Fact: the air inside our houses can make us sick. The good news is that it is possible to build and operate houses that don’t make us sick.
Actually, there are several ways to improve the air quality indoors—but all the solutions ideally should to be coupled with better ventilation. And that is what this article is all about—ventilation—supplying houses with fresh, clean air and removing the stale, polluted air. But before discussing how to ventilate, it is important to understand what air is composed of, how air gets polluted, and why polluted air is not good to breathe. In other words, the problem needs to be understood before an attempt is made to solve it.
The atmosphere
Like all living creatures, human beings require oxygen to survive. The atmosphere contains about 21% of this vital element. High-school science textbooks tell us that the remaining 79% of the air surrounding our planet is almost entirely nitrogen, with small amounts of gases such as argon, carbon dioxide, helium, krypton, neon, and xenon. Air also contains a certain amount of water vapor.
A wide variety of naturally occurring air pollutants also float around us waiting to be breathed. For example, volcanoes spew out dust and sulfur compounds. Forest fires contaminate the air with soot and smoke. Radon is a naturally occurring radioactive gas found in small quantities in the air virtually everywhere. Swamp gas, sea salt, sand particles, and petroleum emanating from the surface of the earth can also “naturally” affect air quality. Living creatures pollute the air with their waste products and the by-products of their metabolism—carbon dioxide, ammonia, methane gas, water vapor, etc.—and when living creatures die, their decomposition releases additional pollutants into the air.
“Natural” pollutants also include pollen and mold spores—tiny particles that can aggravate a variety of symptoms in allergic or asthmatic individuals. The characteristic odor of mold or mildew is something many people find irritating or offensive. Disease-causing viruses and bacteria are also considered air pollutants.
Ozone is a gaseous pollutant that has been around for millennia. It is a powerful oxidizing agent—meaning it can react with a variety of substances, including human tissue. As a result, it is very irritating to the respiratory tract. Ozone consists of three oxygen atoms bound together, while the oxygen we need to breathe is found in atomic pairs. This may seem like a minor difference, but our bodies were not designed to metabolize the three oxygen atoms of ozone. In the lower levels of the atmosphere, ozone is often a component of smog, where it can cause burning of the eyes and breathing difficulties. Paradoxically, in the upper atmosphere—where there are no people—ozone is vital to our survival. High above the earth’s surface, a thin layer of ozone blocks a portion of the ultraviolet light from the sun. Where the ozone layer in the upper atmosphere is thin or missing, excess ultraviolet light can pass through the atmosphere and reach the earth’s surface. Too much ultraviolet light increases incidences of skin cancer.
Man-made pollutants
In addition to all the naturally occurring air pollutants, many man-made contaminants exist as well. One of the earliest examples of man-made pollution was a campfire. When the fire was brought inside a cave, it was likely responsible for the first case of indoor air pollution. Over the centuries, human beings have discovered many more ways to contaminate the air. Today, primarily as a result of the industrial revolution and “better living through chemistry,” the air we are breathing contains hundreds of new pollutants, some of which are difficult to pronounce. We have all heard of formaldehyde, but other compounds such as limonene, 4-Phenylcyclohexene, and ethylbenzene rarely pop up in daily conversation.
The air in a typical house is filled with hundreds of gaseous pollutants that have only been synthesized in the last several decades. Many of these contaminants originate in paints, caulking, adhesives, cleaning supplies, building materials, home furnishings, printing ink, and dozens of other products that we come in contact with daily. While these substances may not have an immediate and noticeable effect on many of us, people who have an illness called Multiple Chemical Sensitivity (MCS) often exhibit severe symptoms when exposed to these common sources of indoor pollution.
Since the advent of the industrial revolution in the late 1800s, everyone on the planet has been exposed to ever-increasing amounts of combustion by-products. While emissions from factory smokestacks and automobiles are now strictly regulated—at least in industrialized countries—we still breathe far more pollution from combustion processes than is good for us. As a result, some parts of the U.S. have restrictions on the use of wood stoves and fireplaces, but for the most part, pollution from these devices is very poorly regulated. Gas or oil furnaces and water heaters, space heaters, and gas ranges are also common sources of combustion gases. While their contribution to atmospheric pollution is certainly a serious concern, the greatest immediate danger to human health is when they directly pollute the indoor air—something that, unfortunately, happens on a regular basis.
Indoor pollutant categories
Literally thousands of possible air pollutants can be found inside a house. They range from lead paint to asbestos, radon to carbon monoxide, formaldehyde to dust mites. With new chemical compounds being developed daily, the list continues to grow. Following is a brief rundown of what types of materials contaminate the air we breathe indoors.
