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Building Science

Understanding Indoor Air Quality (1) – Building Science Podcast

People, Pollutant, Path, Pressure

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This podcast series is excerpted from a two-day class called “Building Science Fundamentals” taught by Dr. Joe Lstiburek and Dr. John Straube, of Building Science Corporation.

Last week, Dr. John discussed humidity and the damage it can do to a building. This week, Dr. Joe explains his preferred method of evaluating indoor air quality and why it works. He also points out that air contaminants are usually a symptom of the breakdown of building materials by water, heat, ultraviolet light, and ozone.


Practical air quality evaluation

We’re going to do the four p’s: people, pollutant, path, pressure. That’s a way of looking at indoor air quality, it’s a completely different way of looking at things from the traditional industrial hygiene community view of indoor air quality, which is based on concentration of contaminates versus dilution. That approach is completely correct, completely accurate, and totally useless. The approach that I’m going to propose is not completely accurate, not completely precise, but extremely useful. And so, as an engineer, this is going to be approximately correct, but not precisely or strictly correct. So it’s gonna get you eighty, ninety percent of the way to understanding what’s going on with very little effort; and once you’ve got the big picture sort of figured out, you can then transition into the traditional, industrial hygiene approach.

How it works

Okay. What’s this based on? If you have people over here, and you have a pollutant over there, you don’t have a problem because the people and the pollutant aren’t in the same place. You need a way for the pollutant to get to the people; so you need a connection. You need a path. If you have people here, the pollutant here, and you have a path, you have a problem. Well, you don’t yet — you still need something to push the pollutant down the path to the people. You need a pressure. So you need all four things for there to be a problem: people, pollutant, path, pressure.

Many different people and pollutants

The trouble with people is that they’re people! No two people are alike, so it’s difficult to get a uniform or identical response from people to a similar exposure. Some people are more sensitive than others to the same exposure. I’m a very sensitive person and you know, even to modest exposures of things I react in strange ways. You know a very small amount of alcohol makes me do very, very, very strange things. So anyway, people don’t act the same way to the same exposure. So it’s very difficult to make recommendations on exposures, because, how do you choose which group of people, or what type of person you’re dealing with? In terms of pollutants, there are a million of them available to deal with! Which one do you pick? You would go broke if all you did was try to measure pollutants. The problem with pollutant measurement technology is that we don’t have the ability to go into a space and measure the pollutants in the space. Now I’m going to be very careful about what I’ve just said. If you go into a space, we don’t have the technology to measure the pollutants in the space. You don’t go into a space and ask: “what’s in the air in this space?” You go into a space and you ask: “is this in the space? or this in the space? Or is this in the space? You’ve got a million choices to ask about. So when somebody says: “We’ve tested the air in this space, and we didn’t find anything.” No, what they’re really saying is, “What we tested for we didn’t find within our limits of accuracy of the test procedure that we used.” That’s very, very different from saying, “There’s nothing wrong with the air.” Just means what we tested for, we didn’t find within the limits of the test.

Don’t start with air testing

Well how the hell do you know what to test for? Well, you shouldn’t test for things until you already understand the problem. And so, that’s why all of the people who really know about indoor air quality, like the EPA and the Center for Disease Control in Atlanta, say that the last thing you should do is start off an investigation of indoor air quality by doing air testing. Because any of the money that you use on testing won’t be available to you to fix the problem. And the air testing is always going to find stuff in the air, you’re just not going to know what it means. And it gets even worse. If you find stuff in the air you have to ask yourself, is it bad or good? And more fundamentally, where did it come from? We don’t know whether it’s bad or good because very few contaminants have known dose/response relationships so we can actually set acceptable or unacceptable limits to it. So it’s very difficult to measure something in the air, and say, “This is actually bad, and it’ll be good if it drops below this concentration.” We can only do that with very few things. Usually it’s stuff that people don’t manufacture, make, or distribute. So we have really, really good standards for radon. Why? Nobody makes it. Who are you going to sue, God? Right? So let’s say you find something in the air. Where the hell does it come from? Well, that’s pretty fundamental, or pretty important.

