All About Indoor Air Quality

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All About Indoor Air Quality

Which materials, substances, and practices are important to keep your indoor air fresh and healthy?

Posted on Mar 11 2016 by Martin Holladay

Many owners of green homes are concerned about indoor air quality. often receives questions from homeowners who worry that some building materials emit dangerous chemicals. For example:

  • Will the glue in my plywood or OSB subfloor emit dangerous fumes?
  • Will borateBoron-containing chemical that provides fire resistance to materials such as cellulose insulation and provides decay and termite resistance to wood products. Borate is derived from the mineral borax and is benign, compared with most other wood treatments.-treated cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. off-gas enough to affect the health of my children?
  • What type of clothes dryer is best from the perspective of indoor air quality?

We do our best to provide answers to these questions. But if there is a theme running through these questions — and I think there is — it would be this: homeowners are worrying about the wrong materials and substances.

Occupant behavior matters more than construction specifications

Indoor air quality (IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness.) is a big topic. To get a handle on it, we need to break it down into small bites.

Here’s how I’d like to proceed:

  • First, I’ll provide an overview of the issue.
  • Then I’ll create three lists of substances that are worrisome. The first list will include worrisome substances that are only found in older houses. The second list will include worrisome substances arising from construction methods sometimes used in new homes. The third list will include worrisome substances that homeowners are exposed to because of occupant behavior.
  • I’ll share a list of new home specifications that are important for anyone concerned about indoor air quality.
  • Finally, I’ll share researchers’ findings about which chemicals are most concerning.
    • Indoor air is low-quality air

      In most U.S. locations, indoor air is more polluted than outdoor air. According to the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), “Studies from the Environmental Protection Agency on human exposure to air pollutants show that indoor levels of pollutants may be two to five times, sometimes more than 100 times, higher than outdoor levels.”

      There are exceptions to this rule, of course. If your house is located near a busy intersection or a location where buses or trucks often idle their engines, the outdoor air near you house may be more polluted than your indoor air. If you live in such a location, increased ventilation may not improve the quality of your indoor air. For the rest of us, however, ventilating a house with outdoor air usually improves the situation.

      If I’m worried about IAQ, what tests should I perform?

      The most important test you should perform is a radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. test. If you’ve tested for radon, you’re done. In most cases, further testing isn't justified.

      For more information on testing, see Indoor Air Quality Testing Should Not Be The First Move.

      Worrisome substances that are only found in older homes

      AsbestosMineral fiber once commonly used in many building materials, including insulation, fireproof siding, and resilient flooring. Inhalation of invisible asbestos fibers can lead to chest and abdominal cancers as well as scarring of the lungs. The use of asbestos in some products has been banned by the EPA and the U.S. Consumer Products Safety Commission; manufacturers also have adopted voluntary limitations on its use. When found in older buildings (most commonly in floor tiles, pipe and furnace insulation, or asbestos shingles), the product's friability is a major determinant in how it must be handled during renovations. More information: fibers. Most building products containing asbestos were removed from the U.S. market by the mid-1980s. If you live in a house built before 1985, however, it may include building materials (for example, pipe insulation or vermiculite) that contain asbestos. For more information on this issue, see:

      If your house was built in 1985 or later, you don’t have to worry about asbestos.

      Lead. Lead-containing paint is very common in houses built before 1950. According to the Centers for Disease Control and Prevention, more than 250,000 American children under the age of 6 have elevated blood lead levels. Lead-containing dust can originate from lead paint or lead-contaminated soil; the dust can be ingested through the lungs or the stomach (by hand-to-mouth contact). Lead-containing dust can be generated by scraping, sanding, or demolition activities; it can also be generated by opening and closing old window sash. (Intact and undisturbed painted surfaces are usually not a health hazard.)

      Although it’s possible for adults or children above the age of 6 to be poisoned by lead, young children and pregnant women are the most vulnerable. In the United States, lead paint for residential use continued to be sold until 1978. If your house was built after 1979, you don’t have to worry about lead paint.