Biological pollutants
Biological pollutants are sometimes appropriately called “bio-nasties” in indoor pollution circles. They include such agents as dust mites, mold, mildew, Legionella pneumophila bacteria, decay-causing fungi, viruses, pollen, etc. These are all organisms that can negatively affect health. There are currently some 41 million people in the United States who have conventional allergies, so there is a good chance that someone in a majority of households will end up with a runny nose, itchy eyes, sinus congestion, or a headache if these pollutants are allowed to proliferate indoors. When someone suffers from an allergic reaction, their body is weakened, making them more susceptible to many other air pollutants.
Of course, even more devastating health effects than allergies are attributed directly to biological pollutants. Legionnaire’s disease, a vivid example, took its toll in human life. Some molds can be equally deadly and an asthmatic attack can be quite frightening.
One of the most common indoor biological pollutants is the dust mite. These microscopic creatures find bedding and carpeting to be a comfortable home where they enjoy their favorite meal—tiny flakes and particles of dead skin we all shed every day. If someone is allergic to dust mites, he or she typically reacts to the extremely small pellets of mite feces that can be stirred up by human motion.
Mold and mildew are also biological pollutants prevalent in homes, especially where water or high humidity is present. Basements and bathrooms are particularly vulnerable. If a moisture problem is long-lasting, it can progress to decay, resulting in structural damage to the house itself. Even in a house that seems dry, areas of high humidity—microclimates—always exist near cold surfaces. Because of this, the center of a room can be fairly dry, while mold may be growing on a cold window frame or in the corner of a closet. A variety of moisture sources affect the humidity in a house: bathing, laundering, washing dishes, exhaled breath, defective gutters, plumbing leaks, inadequate damp-proofing of basements or crawl spaces, etc.
Histoplasmosis is a serious infectious disease caused by a fungus that can be associated with pigeon droppings in attics. It should be of concern if you plan on doing any demolition during a major remodeling project. Pets are another common source of allergic reactions because of their urine, dander, shedding, and saliva. More people are probably allergic to cats than to any other pet, but sensitivities to pets ranging from birds to dogs are well-documented. And allergies to insects, such as cockroaches, isn’t unusual.
Pesticides
Sometimes, biological pollutants themselves don’t have a direct effect on human health, but the toxic pest-control chemicals used to eradicate them cause problems. A good example involves termites—small creatures we seldom see. The termite workers that actually do the damage to the wood structure of a house can pass through a crack as small as 1/32", so it is quite easy for them to remain hidden from view. The chemicals that have typically been used to control termites have impaired the health of many people around the world. These pest-control chemicals are literally formulated to kill, and most don’t differentiate between termites and other living things—they affect all forms of life. Because termites are so small, they are easily killed with these toxic solutions. Small amounts of termiticides have caused a variety of negative health effects ranging from immune-system damage, headaches, nausea, and dizziness to muscle spasms, confusion, and Multiple Chemical Sensitivity (MCS).
In Safety at Home, the National Coalition Against the Misuse of Pesticides (NCAMP) reported that consumers in the U.S. buy and use a phenomenal 285 million pounds of toxic pesticides every year. The Coalition says these chemicals “are nerve poisons, can cause cancer, respiratory problems, birth defects, genetic damage, injure wildlife, and pollute the environment and drinking water.” When used indoors, pesticides can be serious indoor air pollutants. Fortunately, less-toxic methods of pest control are available.
Gases
Gases are another major category of air pollutants. While it is obvious that an automobile’s exhaust pipe is emitting noxious combustion by-products, many houses contain unvented furnaces and space heaters that expel combustion gases directly into the indoor air. Natural gas and propane kitchen ranges can emit carbon monoxide, carbon dioxide, and nitrogen dioxide. Even furnaces and fireplaces that are connected to a chimney can be problematic because it is not uncommon for the combustion gases to backdraft, that is, flow back down the chimney. This is something that can actually be caused by an improperly designed ventilation system.
Volatile organic compounds (VOCs) are gases that are emitted, or outgassed, from a wide variety of modern materials. The word volatile means these gases evaporate easily—usually below room temperature—and the word organic means they contain one or more atoms of carbon. Many VOCs are difficult to spell and pronounce: cyclopentadine-ethenyl-2-ethylene, hexamethylene triamine, tetrachloroethylene, 4-phenylcyclohexene, etc. Formaldehyde is probably the best known. It is colorless, only has an odor at high concentrations, and is a probable human carcinogen as well as a sensitizer. Exposure to formaldehyde can sensitize you so that your body will react to very tiny amounts of it—amounts that were previously not a problem. Once sensitized, you may begin to react to a wide range of other VOCs as well, and a typical house could have a hundred or more different VOCs floating around in the air. These include compounds that are neurotoxins (toxic to the nervous system, spinal cord, brain, etc.), carcinogens (cancer-causing), mutagens (alter chromosomes, genes, etc.), teratogens (interfere with fetal development), and irritants (cause part of the body to become overly sensitive). The characteristic odor of a new car’s interior or a new vinyl sofa is composed of dozens of different VOCs.