Pollutants often come from damaged materials

And so I’m going to say: forget about that. Most pollutants come from things that get really hot, or really wet, or exposed to ultraviolet light or ozone. We call these things damage function. When damage functions act on materials they damage them. That’s why we call them damage functions. And one of the things that results from the damage is that the stuff falls apart. And as it falls apart, or breaks down, the break down particles are often contaminants in the form of gasses and particles. So as you expose materials to damage functions, your break down products can be viewed as contaminants that are in the form of gasses and particles and the particles are very tiny, they have a very tiny aerodynamic diameter that they can get into the air, and be transported in the air as the solid form.

What’s the biggest source?

And so what part of your building gets the hottest, and wettest, and most exposed to ultraviolet light and ozone? It’s the roof. So what’s the typical roof? You know, a black EPDM membrane. And you’re on the top of that black EPDM membrane roof in July. And the temperature is what? One hundred and eighty degrees. So you go up on that roof, and you’re standing there, and you go (inhale) — what do you smell? The roof! The roof is emitting huge quantities of VOCs — volatile organic compounds. If you take your hand and you rub it on the top of the roof it’ll actually fall apart in your hand. If those particles get into the air we have an aerosol. So you can actually sense — feel with your own nose because your nose is actually more accurate for detecting chemicals than a gas chromatograph — and you can actually smell the roof breaking down while you’re on it. Which is why it’s always a good idea to put you’re building’s fresh air intake up on the roof. Because you then actually hoover up the VOCs and eject them right into the breathing zone of the occupied space.

The pollutant is the symptom, not the problem

Who are the mold folks here? There are a couple of mold folks here. What we’ve learned is that you don’t look for mold because you’re always going to find mold in the air. What you look for is the wet spot, because that’s where the mold is coming from. So you search for mold by looking for water. Alright? So you search for VOCs not by looking for VOCs; you search for VOCs by looking for something that’s hot that would be giving off the VOCs. So you look for pollutants by looking for the thing that’s hot, or wet, or exposed to ultraviolet light, or to ozone. And then ask yourself, are those things connected to the breathing zone of the occupied space by a pathway and pressure relationship? I only sample after I know what’s going on, in order to shut up people that don’t know what’s going on. So you don’t look for pollutants, you look for the hot spot, the wet spot, the spot that’s exposed to ultraviolet light and ozone and work backwards via a pollutant, pressure, and pathway relationship. So you do an indoor air quality investigation by thinking in two dimensions at the same time. On one axis is what? People, pollutant, path, pressure. On the other axis is hot, wet, UV. Now, there’s ozone in there, but the first three damage functions are in order of magnitude more significant than the fourth one. So the really, really important ones are hot, wet, UV; ozone is kind of important, but it’s not in the same league as the other one. Why even mention it? Because it’s not insignificant. In other words it’s not the big three, but it can cause you some grief. Are you with me on this?

How damaging are these effects?

Now, each one of these effects by itself is annoying. Two or three of them acting together is really, really bad. We view these as being exponential and synergistic. Exponential means that each affect by itself gets very, very, very bad, but nor on a linear increase, but on a really ramping increase. So we know with temperature, every ten degree rise Kelvin in a material doubles the off-gassing rate. That’s eighteen degrees Fahrenheit. So you double the badness of a material every 18 degrees. Using Joe math that’s approximately what? Twenty. So every twenty degree increase you double the off-gassing. Now, it doesn’t take very long to have a factor of four. That would be how many degrees? Forty. Cause it’s two by two, so the first twenty degrees doubles it, the second twenty degrees doubles the double. So that would be four. See you want your salary to be increasing exponentially, not linearly. Right? Ok. Now the same thing with humidity. It turns out as you double vapor pressure, you basically double off-gassing. And that’s approximately eighteen percent relative humidity. That’s purely by chance. They’re not connected, the eighteen and eighteen — it just happened serendipitously … accidentally … I can’t even frickin’ pronounce it. Alright. Now the same thing with UV light. Every ten percent increase in UV light results in the doubling of the factors. Now when any two of them are acting together, they’re not additive affects, they’re multiplicative affects, so if you really what to trash something, make it hot, make it wet, and expose it to sunlight. And then zap it with a little bit of ozone.

Why should people care?

