tell you that you do have to change almost all bulbs once a year, although the bulb still appears to be ”on” it will not emit the UV-C after one year). Here also is an opportunity to tell you that you must not look at the bulb with the naked eye. Just like looking at the sun, looking at a UV light can damage your eyes. Plastic lenses, glass glasses all block out the UV rays, also indirect viewing of the blue effect will not harm you, (just don’t look directly at the lit bulb unprotected.) Consequently a single bulb mounted below, inside or above your AC coil will provide all the “killing” power you need for the surfaces.
Sterilizing the moving air is another science in itself. Suffice it to say killing everything first pass would be more expensive than the consequences of letting some of them pass through, unless special medical needs are present, such as an operating room, or special recovery areas and such. The speed the air passes the bulbs, how far they are away, and the temperature all are important issues in the equation. Click here for a chart on kill time for various organisms.
So far the best product for this application is the GeneralAire PCO 2450, and the APCO by Fresh-Aire UV.   If you want to do some additional “air cleaning” with UV-C you need to place a second bulb arrangement in the return air area, where the air is moving it’s slowest, place the bulb centered to provide minimum air space with the sides of your ducting and place it “long ways” so the bioaerosols get maximum exposure on their trek through the duct system. 
There are numerous variations and styles of lights available; we recommend the GeneralAire, Dust Free, Fresh-Aire UV and the inexpensive Gorrilla Aire families of VU-C products. They offer everything from easy mount 110V single bulb units, “plug in units”, to four bulb remote mount 2-year life span units with odor counteractants, to commercial racks with multi-bulbs for large duct sizes.
            There are four other items we need to touch lightly on. The first is vacuums, we’ll talk more in the particles section but you cannot use a normal vacuum cleaner or shop vac when dealing with microbials. They just pass right through, enter the air under pressure and start the process all over again. The only vacuum cleaners acceptable are the one’s labeled HEPA VAC’s and you still need to be sure it really is a Hepa Vac. Remember, these bioaerosols are tiny.
Second is duct cleaning and fogging. We’ll discuss duct-cleaning mores under particle removal but fogging with a product like Oxine is sometimes a necessary step in the remediation of microbial proliferation. This is when your IAQ professional hooks up a “fogger” to your duct system and  blows a cmicro-biocide down your duct to kill anything in your ducts. My advice is to have a professional only do it if necessary and you and your pets must leave the house. This is one of those worst-case scenario procedures and is not used very often.
Better filtration is the third subject. We will go into filtration at length when we examine that “pane of glass” but suffice it to say the better the filter system you use the better you will do with your battle with microbials/bioaerosols. I don’t believe any filter system alone designed for residential forced air systems can stop all microbials / bioaerosols on the first pass. Consequently filtration is part of the solution, but not the catchall.
The last issue under Microbials is Ozone. Once thought to be a cure all, ozone treatment has fallen into disfavor, mostly because of misuse, misapplication, and it’s over promotion. Ozone is the by-product of O2, which is an oxygen molecule (that we breathe in every day) and an open electrical reaction. Three O2’s with an electric arc produce two O3’s. This triangle molecule O3 is unstable, has a half-life and reacts favorably (i.e. breaks them down) with carbon-based molecules/life forms (bioaerosols) and odor causing molecules causing their demise through oxidation. Though effective on a molecular level, too much ozone itself is considered an indoor air quality hazardous situation. So adding it to your system makes no sense and can lead to other problems. 
The only time we “like” mechanically produced ozone is after you have done a microbial / bioaerosol remediation and you want to “sterilize” the air in the home and ducts for fear that you have broken some bioaerosols loose i.e. the dandelion effect. Take the occupants out of the structure, lock the doors and place a do not enter sign on them. Then set a commercial ozone generator  for 5 to 7 hours. After the ozone machine has shut off for an hour or so, your IAQ professional would then enter and ventilate the house to be sure unsafe levels are no longer present. The EPA sets 3 parts per million as a permissible level for ozone with about 100 part be million being normal on a Nuclear Submarine running under extended submerged situations.
We could talk for hours on this one subject alone but let’s move on and see what else makes sense with our “broken window” effect. 