Most VOCs have not been studied for their precise health effects, but an evaluation of 52 compounds released from common building materials found that 25% were known or suspected carcinogens and 82% were known or suspected irritants. To learn how VOCs act in combination with each other (synergism) is virtually impossible because the concentrations and specific compounds vary so much from house to house. What is known is that, in general, it is wise to limit our exposure to VOCs.
VOCs from products such as fresh paint will dissipate relatively quickly, often in a matter of days, but other compounds can linger for weeks or months. Building materials such as particleboard and medium-density fiberboard will outgas formaldehyde into the air for years. The outgassing rate for VOCs changes with the seasons: it increases as the temperature and humidity go up. In other words, VOCs outgas faster during a hot, humid summer than in a cold, dry winter.
People and animals also release several different gases. These are the pollutants we all give off just because we are alive—they are normal by-products of metabolism, but they are pollutants nevertheless. They include such gases as acetaldehyde, acetone, ammonia, carbon dioxide, hydrogen sulfide, methane, toluene, and water vapor. Sometimes we release more pollutants than normal, for example, after heavy exercise or eating certain foods.
How pollutants get into houses
Because air pollutants originate from a wide variety of sources, a control strategy effective with one particular contaminant may not be very effective with another; in fact, some control strategies can reduce the concentration of one pollutant but increase another.
Ventilation is often considered a universal method of controlling indoor pollution. However, ventilation can only reduce the concentration of airborne pollutants, and not all forms of pollution are airborne.
Sometimes particulate pollutants (as opposed to gaseous pollutants) such as lead dust, asbestos fibers, or mold spores will be airborne for a while, then will settle out of the air when the air is still. When they aren’t floating in the air, you can’t breathe them—but you can still get them on your fingers by touching a contaminated surface. If a ventilation system stirs up these pollutants and causes them to become airborne, two things can happen: 1) your exposure will increase (while they are airborne) and 2) the ventilation system will help to blow them out of the house, thus reducing the duration of your exposure. If a forced-air heating/cooling system stirs up these pollutants, your exposure can also increase, but instead of blowing them out of the house, it can circulate them to other rooms. Of course, some forced-air heating/cooling systems contain a filter that can capture airborne pollutants.
In order to determine the best indoor-pollution control strategy, it is helpful to place airborne indoor contaminants into three categories: those that are released from materials inside the house, those that can be sucked into the house by air pressure differences, and those that are released by people. Pollutants can also diffuse from outside the house through solid building materials into the living space, but this is a far less significant route of entry.
Pollutants can be released from materials in the house
Many cleaning products and household furnishings release contaminants directly into the indoor air. Formaldehyde is given off by kitchen cabinets. Wallpaper is treated with fungicides. The odor associated with new carpet consists of different VOCs. Disinfectant and pesticide aerosol sprays often contain hazardous ingredients. Bothersome chemical treatments are found on upholstery fabric. And on and on. When you analyze everything found inside a typical house, it quickly becomes apparent that we may be surrounding ourselves with unhealthy materials. The good news is that there are many alternative products on the market that are much more benign that can be used to build, furnish, and maintain our houses. If we used more of these healthier products, our indoor air quality would be much better. When indoor air quality is fairly good to begin with, a small-capacity ventilation system is often enough to keep it clean.
Sometimes, pollutants released from materials in a house are described as being either avoidable or unavoidable. In reality, all the pollutants in this category are avoidable because there are low-tox choices on the market for virtually everything—from less polluting building materials and furnishings to healthier cleaning products. However, from a practical standpoint, it can be expensive to remove all the polluting materials from an existing house and replace them with healthier alternatives. So, even though many pollution sources are technically avoidable, in the real world, some are more avoidable than others. For instance, because books, newspapers, and magazines release VOCs as the ink ages, they could be avoided by banning them from the house, but most people would be unwilling to do so. Even though avoidance is still generally more effective, for pollutants that are less avoidable, a ventilation system that blows them outdoors can minimize their negative effect.
Biological pollutants can grow on surfaces inside the living space and be released into the air. For example, mold can grow on the walls of a room with a high relative humidity and dust mites can live and grow in bedding and carpeting. In these instances, the pollutants are released from living creatures that grow on materials in the house.
