Feature Article

The IAQ Challenge: Protecting the Indoor Environment

Are our buildings making us sick? Yes, say an increasing number of indoor air quality specialists in government agencies, academia, and the emerging industry working to solve these problems. By some estimates, direct medical costs associated with IAQ problems in the United States are as high as $15 billion per year, with indirect costs of $60 billion. These estimates do not include problems like asthma, which may be triggered by IAQ problems—and which has increased 42% between 1982 and 1992 in the U.S., according to the Centers for Disease Control.

Sick building syndrome(SBS) is a relatively new term used to describe a collection of widely varying symptoms incurred by building occupants—including headaches, irritated eyes, nausea, fatigue, dizziness, coughing, and throat irritation—that tend to go away or improve significantly when they leave the building. SBS is different from building-related illness, in which the sickness is something recognizable by physicians that comes from the building, such as Legionnaire’s disease.

A number of SBS cases have been highly publicized: in 1987-88 workers at an Environmental Protection Agency office building in Washington, D.C. became sick following remodeling work in which new carpeting was installed; in 1990 more than 20% of the workforce at a brand new National Oceanographic and Atmospheric Administration (NOAA) office in Silver Spring, Maryland became ill; and in 1994-95 a whopping 518 workers (out of 640) at the Massachusetts Registry of Motor Vehicles building in Boston became ill. Not surprisingly, the legal profession has taken note of this trend—dozens, if not hundreds, of lawsuits are making their way through the courts.

IAQ problems, including sick building syndrome, are among the most challenging issues facing building science experts today: symptoms are inconsistent; causes are complex and poorly understood; and without fully understanding the problems, solutions are often very hard to find.


Chemical sensitivity

The issue of sick building syndrome is complicated by enormous variation in how individuals react to pollutants. Many people seem unaffected by relatively high levels of contaminants, while a certain percentage of the population is extremely sensitive to even tiny quantities of contaminants. This condition is variously referred to as multiple chemical sensitivity (MCS) or environmental hypersensitivity.

Many individuals suffering from MCS can trace their sensitivity to a particular event in which they were exposed to a high dose of a toxic compound, which greatly weakened their immune systems. Some experts believe that cumulative exposure to chemicals over a long period of time gradually weakens our immune systems and can result in chemical sensitivity. Indeed, some suggest that incidence of MCS in our population is increasing at an alarming rate. In this sense, today’s chemically hypersensitive individuals may be serving as canaries in the office, warning others of the eventual consequences of continued exposure to contaminated air.


Types of indoor air pollutants

We define an indoor air pollutant as something foreign in the indoor air that has an adverse effect on human health. These are substances we breathe in: small airborne particulates, tiny suspended droplets of liquid (aerosols), radioactive isotopes, and gases.

Understanding and avoiding indoor air pollutants

To develop effective strategies for avoiding or eliminating indoor air pollution sources, we need to understand where they come from. If we understand the pollutant sources, or the conditions under which they may be generated, we can often come up with effective solutions relatively easily. Major categories of indoor air pollutants are addressed below with information on sources and general elimination or control strategies. A checklist of specific recommendations is presented at the end of the article.


Particulates – non-biological

Tiny particles or particulates suspended in the indoor air, including dust and fiber, can cause respiratory, throat, and eye irritation, or illness as the body tries to rid itself of these foreign particles. The health effects can be simple irritation, allergy, or even a toxic reaction, depending on the substance and the individual. Biological particulates are discussed separately below.

Concerns about health threats from building materials are driving the market for alternative materials, such as this Miraflex fiberglass, which has no chemical binders and is less friable.

Photo: Owens Corning Fiberglas
The fact that manmade mineral fibers can break off from fiberglass or mineral wool insulation and acoustical ceiling tiles has been given a great deal of attention.

The concern is that tiny airborne respirable fibers may be able to lodge in our lungs and cause cancer, as has been demonstrated among mine workers with asbestos. Fiberglass insulation, for example, must carry an OSHA cancer warning label and is listed as a likely carcinogen by the Health and Human Services Department’s 7th Annual Report on Carcinogens (see EBN Vol. 3, No. 5).