Broken Pane #2 Filtration
Our second broken windowpane is the removal of particles; all the filter manufactures have strived to make filtration the “player” in the IAQ market. They have taken huge steps is the last couple of years to “police” or standardize their marketing and upgrade their standards. The latest and greatest is the new MERV (Minimum Efficiency Reporting Value) rating system. Suffice it to say any manufacturer who has embraced the new rating system is giving you a fair and honest product evaluation. In the past there were several ways to mislead you. The new ratings make good sense and are logical. A MERV 6 is an okay filter that will provide specific results, specifically 35% to 50% efficient in capturing particles in the size range of 3.0 microns to 10.0 microns while a MERV 11 is really good and captures 85% to 95% of the same size particles. There is by definition no perfect filter, because if you were to trap everything not enough air would go through your system. The MERV rating addresses particle sizes that matter: specifically group 1 (bowling balls) 3-10 microns, group 2 (baseballs) 1-3 microns and group 3 (marbles) .03-1 micron. With is in mind, what is best for you? The old buy at Wal-Mart $2.00 filter gets about 3% of particles by weight but catches about nothing other than large pieces of flying debris (bowling balls). The new standard pleated 1” filter is usually close to a MERV 8 and is always a good choice to begin as a basic starting point.
Lets step back a moment and look at particles in the home environment. Think of them as being in a size class from “bowling balls to BB’s”. Bowling balls and the next size down softballs are those big floaty things you see drifting in the air when you catch the light streaming through a window just right and you are amazed that they are there at all. Be assured, they are there all the time. Particles can come from a myriad of sources; bed linens, that new sweater you just bought, the pets, shoes, blankets, sofas, on and on. Hard floors seem to make your house more dusty, but be aware most carpets are actually worse since they not only catch and release particles the also produce particles as they wear. Next in size are baseballs and golf balls. They are harder to find and fall into the size stuff you have to dust. You know, the stuff you rub off with your finger on the mantel and then accuse your wife of shoddy house cleaning. Pollen and many outside particles that effect allergy suffers fall into these 2 un-scientific categories and are all pretty much handled by our MERV 8 products and definitely by MERV 11’s
This is where is gets a little scary, bowling ball to golf balls only make up about 10% of the actual particles by count that are actually floating around in your air. That leaves 90% filling the spectrum from marbles to BB’s. Here is where the molds, mildews, bacteria’s, viruses, fungi (I think that’s more than one fungus) and their reproductive spores are located. In this area MERV 11 helps a lot but only MERV 13, the best electronic air cleaners or true HEPA filters are effective.
All things being equal, no allergies in the family, mold/mildew bioaerosols not being a factor, you have no pets, and use a HEPA rated vacuum cleaner a couple of days a week, the MERV 8 standard 1” pleat changed once a month or at worst every two months is adequate. This will in general keep your evaporator coil clean and prevent most air borne “stuff” from traveling downstream through you duct system and blowing back out into the home. A nice upgrade is to use a longer duration upgrade product. GeneralAire's Mac SeriesAprilaire's Space Quard, Honeywell's F100 and F300 series Media Air Cleaners and Quality Filters , who have a nice 5” deep Grill Filter . These typically are MERV 8 to 11 products (and can be upgraded to the newer MERV 13 standard) that come with a steel insulated case of some kind and range 4” to 6” in filter depth instead of the above mentioned 1” pleat. These products provide you with the ability to change the filter every 6, 8 even 12 months. Their added benefit is that they tend to collect more and smaller particles later in the filter life and do not restrict airflow in the process.
 Upgrading this longer duration 5” pleat to a MERV11 or MERV 13 product is a big step in that you now catch more and smaller air borne particle such as pet dander, dust mites, pollen and further reduce downstream particles plus begin the difficult process of dealing with the mold/mildew bioaerols issues so prevalent in the more humid climate locations.