      For more information on avoiding lead exposure, see EPA: Lead and National Center for Healthy Housing: Lead.

      Worrisome substances related to construction methods

      Radon. Radon is a naturally occurring radioactive gas that is present in many soils. It can enter a house through cracks in the foundation. If exposure levels are high enough, radon can eventually cause cancer. Fortunately, construction details have been developed to reduce or eliminate dangerous levels of radon. Unfortunately, some builders still build homes without these necessary details.

      For more information on this topic, see All About Radon.

      FormaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen.". In the past, products made of particleboard (especially inexpensive kitchen cabinets and inexpensive furniture) and some other composite wood materials contained worrisome levels of formaldehyde. Formaldehyde is a gas; at high concentrations, formaldehyde can cause health problems, including cancer.

      The good news is that federal legislation enacted in 2010 established new lower limits for permissible formaldehyde emissions from composite wood products (including hardwood plywood, medium-density fiberboard, and particleboard). Since this legislation (the Formaldehyde Standards for Composite Wood Products Act) was passed, health risks associated with cabinets and furniture made from these composite wood products have been greatly reduced. That said, concerned homeowners may want to specify formaldehyde-free cabinets and furniture.

      In the past, manufacturers of fiberglass batts used a formaldehyde-containing binderGlue used in manufactured wood products, such as medium-density fiberboard (MDF), particleboard, and engineered lumber. Some binders are made with formaldehyde. See urea-formaldehyde binder and methyl diisocyanate (MDI) binder. (glue) to help hold the fibers together. Last year, however, the last fiberglass manufacturer in the U.S. using a formaldehyde-based binder switched over to an acrylic-based binder — so we can scratch that worry off our list.

      Manufacturers of mineral wool still use formaldehyde-based binders. Where this fact is worrisome depends on your appetite for risk; the level of formaldehyde that remain in the product after the glue has cured is low, and in most homes, there is an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. between the insulation and the indoor air. For more information on this issue, see Better National Distribution for Mineral Wool Batts.

      Formaldehyde is also a component of tobacco smoke and the fumes given off by a gas range. Smoking tobacco indoors (or anywhere, frankly) is a very bad idea. And anyone concerned about the formaldehyde emissions produced by a gas range should choose an electric range (either one with electric-resistance burners or an induction range).

      For more information on formaldehyde, see Formaldehyde in the Home and EPA: Formaldehyde.

      Volatile organic compounds. Volatile organic compounds (VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production.) are found in many liquid products, including solvents, household cleaning products, paints, varnishes, and fuels like gasoline. Reducing the levels of VOCs in your home is mostly a matter of occupant behavior: to the best of your ability, choose paints and cleaning products that are low-VOC or zero-VOC.

      That said, there is one aspect of VOC exposure that depends on construction specifications: namely, whether or not your home has an attached garage. Homeowners often store products that emit VOCs — including gasoline, solvents, and old cans of paint — in their garage. If you have an attached garage, air from the garage can enter your house through leaks in the common wall.

      There are two ways to avoid this problem: either specify a detached garage, or make sure that your builder pays extremely close attention to the air barrier between your garage and your house.

      Water vapor. At high levels, water vapor lowers your indoor air quality. While water vapor doesn’t directly cause health problems, at high concentrations it can encourage the growth of mold and dust mites. If anyone in your family has asthma, indoor relative humidity levels matter, because mold and dust mites can aggravate asthma symptoms.

      Sloppy construction details — especially, bad basement details or crawl space details— can contribute to high indoor humidity levels. For more information on details that keep your basement or crawl space dry, see these three articles:

      • Preventing Water Entry Into a Home
      • Fixing a Wet Basement
      • Building an Unvented Crawl Space
        • Worrisome substances related to occupant behavior

          Smoking tobacco. Smoking threatens the health of smokers, and indoor smoking threatens the health of everyone who lives in the house. Smoking tobacco belongs near the top of any list of habits to avoid. Do it outdoors, or don’t do it at all.