It’s the same thing for humans, not just materials. It’s true. If you want to live a long time, you go to some place that it’s cold, that it’s dry, you protect yourself from UV light, you stay away from urban areas, because urban areas have what? Ozone! And you eat nuts and berries. So you’ll live a very long time but you’ll feel miserable and wish you were dead. So where is the worst place to retire? Florida! Cause you’re going to a place that has what? Hot, wet, humidity, and it’s an urban area. So you’re going to frickin’ die. Alright, let’s go to hospitals. The worst thing to do if you’re sick is to be in a humidified space. The dryer it is, the healthier it is. So why do we humidify hospitals? Cause they’re filled with explosive gasses and we don’t want them to blow up. That’s very unhealthy. Unfortunately, you get lots of mold and bacterial growth and that’s really bad, cause if you’re sick, your immune system is weak, and you’re exposed to these opportunistic pathogens. So the most dangerous place to be if you’re sick is in a humidified hospital. It’s part of the new health care system. Alright. People, pollutants, path, pressure. Cause there’s always going to be a pathway, and there’s always going to be pressures. You can never get rid of all the holes, you can just get rid of the big holes.


  1. Tom Lofft | | #1

    IAQ vs. Passivhaus
    So, to what extent does this concept of 'dry air is healthier air' conflict with the Tight House concept promulgated by Passivhaus? A tight house keeps both energy and humidity inside. How do each of these concepts mesh with the IBC requirement for frequent external air changes, e.g., in schools, and everyone's desire to spend as few $$ as necessary on buying more energy to reheat makeup air?

  2. GBA Editor
    Martin Holladay | | #2

    Indoor humidity and ventilation
    There is no conflict. Everyone advocating tight construction practices, including Passivhaus advocates, agree on the need for a functioning mechanical ventilation system that provides adequate air changes.

    During the winter months in cold climates, high indoor humidity levels are easily addressed by raising the ventilation rate. While you wrote that "a tight house keeps both energy and humidity inside," your statement is true only during the winter. During the summer -- especially in hot, humid climates -- a tight house keeps the DRY air inside. Under those conditions, increasing the ventilation rate brings moisture into the house and increases the load on the home's air conditioner or dehumidifier.

    As you can see, the issue of determining the correct ventilation rate is much trickier under hot, humid summer conditions than under winter conditions. Experts disagree about whether homes in hot, humid climates should ventilate at the ASHRAE 62.2 rate or at something closer to 50% of the ASHRAE 62.2 rate. Fresh-air fanatics can, of course, ventilate at a very high rate if they choose -- as long as they are willing to pay the energy penalty (higher energy costs for air conditioning and dehumidification).

    To read more on this issue, check out .

  3. Laura | | #3

    What about formaldehyde?
    What about formaldehyde. You can buy beautifully crafted cabinetry, made from UF particle board or MDF or plywood that are sealed, and they will still off-gas formaldehyde in sometimes alarming amounts, for years. They don't have to be damaged or heated or wet. Of course if you do heat them, they'll emit even more. And in a tight home this doesn't get circulated.

    A solution is to bring only products, furniture, flooring, etc that you know are safe. You can ask for formaldehyde-free plywood to be used in your cabinets, closets, built-ins etc. Use only non-toxic insulation. Ask questions when you plan and purchase. Avoid bringing VOCs into your home to start with.

  4. GBA Editor
    Martin Holladay | | #4

    What do you mean by a tight home?
    Problems with formaldehyde or other sources of indoor pollution are not worse in a tight home, as you imply. They are worse in a poorly ventilated home.

    No one disagrees with your recommendation to choose products with low levels of formaldehyde. However, a tight home with a good ventilation system will provide better IAQ than a leaky home without a ventilation system. Building a leaky home does NOT guarantee that fresh air will enter the house, nor will it guarantee that fresh air will reach the rooms where the air is needed.

    I'm sorry, there really shouldn't be any exceptions to this rule: every new home should be built to be as tight as possible, and should include a well designed mechanical ventilation system.

  5. John Zito | | #5

    Tight Homes
    Before we start any remodel/addition project that will involve the building envelope/mechanical systems, we blower door/infrared as a baseline. Many homes that are 20-25 years old are well below AHRAE 62.2, even in the starter home category (alot of polyiso over osb). Probably as no strange coincidence, the oft quoted rates for asthma rates (as well as chemical sensitivities) rising start with that period. I agree to make smart product choices, but we needed mechanical vetilation then, and need it now.

  6. Thirteen | | #6

    good stuff
    I just really appreciated reading this and wanted to say thanks for posting it online. Lots of good information!
    From Sunny Humid Suburban Florida,

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