Pollutants can be sucked indoors by air-pressure differences
Some air pollutants originate outdoors but get sucked indoors by air-pressure differences. The pressure differences we are talking about are quite small—usually too small to even feel—but they are quite common. For example, when you turn on a clothes dryer, it blows a certain amount of air out of the house. This creates a slight negative pressure in the house, and an equal volume of air gets sucked in (infiltrates) from the outdoors through small hidden gaps and cracks in the house. This infiltrating air can bring pollutants with it.
Actually, it is quite common for infiltrating air to bring pollutants indoors. The prime example is radon. It is found in the soil but it gets pulled through cracks in the foundation into the basement if the air pressure in the basement is less than the air pressure outdoors. This can also happen with other pollutants found in the soil—things such as lawn chemicals, termiticides, and biological pollutants such as mold. Air-pressure differences can cause all these pollutants to be pulled from the ground, through a crawl space, into the living space, or from the ground directly indoors through cracks in a basement wall or concrete-slab floor. Water vapor can also be pulled indoors from the soil when a house is depressurized.
Combustion gases often migrate into the living space from a furnace, water heater, or wood stove, even though they are supposed to be expelled through a chimney. If the air pressure indoors is less than that outdoors, the gases will have difficulty going up the chimney and can remain in the house.
Particles or gases from insulation can also be sucked indoors by air-pressure differences. This is more of a problem with very potent insulating materials such as the rarely used urea-formaldehyde foam insulation (UFFI) than with today’s commonly used insulations. Fiberglass, cellulose, and the various foam boards—though there are negative health effects associated with most of them—are more benign by comparison.
With an understanding of how and where these pollutants enter a house, it is possible to control them by either manipulating the air pressures, or by blocking entry points. With proper attention to detail during the design and construction of a house, this category of pollutant can be kept out of the living space in the first place. Therefore, the ventilation system won’t need to dilute them, so it can have a smaller capacity.
Pollutants can be released by human and animal metabolism
All living creatures release a variety of pollutants as a part of their metabolic processes. Oxygen, water, and food are consumed, and by-products of the life process are released. For example, the odor associated with mold is given off by the tiny creatures as they live and grow. We are all familiar with the unpleasant odor of a sweating body, or the smell that occasionally permeates a bathroom. Human beings (and animals) give off a wide variety of pollutants. Our exhaled breath contains dozens of chemical compounds. These are normal by-products of our metabolism, and they all contribute to indoor air pollution.
The pollutants given off by people are usually much less noxious that the pollutants given off by building materials or cleaning products, or the contaminants that are sucked indoors by air-pressure differences. Even though it is possible to eliminate all the toxic materials in a house and prevent pollutants from entering a house because of pressure differences, the only way to counteract the pollutants given off by people is to either eliminate the people (usually not a viable solution!) or dilute the pollutants with ventilation air. The best way of dealing with the metabolic by-products from mold is to prevent the mold from growing by controlling the moisture it needs for survival.
The concentration of “people pollutants” in a house depends on several factors. The number of people inside a house is very important, as is the size of the house. Six people in a small house will contaminate the indoor air faster than six people in a large house. Behavior patterns are also significant. Someone who exercises daily will release more by-products of metabolism than a couch potato. A household with several teenagers may contain more moisture as a result of frequent showers than a household with young children, so the age of the occupants is also a consideration. Because each of us is biologically unique, each of us has a different comfort level and degree of sensitivity, so we tend to perceive different levels of people pollutants as being comfortable or tolerable. Therefore, the amount of ventilation air that we need indoors to feel comfortable can vary slightly from person to person.
People also bring pollutants indoors attached to their bodies. We are all familiar with how tobacco smoke can cling to our clothing and hair. Our lungs also take up the pollutants as we inhale. In fact, many air pollutants—VOCs, perfume, exhaust gases, etc.—can be carried indoors on our bodies. Once contaminated clothing and bodies are indoors, the pollutants will be released slowly, contributing to indoor pollution. Hypersensitive people sometimes have trouble being close to family members whose bodies have become contaminated with heavy doses of pollutants. People can also track pollutants indoors on their shoes (e.g. lawn chemicals, animal waste, road dust containing asbestos, lead, rubber, etc.), and deposit those pollutants in carpeting.
The bottom line
We are all surrounded by air pollutants every day, both indoors and outdoors. While a human body is capable of tolerating a certain amount of contaminated air, evidence from a variety of sources tells us that we are being exposed to more pollution than our metabolism may be able to adequately process—especially when we are indoors—and it is often making us sick. But there is no reason for this trend to continue. After all, a variety of strategies can be used to build houses with minimal indoor air pollution, and then to ventilate them to keep the indoor air as healthy as possible.
Reference: Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems by John Bower, originally published by The Healthy House Institute.