Some suggest that the health risk of airborne fibers has been exaggerated. The fiberglass industry, as would be suspected, falls into this category, but so do some prominent building science experts, including Jim White, Senior Advisor on Building Science with the Canadian Mortgage and Housing Corporation (CMHC). “The cancer risk has been blown way out of proportion,” says White, who claims that most building occupants have very little fiber exposure.

Asbestos, which is a known carcinogen, was long used for insulating boilers and steam/hot water pipes, as well as in fiber-cement building materials. Though some roofing products still are made using asbestos, it is banned from other building products in the U.S. and Canada and is not an issue of concern in new construction. In existing buildings, any exposed asbestos insulation should be encapsulated or removed by qualified contractors. Encapsulation is preferable, according to White, since “exposure to asbestos almost always goes up after removal.” Old vinyl-asbestos tile is another potential source of asbestos, but the fibers are less likely to become airborne.

One of the most significant sources of mineral fibers in some buildings is the HVAC system. In residential and light-commercial buildings, improperly sealed return ducts in attics that are covered with fiberglass or mineral wool insulation can introduce fibers into the air distribution system and spread them throughout the living area. Fiberglass duct board may be another source of respirable fibers. With standard duct board, the fibers are fully exposed to the air stream. In response to this concern, at least two duct board manufacturers have introduced duct board products with a containment layer or coating on the inside (see EBN Vol. 4, No. 2).

In larger commercial buildings, return air plenums above acoustical ceilings can also result in fiber dispersal. Fibers can be released from the ceiling panels or from fire proofing in the plenum. In the Registry of Motor Vehicles building in Boston, mentioned above, moisture introduced through the ventilation system is believed to have waterlogged mineral wool fireproofing insulation in air distribution plenums and allowed the loose fibers to be distributed throughout the building—in this case carrying toxins generated in another reaction with them.

Combustion particulates, especially those from tobacco smoke and wood stoves, are a common problem. Smoking is now banned in many public buildings, and those spaces where smoking is permitted are often being designed with higher levels of ventilation and direct exhaust of return air to the outside. Wood stoves and fireplaces are generally discouraged from homes where indoor air quality is a concern. If they are installed, they should have dedicated combustion air supply to reduce risk of combustion gases spilling into the indoor air.

Regular vacuuming can effectively remove most particulates from a building, but many vacuums do not adequately filter collected dust—they can even worsen air quality problems by making the smaller particles airborne. More effective are central vacuum systems, in which the exhaust is dumped outside of the occupied space.


Particulates and aerosols – biological

In humid climates the use of galvanized ducts is recommended, because the zinc coating deters mold growth. When ducts run outside of the insulated space, they should be well insulated themselves.

Photo: Florida House Foundation
Biological or microbial pollutants are a hot issue in IAQ circles. Included in this category are fungi (including molds, mildews, and yeasts), dust mites, pollen, bacteria, and viruses. Molds and mildews are among the most widespread biological sources of indoor air quality problems in buildings and almost certainly one of the most significant. Molds and mildews can cause problems in several ways. Mold spores or fruiting bodies released when the organisms mature and reproduce can cause allergic reactions in some individuals. Up to 20 million people in the United States may be allergic to fungal spores, according to the authors of Your Home, Your Health, & Well-Being (Ten Speed Press, 1988). The book claims that even people without previous allergies may become sensitized to such contamination if exposed to high concentrations.

Potentially even more significant than allergy to mold spores, according to Virginia Salares of CMHC, is the toxicity of certain molds and mycotoxins produced by molds. Chemicals from mold cell walls, including beta 1,3 glucans, have various toxic effects, and these chemicals can be released even after the mold colonies die and the cell walls are disintegrating. The mold species Stachybotrys atra produces a group of chemicals called tricothecenes, which have been implicated in infant deaths. More and more experts agree that molds are a lot more significant in terms of overall indoor air quality concerns than had previously been thought. According to Dr. David Miller, a mold expert with Agriculture Canada, the impact of mold “is equivalent roughly to the effect of second-hand cigarette smoke.” Jim White told EBN that in a recent study of moldy basements by CMHC (as yet unpublished), all of the basements were growing toxigenic or pathogenic molds.

Mold and mildew can become a problem whenever adequate growing conditions exist. Their basic requirements are few: moisture, free oxygen, comfortable temperatures (between 50° and 90°F – 10-32°C), and a source of food. Because oxygen and suitable temperatures are present in most buildings, and the necessary nutrition—basically carbon and nitrogen—can be found in many building materials, the only practical way to control mold growth is to control moisture, especially relative humidity. Biocidal treatments can also be used, but their health effects can sometimes be as bad as those of the mold.