          Using insecticides or pesticides indoors. If you have ants in your kitchen, research safe ways to discourage them without the use of insecticides.

          Using solvents or industrial cleaning products indoors. Some Q&A columns advise homeowners to clean their bathtub with paint thinner or acetone. Sure, it works — but you don’t want to try this at home.

          Hobbies that require using chemicals or solvents. If you refinish furniture indoors, you may be using varnishes that emit VOCs. Do this type of work in a detached garage, or get a powerful exhaust fan.

          Plug-in air fresheners. Plug-in air fresheners release volatile chemicals into your indoor air. If your bedroom smells like dirty socks, you should collect the dirty socks and wash them. Don’t use a plug-in air freshener.

          Forgetting to operate an exhaust fan. Your bathroom and range hood have exhaust fans for a reason: to exhaust humidity, particulates, and aerosols from your house. Use your fans.

          Using an unvented gas heater. Gas space heaters and gas fireplaces emit lots of water vapor and, if the burners are not adjusted properly, can emit carbon monoxide. If you want a gas space heater in your house, make sure that it is vented.

          Wood smoke. If you have a wood stove, you know that smoke sometimes enters your house when you open the stove to load fuel. Wood smoke contains acrolein, formaldehyde, and small particulates that aren’t good for your lungs. Some homeowners accept this level of risk; but for others, this type of indoor pollution is unacceptable.

          Pets. People who keep pets usually love them, but cats and dogs can add lots of pet hair and dander to your home. Is this a problem? You decide.

          Flame retardants in furniture cushions. The foam used in furniture cushions usually contains brominated flame retardants, and some experts worry that these flame retardants may have negative health effects. If you’re worried about flame retardants, you may want to seek out furniture that is flame-retardant-free.

          For more information on this issue, see Brominated Flame Retardants.

          Emissions from new carpeting. In the 1990s, many magazine articles proposed a link between “new carpet smell” and health problems. Since then, carpet manufacturers have made a concerted effort to identify chemicals of concern and to reduce emissions. Research sponsored by the U.S. Consumer Product Safety Commission (CPSC) determined that none of the chemicals found in new carpeting are emitted at levels known to be hazardous.

          That said, if you are worried about emissions from carpeting, you can certainly specify hardwood flooring or tile.

          For more information on this issue, see Carpeting, Indoor Air Quality, and the Environment.

          Meth labs. If you are reading Green Building Advisor, you probably don’t have a meth lab in your kitchen. If you’re thinking of setting one up — don’t.

          Aerosols and particulates from cooking. Cooking releases acrolein and tiny particulates that can harm human health. Cooking over a gas flame releases nitrogen dioxide, formaldehyde, and carbon monoxide.

          From an IAQ perspective, cooking with an electric appliance is far preferable to cooking with a gas appliance. No matter what type of fuel you use, it’s important to operate an exhaust fan that vents outdoors when you are cooking.

          For more information on this topic, see The Hazards of Cooking With Gas and A Method to Estimate the Chronic Health Impact of Air Pollutants in U.S. Residences.

          Burning candles. Burning candles releases small particulates into the air. Unless it’s someone’s birthday, don’t light candles indoors.

          New home specifications and advice

          Just because I have included an item in one of my lists above, doesn’t mean that the items on these lists have equal weight. If you smoke a pack of cigarettes a day indoors, you are endangering the health of your family. On the other hand, there isn’t much evidence that you have to worry about carpeting (as long as you keep your house clean).

