Archive for the ‘Indoor Air Quality’ Category

Central vs. Portable Humidification Systems

November 14, 2010

In previous posts, I discussed how humidification systems work and how effective a central humidification system might be.  In that post, I hinted at differences between central (whole house) humidification systems and portable (local) humidification systems.  In this post, I want to discuss more about the differences between the two systems.

If you have not realized yet from previous posts, I am biased–I am not a fan of central humidification systems.  I see them often in the homes I inspect; but I suspect that most do not work as intended.  Yet, there they are.  I have to wonder if the installation company actually analyzed the need for a humidification system or was just selling a product that is quite profitable for the company.  Simply asking the homeowner whether he and/or she wanted a humidification system is not the analysis about which I am talking, by the way.

Following are the reasons that I believe a PHS is better than a central humidification system.  For simplification, CHS is used for central humidification system, while PHS is used for PHS.

First is the big picture.  Why try to humidify a whole home when all of the home occupants are not in all of the home’s areas at once?  Let me put is bluntly, injecting a gallon of water into a room is going to be more effective at raising the humidity than injecting a gallon of water into the whole home.

Second is effectiveness.  I believe that a portable humidification system providing spot moisture would likely work better than a CHS.  With a PHS, moisture from a judicially placed unit can be directed into the air around a person’s head (the area that industrial hygienist types call the breathing zone or when they really want to be cool—the BZ).  Even if the PHS cannot be aimed to direct moisture into the occupant’s breathing zone, it can be located close enough to elevate the moisture in the air people are breathing.  If the room can be closed up, such as a bedroom, humidification will likely be more successful than trying to humidify an entire home.  In fact, in the previous blog where I discussed virus and humidification, a portable humidifier was used in that research.  As my previous blog post showed,, CHSs are probably not effective at significantly elevating humidity levels in some homes, particularly air leaky homes.

Home leakiness leads to the third reason portable is better than CHSs—losses.  Moisture in the air is in the form of water vapor, which behaves like the other gases in air.  As such, if the amount of airborne moisture in one area is greater than in another area, moisture in the first area will travel to the second area as long as the two areas are connected.  On cold winter days, the amount of moisture in the outdoor air is usually lower than the amount in indoor air (a subject for another post).  In air leaky homes, indoor moisture will likely move outdoors, even if outdoor air is moving in the opposite direction.  So, most of the water a CHS is putting into the air could be traveling directly outdoors, barely elevating the indoor humidity levels.  Over an entire home, the total area of the air leaks is less than those in a single room.  Therefore, the amount of moisture being lost from one area will be less than throughout all areas.  If a PHS is supplying more moisture to a given area than the central system is supplying to the entire home area, moisture levels in the area with the PHS will be greater.  Even if the moisture from the PHS is also traveling outdoors, it has a better chance of being effective as it travels through people’s breathing zones on it trip outdoors.

A fourth reason is condensation areas.  Most homes have cold surfaces in the winter where condensation can occur.  The chances that condensation surfaces are in the same room with a PHS are less than the chances of moisture from a CHS seeing a condensation surface.

A fifth reason is better humidity control.  Some PHSs now come with their own humidistats.  Basically, the portable unit is sensing the humidity right in the space where the person is.  CHS humidistats are installed in the return air duct in an effort to sense the “average” humidity in the home.  What happens if the return air system is pulling more air from some areas of the home than others, meaning that it is not sensing the true average humidity levels?  That issue is more common than you might think.  Some central system humidistats are placed on a wall in the home; but those also have the same issues as far as sensing the “average” humidity levels.

A sixth reason is operation.  CHSs, if they are working right, only humidify air when the air handling system is working.  During the rest of the time, the CHS is at the mercy of the thermostat.  The central humidification control system has to wait until the thermostat calls for heat before it can work.  Just in case someone is thinking that the CHS can be set to operate without the furnace, remember that the CHS needs air moving through it to work.  Someone is likely also thinking that the thermostat can be set to ON so that the fan is operating all the time.  Then, if the humidistat calls for humidification, air will be flowing through the CHS.  It could; but, the reason air downstream of the furnace is passed through the CHS is because heated air can hold more moisture than cooler air. So, more water will be lost with the CHS if unheated air is passed through it than heated air.  With PHSs, the unit is always injecting moisture into the air without the need for moving air to transport the moisture.