Collected dirt in supply ducts can contaminate delivered air and breed microbials.

Photo: Enviro Team Inc.
Almost any surface that remains moist for a period of time—usually 24 hours is long enough—can harbor mold growth. Buildings have many potential sources of moisture, including rainwater penetration, leaky plumbing, spills, cooking, bathing, and unvented combustion appliances. Any of these sources can result in problematic mold growth.

HVAC systems are particularly troubling sources of mold growth because they have a ready means of distributing mold spores. Horror stories abound of mold growth in ducts, particularly in warm, humid climates.

Fungal spores can be hard to detect and measure in a building because their levels in the air fluctuate, and they don’t tend to be distributed evenly in a space. Recently, scientists have discovered that certain types of fungi emit volatile organic compounds, or VOCs, and that measuring these can be a more accurate way to assess fungal contamination in a building. Some of these biological VOCs, including quinones and terpenes, pose a health risk similar to that of synthetic VOCs found in buildings (see discussion of VOCs below); others are endotoxins that may be even more toxic than regular VOCs.

Another very common biological indoor health problem is posed by dust mites, which are a type of arachnid—the class of animals that includes spiders, ticks, and scorpions. Dust mites feed on dander (dead skin and other organic matter) and are extremely common in carpets and upholstery. From an IAQ standpoint, the problem is usually the feces produced by dust mites, rather than the mites themselves. If the feces become airborne, they can cause respiratory ailments, particularly among people with allergies. Minimizing carpeted areas, controlling humidity, and providing regular maintenance with a high-efficiency vacuum or central vacuum system are the most effective control strategies. Pollen, a common allergen, can also be an indoor air contaminant if unfiltered or poorly filtered air is brought in from outside during the pollen season.

Less common but potentially more dangerous than most of the biological pollutants described above are various types of bacteria and virus that can cause infection. The most famous case of this type of building-related illness occurred at a 1976 meeting of the American Legion in Philadelphia. Twenty-nine Legion members were killed by what was shown to be a bacteria that contaminated water in the cooling system. The bacteria, named Legionella pneumophilia after the outbreak, is a common contaminant of water systems that is now suspected as the cause of mysterious outbreaks of illness going back as far as 1947. People can become infected with this type of pneumonia when they inhale water droplets contaminated with Legionella bacteria. Hundreds of cases of the sickness are confirmed each year, with estimates of actual cases as high as 116,000 per year, according to Dr. Shirley Hansen, author of Managing Indoor Air Quality (The Fairmont Press, Lilburn, Georgia, 1991).

Viral infections may also be considered indoor air contaminants. There are a few isolated cases in which viral infections are believed to have been spread through an HVAC system, such as a measles outbreak in a school attributed to the air conditioning system. But most viruses can survive for only a few minutes and are typically transmitted directly from person to person through sneezing or physical contact. Fungal and yeast infections can be spread through contaminated ducts.


Combustion gases

Combustion gases are considered by many experts to be among the most harmful contaminants in indoor air, but they are also among the easiest IAQ problems to solve. The primary products of hydrocarbon combustion are carbon dioxide and water vapor—shown for methane (the primary constituent of natural gas) as follows:

CH4 + 2O2 ‘ CO2 + 2H2O

In reality, however, combustion is seldom so simple. If oxygen levels are inadequate, some carbon monoxide (CO), a poisonous gas, will be produced in place of some of the CO2. Also, some atmospheric nitrogen is typically combusted to form nitrogen dioxide (NO2), and small quantities of other nitrogen oxides, sulfur dioxide, formaldehyde, and various polycyclic aromatic hydrocarbons (tar-like substances) may be produced.

Fortunately, it is fairly easy to eliminate combustion gases from buildings. IAQ experts are arguing more and more strongly that all sources of open combustion should be eliminated from buildings. This means installing only combustion products with active venting directly to the outdoors. Among furnaces, boilers, and room heaters, the best option is sealed combustion, in which outside air is brought directly into the combustion chamber and exhaust gases are vented directly to the outside, with no opportunity for interaction with the indoor air. The second-best option is direct-vent or power-vented equipment, in which indoor air is used for combustion, but a fan forces exhaust gases to the outside.