          Here are a few key points for anyone concerned about IAQ:

          • Every bathroom should have either an exhaust fan or an exhaust grille connected to an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. or ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV..
          • Your kitchen range should have a range hood equipped with an exhaust fan that is ducted to the outdoors.
          • If your home has a furnace, boiler, or water heater that burns propane, natural gas, or oil, make sure that these appliances have sealed-combustion burners that resist backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney..
          • Your house should have a ventilation system capable of meeting ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. requirements. In some cases, a bathroom exhaust fan, properly controlled, can perform this function. For more information, see Designing a Good Ventilation System. To limit the entry of small particulates into your home, supply ventilation systems and balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). systems should include a MERV 13 filter. If you choose to install an exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. system, most of the particulates in the outdoor air that enters your home through building cracks are likely to be filtered out as the air passes through building assemblies on the way to your home's interior.
          • If your house has a basement or crawl space, make sure that it is detailed to limit the entry of moisture and radon. For more information, see All About Radon and Preventing Water Entry Into a Home.
          • A detached garage is preferable to an attached garage. If you have an attached garage, make sure that the common wall has an impeccable air barrier, and that the door to your garage has excellent weatherstripping.
          • Never install an unvented gas space heater or an unvented gas fireplace in your home.
          • Remember that most IAQ problems are related to occupant behavior. If you want clean indoor air, you will need to police your family.

          What do the researchers say?

          Max Sherman, a senior researcher at Lawrence Berkeley National Laboratory (and a recipient of ASHRAE's Louise and Bill Holladay Distinguished Fellow Award, named after my grandparents), has helped develop a list of the top nine contaminants that lower the quality of indoor air. These contaminants are both relatively common and relatively concerning (due to their known deleterious health effects).

          Most of the chemicals that show up in the questions posted on GBA don’t make the list. What matters most? It turns out that fine particulates — PM2.5 — cause the most health damage. In fact, these fine particulates are three times more concerning (from a health perspective) than the next most worrisome contaminant on the list. Some of these particulates come from outdoor air, due (for example) to diesel exhaust or wood smoke. The rest come from indoor activities like cooking and burning candles. (For more information on the researchers' findings, see “Hidden Dangers in the Air We Breathe.”)

          If your home has a supply ventilation system (for example, a central-fan-integrated supply ventilation system), it's a good idea to include a MERV 13 filter. This filter should be installed at your air handler or furnace. If your home has a balanced ventilation system (an HRV or ERV), choose a ventilating appliance that offers a MERV 13 option. A MERV 13 filter will eliminate most of the PM2.5 particles entering your home.

          According to Terry Brennan, a building scientist who serves on the ASHRAE 62.2 committee, “The kitchen range is the biggest stationary source of fine particulates in a residential setting. It's the pyrolizing and burning of food that makes most of the particulates and contaminants.” So install a good range hood fan, and use it.

          After fine particulates, the next two most worrisome contaminants are acrolein and formaldehyde. Acrolein comes from cooking, wood smoke, and tobacco smoke. Formaldehyde is a component of tobacco smoke and is emitted by many types of particleboard.

          Radon may or may not be a contaminant of concern. Anyone worried about indoor radon should certainly test their air for radon. If concerning levels of radon are detected, it's important to install a radon mitigation system.

          There's a lot that researchers still don't know

          Every tight home needs a mechanical ventilation system. Research has shown that operating a ventilation system helps reduce indoor levels of most air-borne contaminants that can injure human health.

          That said, researchers haven't been able to determine the ideal residential ventilation rate needed to optimize occupant health. (According to Terry Brennan, most studies seem to support a ventilation rate in the range of 10 to 20 cfm per occupant.) In fact, researchers haven't yet been able to show any correlation between residential ventilation and occupant health.

          That doesn't mean that we should be skeptical of the need for mechanical ventilation; it only means that we need to be aware of the many uncertainties surrounding advice on optimizing indoor air quality. For more information on this issue, see Ventilation Rates and Human Health.

          Assessing the risk

          Researchers have noted that there are serious health problems associated with inhaling fumes given off by food that is being cooked and inhaling wood smoke. These are some of the most worrisome contaminants found in indoor air.