A seventh reason is maintenance.  If a valve on the CHS sticks open, water will be dumped right down the drain when air is not moving through it.  In some cases, the CHS drain is plugged up, and water is dumped into the air handler and then ultimately onto the floor outside the unit.  If that water travels to nearby furnished areas, moisture-related damage can occur.  Sometimes, the damage is extensive, such as shown in the photos below.  The fact is that CHSs are usually not inspected very often and problems may not found until a serious malfunction occurs.  If a PHS malfunctions, it is usually right in the same room with the home’s occupants, who can then see that a problem is occurring.  Maintenance of a PHS is usually so easy that the home owner can do it.  For most homeowners, a HVAC technician is needed to service the CHS.  Remember too, that the more debris that collects on the media inside a CHS, the less air can get through the CHS and the less effective it will be.  The PHS, on the other hand, can be kept clean of debris.


Water damage caused by a malfunctioning CHS



An eighth is bioaerosols.  One of the more well-known cases with PHSs is humidifier fever caused by a PHS that was not properly cleaned and reservoir water was allowed to sit in the unit and grow yuck.  When the unit was operated, the yuck was injected into the air that people were breathing.  But, CHSs are not without the same problems.  In fact, they are essentially operating as a back-up filter to air handler filter.  The debris collected on the media inside the CHS stays there until the media is replaced and that debris contains bioaerosols that may find the conditions inside the CHS a very nice place to grow.  Nowadays, most PHSs are made so that the owner can readily clean the unit and all of them recommend using fresh water every time the unit is operated.

A ninth reason is cost.  A homeowner can buy a lot of PHSs for the cost of one CHS.  For sure, the initial cost of a CHS is much more than a PHS.  But, consider also that if you are not happy with the operation of the humidification system, replacing a PHS is a lot cheaper than replacing a CHS.  I have found cases where homes had unused or disabled CHSs along with PHSs that were being used.

A tenth and final reason is choice.  For residences, homeowners are mainly stuck with one option—the wetted media CHS.  Although residential steam injection CHSs are available, they are even more expensive than the wetted media CHSs.  With PHSs, the owner has not only choice of the method of humidification, as explained in a previous post, a number of manufacturers produce the various types of PHSs.  Having a range of options also means more competition with PHSs than with CHSs, which further means price and feature competition.  I have seen several CHSs and they appear to be amazingly similar, while I have seen a range of PHS designs and those designs continue to evolve.

I guess I could be faulted in this post for not finding more advantages of CHSs over PHSs.  The truth is, none comes to my mind other than the fact that water is supplied to the CHS, while the owner has to carry water to the PHS.  Even though that difference could be considered an advantage of a CHS over a PHS, I think it can also be considered a disadvantage because when changing water, the owner actually is inspecting the PHS and likely keeping it clean.

If you have another opinion, let me know.

The basics of humidification systems

October 6, 2010

In a previous post on virus and humidification, I promised to discuss more about humidification systems.  In this post, I am going to discuss the basics of humidification systems.  Humidity is a measure of the moisture in the air.  Technically, the humidity that everyone is usually talking about, including the weather guys is relative humidity.  Every so often, the term absolute humidity sneaks out of the scientific and engineering circles into the public.  This is a different measurement of moisture in the air; but that is a subject for another day when I pretty much run out of other topics—and I do have a list.

Humidification systems are simply devices for putting moisture into the air.  In our area, we only want to put moisture in the air during the winter.  In the summer, we are more concerned with getting out of the air.  The reason that people want to humidify air is that the air in our homes is extremely dry during the winter.  Even though outdoor air during the winter may have what seems like a normal humidity level, when that air comes indoors and is heated, the relative humidity levels drop to levels seen in a desert—somewhere around 10%.

For homes, a homeowner has really two basic choices in humidification systems:  central and portable.  A central humidification system supplies water to the air going into or coming from the air handler.  The humidified air is then distributed to all areas of the home.

Central humidification systems typically are recognized as a plastic box mounted on a return air duct near the air handler as seen in the photo below.  However, I recently saw on system mounted on the supply air duct, which may have been the work of an installation instruction-challenged installer.

A typical central humidification system

Inside the box is a web-like media through which air flows.  The air that flows through this media usually comes from the supply air duct just downstream from the air handler (the duct coming into the side of the humidifier in the above photo).  A humidistat connected to a sensor normally in the return air duct upstream of the humidification system (the little white box above the humidifier in the above photo) operates an electronic water valve that controls the water flowing onto the media (the valve can be seen alongside the humidifier in the photo above).  The humidistat is a control system for sensing moisture in the air similar to the way a thermostat senses temperature.  The water valve is connected to the home’s water supply.