 Let’s say someone in the family has health issues or you want IAQ to be an important part of your household/office life.  You want to upgrade to at least a MERV 13 product as described above or consider an Electronic Air Cleaner. Again the major manufactures all have quality products that fall into this category. If you are looking for a all-in-one top of the line product, theGeneralAire Tersus 2000 is a hybrid product that combines a  4” deep MERV 11 media, with 2 anti-germicidial lamps VU-C lamps that "charge" 70 sq. feet of reactant media to provide photo-catalytic oxidation for complete control of all air polluants. It is a reasonabably priced product that gives the homeowner the most bang for their bucks..
Traditional electronic air cleaners by GeneralAire, Honeywell, Emerson, Nordyne and Goodman are also effective in the high MERV category and easily and consistently collect the smallest of particles. There is some questions as to how often they need to be cleaned and how non-cleaning effects performance, but all-in-all the technology is sound and they have been tried and tested over time and are a valuable resource in the IAQ particle battle.
Generally speaking, filters do nothing to very little about catching anything other than particles. Consequently you cannot treat VOC’s (volatile organic contaminates, see more under the ventilation section) or any type of “smoke” type gases successfully with normal filters. There are several carbon type filters like OdorBan from Quality Filters that are designed to address this problem.  Frequency of changing and total parts per million of contaminates play a roll in the effectivness of "charcoal" filters..  The science of "photo-catalytic oxidation" is a newer entry in the IAQ market.  Products like  Apco by Fresh-Aire, and GeneralAire PCO2450  by using UV-C and photocatalytic oxidation do a great job on VOC's and household odors.
DustFree out of Royce City Texas released the first cleanable, washable MERV 8 filter called Envirogreen 8. Its frame is warrantied for the lifetime of the product and the removable, cleanable pleated foam core is warrantied for 5 years. It is available in 33 standard sizes and they will make custom sizes up to 30”.
              One last filter we will mention is the electrostatic type. This was thought to be a cheap alternative to “powered” or electrically enhanced electronic filters. Although the science is fine, in actual application they are hard to clean, sometimes lose charge when sitting and are known to release trapped smaller particles during cycles. They do have a place in filtration, but not a formal place when discussing IAQ filtration. The MERV rating system pretty much stifled the higher rating they formally used to advertise these products. They also tended to be very restrictive to the airflow and more often than not negatively affected the forced air AC/Heating unit’s performance. 
          Our last point of discussion in filtration is “frequency of change”. Back in the day before IAQ and our concern for the whole house environment, filters were actually designed to catch the “bowling balls” in front of the heat transfer surfaces. This kept them working effectively so they needed to be cleaned less often. You changed the filter when it was stopped up, dirty, or had “sucked in” (really dirty). Our surge in concern over IAQ has led to these better filters as discussed above. The overall rule of thumb for 1” non-extended life filters is once a month in seasons where your system runs full time day and night (summer and winter) and every other month is the off seasons. For extended use filters 2” thru 4”, the rule of thumb is to check them every 2 months or so and change before you think it is loading up. For the deeper filters 5” and 6” or more, checking every 4 months and changing every 6 months is the best practice. On electronic air cleaners follow the manufactures instruction for the specific brand you have. Take our advice and when cleaning do a “great” job instead of a poor one. It makes a real difference on the effectiveness of electronic air cleaners.
           Broken pane # 3 Ventilation
          Our third area of discussion is “ventilation”; it is commonly referred to as “dilution is the solution”. A good word picture is spilling a bottle of perfume in the bathroom. First you wipe it up with paper towel and take it outside to the garbage can; you surely don’t want that smelly soaked paper towel left in the house. Then you walk back inside and “whew” you smell perfume everywhere. The only sensible answer is to open windows, and get fresh air in and the tainted air out. Hence “dilution is the solution”; this can be facilitated several ways. Exhaust air out via a fan or force fresh air in via a fan.
              Another way is to take advantage of negative pressurization by using your forced air central air and heat system.  An example would be to open a window on one side of the house, shut the door to the contaminated area, open a window in that room, and then turn you system's fan "on".  In this situation you would "suck" air into the home via the open window on the other side of the house the equilavent of the air pushed out the window of the room with the door shut.  This quickly removes all the comntaminated air in the "bad" room and does no cross contaminate the rest of the structure.