Natural-draft combustion equipment, in which the warm flue gases rise up a chimney due to their natural buoyancy, should be avoided. Unfortunately, even as building science experts argue with an increasingly unified voice that all open combustion should be eliminated from homes, unvented gas heaters and fireplaces are increasing in popularity rapidly (see Unsafe Gas Heaters Proliferate). Similarly, gas ranges should be avoided, a strategy that is often unpopular with homeowners, and wood stoves and wood-burning fireplaces should only be installed if equipped with outside combustion air.

Polluted outdoor air is a frequently overlooked source of combustion gases and other contaminants in buildings. Locations where air pollutants are generated, such as loading docks, taxi stands, busy streets, and ventilation exhaust vents, should be avoided when deciding where to place the fresh air intake vents. Also, imperfect air-sealing, combined with the high fan pressures found in mechanical systems, can cause significant amounts of outdoor air to bypass the air intake and be drawn directly into the air distribution system. Because such mechanical systems are often located near loading docks as part of the “building services” area, such short-circuiting can also draw pollutants indoors.



Radon is a natural, radioactive gas that can enter buildings through slab floors and basement walls, as well as from certain masonry materials used in the building itself. Radon levels are measured in units of picocuries per liter of air (pCi/l), and the U.S. EPA has determined that radon levels in buildings should be below 4.0 pCi/l. Relative to human health, radon is considered the second greatest cause of lung cancer in the U.S., behind tobacco smoking, but there is considerable debate as to how bad radon really is.

This chart of radium decay shows the problematic radon daughters.

Source: U.S. Environmental Protection Agency
Radon gas, which has a half-life of 3.8 days, breaks down into a string of very short-livedradon daughter products, all of which release ionizing radiation (alpha, gamma, and beta particles), as shown in the figure below.

Of the radiation released during this decay process, alpha particles are the most dangerous—these large, slow particles are sometimes called the “.45-caliber slugs” of radiation. They are stopped quickly by most materials, including the tissue in our lungs.

Some experts argue that the primary way radon and its daughter products get into our lungs—where the ionizing radiation can cause lung cancer—is by being adsorbed onto particulates that we breathe in. The early studies correlating lung cancer with high radon levels were in very dusty uranium mines. If high concentrations of particulates are not present (and if the building occupants aren’t smokers), so this argument goes, the risk of lung cancer may be very small because most of the radioactive particles precipitate out of the air onto walls and other surfaces. In fact, IAQ expert and builder Oliver Drerup, of Carp, Ontario suggests that radon is really a particulate problem. On the other hand, Dr. Hansen suggests that it is the free radon gas that is most dangerous, because it can penetrate deeper into our lungs where damage can be greatest.

This scientific debate aside, radon control and mitigation measures are fairly easy to incorporate into buildings and should be done as a precaution. Because it is very difficult to determine if local soils under or around a building will be a significant source of radon, radon control measures should be incorporated into all buildings. Typical measures, which are detailed in various EPA publications, include providing clean, uniform-sized aggregate under the basement or first-floor slab, providing a layer of polyethylene under the slab, and installing a capped pipe through the slab that can be uncapped and extended up and out through the building (usually with an in-line fan) to ventilate radon if testing shows levels to be unacceptably high.



Volatile organic compounds, or VOCs, are among the most complex and troubling indoor air pollutants—because their health effects are so difficult to pinpoint and because lots of different VOCs are often present in a given situation, making it difficult to determine exactly what might be causing a health problem. Organic compounds are generally classified as “volatile” if the molecules they release create a vapor pressure greater than 1 mm mercury at 20°C. The air we breathe contains hundreds of such compounds, released from natural and synthetic materials, both indoors and out. Once released, some persist in the air for long periods of time; others decompose quickly into smaller molecules, or are adsorbed by (attached to the surface of) other materials. Once adsorbed, the molecules can be re-released in the future.

Natural materials with strong odors, such as citrus oils and pungent woods (cedar or pine, for example), release large quantities of VOCs. Materials and products used in buildings that are made from such materials likewise release lots of VOCs. Some of these VOCs cause health problems, especially among people with multiple chemical sensitivity or specific allergies, but most of the concern about VOCs currently has to do with petrochemical-based compounds.