When the humidistat senses that the moisture level of the air is too low, it opens the valve to let water flow over the media.  When the humidistat is satisfied, it closes the valve.  Air flowing through the media is supposed to pick up the moisture through evaporation and carry it into the return air stream.  Water flowing across the media that is not absorbed by the air is supposed to flow into an overflow that has a drain line to a nearby drain (the black tubing below the humidifier in the photo above).  In a few cases, the humidistat is located in a living area of the home, usually next to the thermostat.  Some humidistats are incorporated into the thermostat.

Most people will be more familiar with the portable humidification system, particularly since they are sold pretty much everywhere.  These units are self-contained humidifiers that have a water reservoir and a means for converting the water into a mist.  Some units also have a small fan to power air through the unit.  Portable humidifiers have three methods for turning the water into a mist.

The first method for turning water into a mist is a nebulizer that is simply a vibrating plate onto which the water dribbles.  The vibration breaks the water into a mist that is usually carried into the air by a fan-powered air stream.  These units are pretty simple devices that use the electric current to generate the vibrations in the plate.

The second method for turning water into a mist is a rotating disk that slings water at a comb-like device, which is supposed to break the water up into a mist.  The spinning disk operates both as a kind of pump to pull water up out of the reservoir and a means of slinging the water at the comb and a fan to push the mist out of the unit.  This method tends to generate larger water droplets than the other methods, which can lead to water falling on surfaces around the unit.  But, they are also commonly the cheapest units.

The third method for turning water into a mist is the old tried and true method from many of us boomers child-hoods is steam generation.  These humidifiers have a electrical resistance element that heats up when electricity runs through it, similar to the elements on an electric stove.  Water in contact with the heated element boils and is converted into steam.  The steam literally is forced out of the unit because of the energy it absorbs as it turns to steam.  This method of humidification is also used in most commercial and higher-end home humidification systems.

So, in summary, humidifiers convert water into water vapor through three methods; but all of them ultimately work through evaporation.  Central humidification systems use direct evaporation of the moisture into the air as it passes through a wet media, similar to the way that we evaporate water from our skin.  The other methods generate a mist with the intent to get it into the room air where the water will evaporate into the surrounding air.  For these mist generators, the smaller the size of the droplets in the mist, the better the water evaporates.  That doesn’t mean that central systems are more effective humidifiers than the others.  Steam systems are actually the most effective method, but take more energy to do the humidification.

I will end this part of the discussion there.  In the next post on humidification, I will discuss the pros and cons of central and portable humidification systems.

Humidification and Virus

September 29, 2010

The Environmental Health Journal published an article this past month titled, “Modeling the airborne survival of influenza virus in a residential setting: the impacts of home humidification“.  The results of the research taken from the article are, “The presence of a portable humidifier with an output of 0.16 kg water per hour in the bedroom resulted in an increase in median sleeping hours AH/RH levels of 11 to 19% compared to periods without a humidifier present. The associated percent decrease in influenza virus survival was 17.5 – 31.6%. Distribution of water vapor through a residence was estimated to yield 3 to 12% increases in AH/RH and 7.8-13.9% reductions in influenza virus survival.”

What the article states that a slight rise in relative humidity in the room caused a slight drop in the influenza viruses in the air.  I have a couple of criticisms about the article, though.  The authors derived this conclusion based on limited data using modeling instead of actual sampling of the virus.  The lack of data does not really show how the trend would go for humidity versus virus concentrations.  Without data from actual virus, the question also exists whether the model is realistic or whether the results are repeatable.  I cannot really fault them because sampling for viruses is difficult at its least, particularly infectious viruses and research is expensive.

But, I am still concerned about whether the risks of a humidification system are worth the beneficial effect on virus.  Other studies have shown that hygiene–hand washing and avoiding sick people–are quite effective in preventing or minimizing virus-related illnesses.  Flu shots are also effective in preventing flu for which the sera are developed.  I can personally attest to that fact.  I also tend to believe that flu shots, even past ones, may help minimize the effects of other virus.

I will confess, though, that I am not a big fan of some humidification systems, particularly central humidification systems.  I have observed too many issues with them from basic design to maintenance-related issues.  In future blogs, I am going to go into more detail about humidification systems and their pros and cons, as I see them.  I even got off on that tangent writing about those issues for this blog.  I decided to reign in this post to stick with the topic of humidity and virus.