             ASHRAE, the governing body for anything to do with AC/Heat/Ventilation, recently facilitated a change in the government’s position on ventilation for homes. Their new ruling is called Standard 62.2 and following is a quick summary by Max Sherman who helped draw the standard.
“Goals of Standard 62.2
The most effective strategy for minimizing indoor exposure to pollutants is to prevent them from being released into the air in the first place. To this end, the standard requires source-control measures that exhaust pollutants (e.g. from combustion appliances, cooking fumes, see Figure 1) from specific rooms before the pollutants enter the rest of the household. In addition, whole-house ventilation brings fresh air into the house, diluting pollutants that are difficult to control at the source.
What 62.2 Covers
Standard 62.2 addresses three primary areas:
  • Whole-house ventilation
  • Local exhaust
  • Source control
Whole-house ventilation. The whole-house ventilation requirements in the standard are intended to dilute contaminant emissions from people, materials, and background processes.
In typical houses, the standard requires a ventilation rate of about 50 cubic feet per minute (cfm) or 25 liters per second (L/s); larger houses must have a higher rate. Almost all houses must have a whole-house mechanical ventilation system rated at 7.5 cfm per occupant, plus one cfm for every 100 square feet of floor area that can be occupied. Houses exempt from this requirement include houses in hot climates without air conditioning, houses conditioned for less than 876 hours per year (e.g., cabins and vacation homes that are occupied for brief periods), and houses in hot dry climates, primarily in the southeast and southwest U.S. where occupants generally ventilate by opening windows.”
            Before going into the means of achieving Standard 62.2, let’s go back to one of the first principals we addressed for our new air tight well insulated homes. Here again are a couple of quick facts for reference:
1.      The average American spends 90% of his time indoors.
2.      The average adult human breathes 3,400 gallons of air each day
3.      ½ of all families have someone directly affected by allergies
4.      85% of homeowners think outside air is of worse quality than the air in their home or office.
5.      In actuality the U.S. EPA (Environmental Protection Agency) states that indoor air can be 3-5 times more polluted that outdoor air.
This in effect means every time you use a chemical cleaner like, Tilex, Clorox, Windex, or every time someone paints their nails, uses hair spray or cooks (think cauliflower), all those smells are mixed together and trapped inside your home.   New furniture, carpets, dogs, cats, on and on, these are all concerns for the indoor air quality of your home. 

The term VOC (volatile organic compounds) is a often tossed about term.  The short explaination is the chemical off gassing of solvents from new product brought into the home or office.  New furniture that has been is sealed containers often give you a "chemical smell" for several days.  New carpet or flooring will off gas and again disrupt the normal IAQ of the home or office.  The only solution for these temporary IAQ issues is "dilution".
There a several great new products on the market that provide great ways to achieve the air turns necessary to meet the new standard. The neatest products are called “perfect windows” and go by the family name of ERV’s or HRV’s. ERV stands for energy recovery ventilators, the ‘H” units usually have an external heat source for cold climates. GeneralAire, Honeywell, AprilAire, Panasonic  (has a unique spot ERV) and others all have multiple sizes and varieties of the ERV. 
They all work on the same principal. Exhaust air out of the house, thru a static type heat exchanger, at the same time bring in equal amounts of outside air thru the same heat exchanger with the two opposing air streams not mixing. What happens is between 65% and 80% of the humidity and temperature is exchanged so the incoming air is pre-treated and does not waste all the characteristics of the outgoing dried and pre-cooled exhausted air. They are very energy efficient and draw minimal electrical watts.
Another new product is the whole house dehumidifier.  Again GeneralAire, Honeywell and April Air lead the way with several other manufactures hot on their heels. This is a compressor bearing unit that filters and dries out the incoming air into the house via high MERV filters and the refrigeration process. The compressors are usually about one horsepower about the size of a normal window unit) and they can use 5 times the energy of the ERV but can effectively dry the incoming air way down to 30% or so. You draw outside air into the unit and treat it. Then it is dumped into the home usually thru the central heating and air conditioning system. Old air is forced out of the house thru a damper system or normal house pressurization.