Manufactured and synthesized products, especially paints, stains, and adhesives that are applied wet, often release large quantities of VOCs. If the products are produced from fossil fuels, some of the compounds they release, such as benzene, styrene, formaldehyde, and toluene, may be irritating, toxic, or even carcinogenic. Most of the VOCs released into buildings have unknown health effects.

Solvent-based finishes and adhesives release high levels of VOC initially, but the emission rates drop off dramatically within days or even hours. Because these VOCs can react with ozone and contribute to outdoor smog, the highest emitting products—including many solvent-based paints and adhesives—are now banned from use in nine U.S. jurisdictions, including New Jersey, Massachusetts, and heavily populated parts of California. Most of these regions are designated by the U.S. EPA as air quality non-attainment areas. California’s South Coast Air Quality Management District has the strictest restrictions in most product categories. The EPA is also proposing national restrictions which, if approved, may take effect at the beginning of 1997. These national standards would not supersede the regional rules where the latter are more restrictive.

The quantity of VOC in finishes and adhesives is typically measured (or, more often, calculated from components) in grams of VOC per liter of product. Some oil-based paints have VOC contents as high as 850 g/l, meaning that over 80% of the product could potentially be released into the air as the paint dries. Water-borne paints, in which water is the primary carrier, have VOC contents that range from zero to more than 350 g/l. Water-borne paints use special chemical additives, known as coalescing agents, to keep the pigments and paint resin suspended in the water—the coalescing agents are needed because the paint solids will not actually dissolve in water. With oil-based paint, the mineral spirits or oils are actual solvents, in which the pigments are dissolved.

Due to pressure from air-quality regulations and concerns about IAQ, paint, varnish, and adhesive manufacturers are scrambling to create lower-VOC formulations of their products (see reviews, EBN Vol. 3, No. 1 and Vol. 4, No. 1). Several lines of Glidden and Benjamin Moore paint are now available in zero-VOC formulations. Some attempts at reformulation have resulted in products that don’t perform as well, or are harder to work with, than their solvent-based predecessors. With ongoing refinements, however, performance is improving. In some cases installers have discovered unexpected advantages of the lower-VOC formulations, such as reduced drying time, which can permit more rapid application of multiple coats. In occupied buildings, the ability to repaint an area without generating strong odors or causing temporary dislocation of residents or employees can provide significant financial benefits.

Office equipment is tested for VOC emissions in a stainless-steel chamber.

Photo: Air Quality Sciences, Inc.
Formaldehyde is one of the most widely discussed VOCs today. While its contribution to indoor air quality problems has dropped off significantly during the past two decades (gone are the extremely high emissions from urea-formaldehyde foam insulation and early particleboard), it is still an issue of concern, particularly among people with chemical sensitivity.

Formaldehyde is one of the most common industrial chemicals and is widely used as a component of building products, upholstery, carpeting, etc.

Manufactured wood products made with urea-formaldehyde (UF) binders typically have higher formaldehyde emissions than products made with phenol-formaldehyde (PF) binders, but the difference is far less than it used to be. Most particleboard manufacturers now control component mixtures much more carefully, so less free formaldehyde remains in the finished product, and formaldehyde

scavengers are typically added to lock up excess. Urea-formaldehyde binders are only used in interior-grade products. Formaldehyde emissions from manufactured wood products drop off gradually over time but may continue for many years. Exposure to high humidity increases emissions because a hydrolysis reaction occurs in which formaldehyde is broken off from the polymeric resin. Products with polyurethane (MDI) binders should emit no formaldehyde other than that present in the wood itself. We can minimize formaldehyde emissions in new buildings by carefully choosing materials and furnishings, or by sealing those products containing formaldehyde.

Carpet installations can be among the most significant sources of VOCs in new or remodeled buildings. A complex group of VOCs, including 4-PC, styrene, vinyl acetate, and formaldehyde, some with interactive effects, make investigations of VOCs from carpeting especially complex (see EBN Vol. 3, No. 6). Also, see page 5 for information on a new Carpet & Rug Institute program to reduce VOC emissions from carpet adhesives—which are often worse than emissions from the carpets themselves.

Acoustical ceiling tiles can also be significant sources of VOCs, including formaldehyde. There are significant differences in the formaldehyde offgassing from different acoustical ceiling tile products, according to industry observer Lena Gill, of Washington, D.C. About half of Armstrong’s ceiling tiles, for example, are low-emitting, she told EBN, so it should be relatively easy to specify low-emitting tiles.