What can be drawn from this and other research.  I believe that humidity levels could have an effect on virus levels in the air.  I have known for a long time that when those sloppy wet infectious particles that are sneezed into the air, or fomites as the medical gang call them, they have a better chance of drying in low-humidity air.  Drying of the particles means that they become smaller and lighter, which in turn means that they can stay afloat in the air longer.  They can also be breathed deeper into the respiratory system where they can more easily set up shop.  The weird thing is that some virus actually survive better in the dried out state.   (Here are a couple of examples of research pointing to that conclusion:  “Dry air might boost flu transmission” and “Flu Mystery Solved? Why It Flourishes in Winter“.)  Elevated humidity is a good thing.  My dispute is with the way to humidify, and that will be the subject of other posts.

Is air sampling for mold a necessity?

February 22, 2010

I lost a home inspection job for this weekend and I believe I know why.  The job involved not just the routine inspection but also had a suspected mold problem.  Although I would have liked the income, I am more concerned that the potential client decided on another inspector because he was convinced that air sampling for a potential mold problem was needed.  The client said that possible mold was present and described construction that could create moisture conditions conducive to mold growth.  But, I told the client that usually air sampling for mold is not needed because no matter what, if you see mold, you clean it up.  If you find moisture problems that could lead to mold growth, even if no visible mold growth is present, you attempt to eliminate them.  A skilled inspector should be able to recognize both without the need for air sampling.

The truth is that no where is there a requirement for air sampling.  In fact, air sampling is usually not recommended.  The main reason is that the complexities of mold and sampling for mold usually creates more confusion than explanation.  The results are usually confusing, and many times do not mean anything.  Over the years, I have found that nearly every time I have collected air samples–and a lot of other types of mold samples other than clearance samples–the results create confusion and misinterpretation.  When I have been an expert consultant in legal cases, I most often do not have trouble discrediting others mold sampling results.  And the truth is to get any kind of statistical accuracy upon which to make significant conclusions, many more samples are needed rather than the two or three most so-called “mold experts” collect.

Over the years, I have found that when so-called experts do not really understand what they are doing, they rely religiously on protocols they learned in their two or three day mold courses.  Those courses teach them how to conduct sampling, but usually do not dig very deep into the logic behind the sampling.  In most cases, the limitations of the sampling are not explained.  Further, I suspect that even when the limitation are explained, most attendees at these classes do not really grasp those limitations because they do not have the background to understand them.  I know my background and all of the various bits of expertise, special training  and experience I have needed to understand those limitations, and it took over 25 years to get it.  So, I suspect highly that a person coming from a non-science background with less than a week’s training probably does not understand them.

The thing is air sampling for mold is a tool, just like many other tools needed to investigate such problems.  In fact, I can think of nearly 20 different types of sampling used to investigate mold problems.  In fact, many various air sampling methods exist besides the usual Air-O-Cell cassette usually used by so-called “mold experts” and I know of at least three air samplers that collect samples similarly to the Air-O-Cell.   I have found that many of the other sampling methods even provide more useful information than any air samples.  With air samples, you HAVE TO understand how air travels throughout an area to determine the validity of the sample and whether it provides information about a risk.

But, the most important tools that an investigator takes into an area is his/her visual acuity and knowledge.  I specifically stated visual acuity because the inspector needs to have an eye for detail.  I have been on many inspections with clients where I have pointed out possible mold or signs of moisture problems that the client did not even see.  The stuff between the ears can only be gotten one way and that is through long hours of learning, knowing the right people and a lot of hard work.  No one is going to stuff that expertise into someone’s head in a couple of days.

So, when it comes right down to it, I lost an job opportunity because someone else was much better at selling a likely unneeded service than I was at convincing the client that the service was NOT needed.  At the same time, the client had a part in my loss and  I don’t mean by just selecting the other person.  No, the client also came into the picture with beliefs–things read or heard.  In fact, I could hear doubt in the client’s voice when I said that I rarely take air samples.   When I get these calls, I try to educate the client.  Sometimes, I succeed and sometimes I don’t.  My only request to anyone reading this is that you listen and learn to ask the right questions.  I also recommend that you also dig deeper into the expertise of the person offering you advice.  It could save you a lot of money in the long run.

Plants as indoor air cleaners

December 9, 2009

Here we go again.  WebMD reported this week that HortScience had an article in August about research on using house plants to remove indoor air pollution.  Interestingly, the article did not report anything about been there, done that.  Well, not me personally.  But, a few years ago, NASA was doing similar research.  At that time, a bunch of people were proposing that plants could be used to clean indoor air.