Along with dehumidifing in the cooling season, humidifing in the heating season is necessary for good air quality.  There are 3 basic types of humidifers: steam, by-pass, and drum type.  The GeneralAire 900A is a good example of the bypass humidifier, while the RS15P is a good example on a high end steam humidifier.
Lastly in this products department there are controlled damper systems. Our friends at Honeywell and April Air have the “neatest” products for this application too. The basic set up is a mechanical damper with a timer and a sensor (either humidity, or temperature) that let the correct amount of air into the residence when the circumstances are best, not the heat of the middle of the day or when it’s raining. These systems are better than just opening a damper by time alone or manually. Remember that bathroom fans, kitchen exhaust hoods and clothes driers all “suck air’ out of your home and force outside air in via a negative pressure effect. This air is raw outside air, hot and damp in the summer and cold and damp in the winter.
The last pane in our “broken” IAQ window is source removal.
Source removal is the primary way to begin all steps in the remediation process for IAQ. Simple in concept, it can get “tricky” if you are not careful. Reference our example in the last discussion of ventilation with the spilled perfume. What would have happened if you had put the perfumed paper towel into a trash can in the bathroom instead of taking it outside? How long would the smell have lingered under those conditions?
In the ventilation discussion we quoted the new fresh air standard 62.2 that stated “In addition, whole-house ventilation brings fresh air into the house, diluting pollutants that are difficult to control at the source”.  Some of the issues come into play when the source is hidden and out of sight. Let’s look at the classic example. We have a two story house and the toilet upstairs overflows. Water is cleaned up but you get a medium water stain on the drywall on the ceiling below the bathroom. You seal it with a product like “Zap” or “Kilz” and repaint. Months later you have a mold problem in the home being spread apparently by the duct system and your air conditioner/furnace. You have it cleaned and "wham" 3 months later it’s back.
The problem in this case turned out to have been initiated by the overflowed toilet and the wet back side of the ceiling drywall below the bathroom.  It had initially become ‘moldy’ because of the overflow but combined with getting just enough dampness daily from the bad grout in the upstairs shower, it became a full fledged mold and mildew factory. Sending air borne spores through the open return duct between the two floors. As it turned out, the culprit drywall ceiling was removed and replaced, the ducts were fogged, the whole house was ozone treated, the AC air handler was recleaned, and then a UV light was installed. The moral of this story was the source was hard to find and the wrong diagnosis lead to extensive AC system remediation that had to be done twice. 
The best plan of attack is to start with the complaint and follow the pathways back to the source of the problem. Confirm the source and remove the problem. Then begin the remediation process including the appropriate steps for your problem: increase filtration (consider charcoal based filters for first batch), dilute with fresh air, hepa vacuum clean up, chemically clean, sterilize, treat with UV-C, or UV-C + photocatalytic oxidation, and if necessary seal with anti-microbial sealants. Never use smelly cover up sprays or light aromatic candles, they only mask odors and add more chemicals to the air.
Let’s add a little discussion on pathways. The main pathway in most residences/offices is the forced air heating and cooling duct work system.   It connects all the rooms through returns and supply registers. Secondary pathways are hallways and chases between floors or rooms. Many homes have only one return grill or access, so treated air is pushed into the bedroom via the supply grills then flows out the door down the hall and into the return air grill. In discerning source problems always check all the options before condemning the HVAC ductwork. It will many times only be the pathway not the source of problem.
We will end this discussion with the main adage of this profession. Remember for professionals Indoor Air Quality is normally considered fine until someone complains. If a problem is found, react quickly using the appropriate technique(s) discussed earlier in this article. (i.e. Remove source of IAQ problem, Clean and Sterilize, Ventilate, Filter, Prevent “re-growth” via UV-C, and pan treatment).  Follow up in a reasonable time frame and evaluate the results of your efforts.  Then, if necessary, take your efforts to the next level or involve a Contractor that has experience with IAQ remediation.
Take some of the procedures discussed in this article and put them to preventative use in your home or office right now. Pan treatment, UV lights, and better filters will go a long way in improving your basic IAQ and keeping your home/office environment safe and healthy. Some circumstances require more and bigger steps, but never fear there are answers to all problems and products to solve them.