Often more significant than emissions from new building materials are VOC emissions from ongoing activities, such as repainting, cleaning, use of air “fresheners,” and pesticide application. Powerful solvents, such as the cleaning agents used to remove chewing gum from school carpets, may release highly toxic VOCs. Any material, such as floor wax, that is spread out over a large area can transfer large quantities of VOCs to the air in a very short time. Air freshening compounds sprayed to mask the odor of cleaning chemicals are among the worst offenders.

Contaminants from carpet can change with time.

Source: CMHC
As mentioned above, VOCs can also be adsorbed onto various surfaces in a building, then released at a future time. Carpets and upholstery are particularly prone to this “sink effect” due to their large surface areas. Some even claim that carpets help to clean the air by acting as a filter for contaminants. Carpeting can also serve as a medium for molds and other biological contaminants. Unfortunately, there is no effective way to clean such contaminants off of wall-to-wall carpets, so they remain as an ongoing sink and source of VOCs and molds until they are replaced.

Contaminants from carpeting are likely to change over time, as illustrated in the graph (previous page). Throw rugs and upholstery covers that can be removed and thoroughly cleaned may be an exception to this problem.

While more building designers are now paying attention to what materials they put inside buildings, few give much thought to what materials go on the outside. This may be a big mistake when it comes to VOC pollution indoors. Failure to seal carefully against air movement between exterior and interior spaces in a building can lead to the introduction of pollutants. For example, in commercial buildings it is common for dropped ceilings to be used as return air plenums, through which indoor air is drawn to the HVAC plant to be conditioned and recirculated. Unless they are extremely well sealed, the negative pressure in these plenums can draw in air from other locations. Building scientist Joe Lstiburek describes a plausible scenario by which air can pick up volatilized toxins from the underside of a hot roof membrane and then be drawn directly into the building’s air distribution system. He believes that this phenomenon may be a significant cause of sick building syndrome in commercial buildings.


Complicating the picture: interactive effects

Researchers have suspected all along that interactive effects between various chemicals may be clouding the IAQ picture. Recent studies by Charles Weschler and colleagues at Bell Labs have found that two common pollutants, ozone and 4-PC, can combine to create more irritating aldehydes. Ozone is commonly generated by fusers in laser printers and copiers, and even intentionally by devices used to mask odors in hotel rooms. 4-PC is released from styrene butadiene latex, the most common carpet backing material. As a result, the combination of the two in indoor air is quite common and may help explain many previously mysterious office headaches.

At the NOAA headquarters in Silver Spring, Maryland, more than 20% of workers became ill, and at least part of the problem is believed to be due to a reaction between phenols given off by an epoxy floor-leveling compound and the PVC backing on the carpet tiles, producing 2-phenoxyethanol and long-chain alcohol VOCs. The bottom line with these synergistic effects, according to Virginia Salares of CMHC, is that low levels of many different types of pollutants can exert effects that exceed the sum of effects from individual pollutants by themselves. “We have very limited knowledge of the effects of single chemicals,” she said. “We cannot begin to determine the effects of having a large number of chemical contaminants.”


The challenges ahead

For many reasons, reliable, high-quality air is frustratingly elusive in many buildings. There are so many independent and interacting factors in these semi-isolated environments in which so many people live and work, that identifying causes can be very difficult.

Even the task of measuring indoor air pollutants is difficult, and measurements from different studies are not always comparable, due to different methods used and different reporting formats. For example, some researchers include unidentified VOCs in measures of total VOCs (TVOC), according to architect and indoor air specialist Hal Levin, while others omit the unidentified compounds and report only the sum of identified VOCs as the TVOC figure.

While providing safe indoor air quality is most important for people with multiple chemical sensitivity, it should be a high priority for all buildings in case current MCS sufferers do indeed turn out to be the proverbial canaries in our coal mines.


The following checklist can serve as a starting point in ensuring a safe indoor environment for your clients.




Published May 1, 1996

Wilson, A., & Malin, N. (1996, May 1). The IAQ Challenge: Protecting the Indoor Environment. Retrieved from https://www.buildinggreen.com/feature/iaq-challenge-protecting-indoor-environment