The research seems to be clear.  Yes, some plants appear to be able to remove some indoor pollutants.  The truth is in the practicality.  Usually, these studies are designed using injection of an initial concentration of a chemical inside an airtight container that contains the plant.  The chemical’s concentrations are then monitored over time to determine how well the plant removes the compound.  The first possible problem with this kind of research is that some of the compound could be lost to surfaces inside the container.  If this research is like most of this kind of research, such losses are not included in the study details.  Another problem is that, over time, chemicals can also break down, depending on the environment and the chemical.  Again, study details don’t usually describe how this loss is accounted for.  And a final detail about the study methodology is sometimes overlooked, and that is loss of chemical to the study.  Each time a sample is removed, the chemical concentration inside the container decreases.

Those facts were about the practicality of the study design.  Reality is probably the least practical aspect of the study results.  We don’t usually have only one chemical floating around in the air.  Does that mean if a whole bunch of plants would be needed if they each have their own chemical they like to dine on?   Realistically, chemicals are not put into the environment in one squirt and then left to decline over time.  Some chemicals are emitted into the air over time, such as those from building materials.  Even more complicating is the fact that chemical concentrations can vary over time because the emissions from materials vary over time, depending on a range of environmental factors.  I have collected this kind of sample data myself, as have many other researchers.  The changes in chemicals floating around a building are pretty amazing to study, actually.

In our buildings, air is constantly being exchanged–moving in and out.  No building is airtight like the containers used for most plant studies.  Air coming in also can carry chemicals into a space and air leaving can taken them out, which is also another reason why chemicals and their concentrations can vary dramatically over time.  Changes in the rate that air enters and leaves a space can be controlled by factors inside and outside the building.  Wind direction and speed, house design, sun and many other factors can influence air movement inside a home.  Operation of the air handler, people movement, heat sources, interior design and many other factors can influence air movement inside a home.  As vapors, chemicals are also influenced by the Ideal Gas Laws, in particular the part about partial pressures which says that vapor pressure of any one chemical in one area will equilibriate with vapor pressure of the same chemical in an adjoining space if the chemical can move between them.  So, chemicals at a higher concentration inside compared to outside a building will eventually equilibriate with concentrations in the outside air.

Materials inside a building also absorb and re-emit chemicals.  Ever smell cigarette odor on non-smoker who has been in a smokey bar?  The fixed things in buildings, the things of which the building is constructed, as well as the solid materials floating around in the air and the other things moving through the building, such as people, pets and their related materials, all absorb and re-emit chemicals.  The researchers call things that absorb sinks and the things that emit sources.  A dynamic exchange is going on between these things all the time.  Further, something that is a sink at this time can become a source when a chemicals concentration changes.  So, what is a plant to do?  Then again, how many plants should a person have when the emission rates could be constantly changing?

But, let’s dig into that question a little more.  How many plants would be needed.  Past research has shown that plants absorb a definable, but minute amount.  Such research predicted that a small forest would be needed to make a significant dent in the chemical concentrations.  Again, we are talking about static chemical concentrations, whereas most buildings have dynamic concentrations.  That brings me to another subject that seems to often overlooked.

Past “Plants Eat Chemical Compounds” researchers also took a lot of criticism for one major overlooked factor.  Plants grow in soil.  Soil contains–dare I say that M word–yes, mold!  In fact, many plants have a symbiotic relationship with certain molds to help them absorb nutrients.  But, many molds themselves emit chemicals into the air.  So, a person has to ask, is it practical to use plants to absorb one chemical when the soil in which they grow could be emitting a whole bunch of other ones?  Probably not.

NASA had a good reason to research whether plants absorbed chemicals or not.  NASA was interested in using plants on spaceships making long journeys where plants might be used for food and to help control the air quality inside the ship.  Currently, NASA uses chemical filters to control chemicals emitted into their ships.  On a long journey, those filters could become saturated, and in some cases then start re-emitting chemicals back into the air.  Having something that absorbs and holds the chemicals would be nice.  Having something that absorbs those chemicals and then turns them into a product that the spaceship residents can use would be great.  Now, given we are kind of like astronauts flying along on this great ship Mother Earth, we still have enough air to take care of plants, chemicals and us.  The amount of air around us is currently adequate to support a recycling system that maintains a reasonably (although I admit debatably) healthy air quality.  Other systems inside out buildings do a better job than plants.

For solid materials floating around in our building air, particle filters do a very good job–as long as air is moving from the building through the filter.  For gaseous materials, chemical absorption filters can work well; but in most cases, they are impractical for many buildings.  More effective, though, is dilution where outside air is brought indoors to reduce chemical concentrations.  And that is where I am going to leave things for right now.  I promise to cover more about control of gaseous chemicals in a future blog.  Suffice to end this blog this way:  use plants to make you feel good mentally; but use dilution or filters to make you breathe well.

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