          Yet we all know that people have been breathing fumes given off by food that is being cooked, and have been inhaling wood smoke, for thousands of years. My mother doesn't know what a range hood is, and would never think of operating an exhaust fan. Moreover, she has spent countless hours — weeks and months, actually — sitting around smoky camp fires. She is 88 years old. Most families include elderly people who have breathed a lot of cooking fumes and wood smoke.

          While these contaminants are dangerous, humans have a lot of experience with them — so it's important to keep these risks in proper perspective.

          Martin Holladay’s previous blog: “Tales From Armenia.”

          Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Eugene Kim / Flickr

Mar 13, 2016 11:20 AM ET

by Charlie Sullivan

This is a great summary. Building can be so overwhelming that it can be hard to gain perspective. This article provides the needed perspective, at least for this issue.

The only danger I see here is that raw foodists will delight in the condemnation of cooking. How long before you get a question from a raw-food devotee who wants help convincing a builder to go ahead with a range-free kitchen design? Oh, wait that already happened.

Mar 13, 2016 11:33 AM ET

Edited Mar 13, 2016 12:10 PM ET.

Response to Charlie Sullivan
by Martin Holladay

Two protesters holding signs in front of a restaurant:

Organic food advocate: "Free-range chickens!"

Raw food advocate: "Range-free kitchens!"

Mar 13, 2016 5:15 PM ET

by Malcolm Taylor

It's hard to keep up. I took CMHC sponsored Healthy Housing courses in the late 90s that emphasized the dangers of particle board and carpets. Good to know progress has been made.

Mar 13, 2016 9:42 PM ET

There's really only one safe cooking method
by Dan Kolbert

Take out.

Mar 13, 2016 10:07 PM ET

by Malcolm Taylor

You've never done prep work in a restaurant.

Mar 14, 2016 8:49 AM ET

Great overview
by Brian Just

This is an excellent overview of an all-too-often overlooked topic.

Mar 15, 2016 1:58 PM ET

Edited Mar 15, 2016 2:25 PM ET.

Range Hood Vent
by Peter Whitman

Great article, thanks Martin!
Somewhat off topic: My range hood is going to vent to the outside with a 7" rigid duct. The range is rated at 650 CFM. This is more than what I wanted but had a hard time finding a quality island hood with lower CFM's. They sell a CFM reducer to lower the rate of exhaust. Should I install it? What's the best back vent system for a horizontal duct to keep the cold air out in the winter: damper? Exterior vent only?
House is new construction, closed cell, HRV/ERV.
Any links or manufacturers would be appreciated.
Thanks is advance.

Mar 15, 2016 4:24 PM ET

Response to Peter Whitman
by Martin Holladay

Most range hoods rated at 650 cfm actually exhaust about 50% (maybe 70%) of the nameplate rating once they are hooked up to ducts and one or more elbows. You might want to test the airflow on your unit after it is installed. What airflow rate are you aiming for? Does the range hood fan have several speed settings (low, medium, high)?

To keep cold weather out in the winter, make sure that the duct termination on your sidewall has a backflow damper.

Mar 16, 2016 11:56 AM ET

Range hood vent option
by Kevin Hogan

Many builders in Wisconsin have chosen to vent down and out the rim joist to successfully prevent cold air from entering the home through the backdraft damper located at the termination. The rationale is cold air will not travel up the vertical run to the range hood because cold air is heavier than the indoor air. Simple solution and very effective.

Mar 16, 2016 12:28 PM ET

Edited Mar 16, 2016 12:40 PM ET.

Response to Kevin Hogan
by Martin Holladay

I hate to disappoint you, but the theory you present is mostly hogwash.

The relevant forces acting on the air in the duct are many, but the temperature of the air is probably the least important. The main relevant forces are the stack effect and wind (as well as pressurization and depressurization forces created by appliances in the house).

If the kitchen range hood is on the first floor of a two-story or three-story house, then the stack effect in winter might be depressurizing the kitchen with respect to the outdoors. This will tend to pull air into the range-hood duct (if the range hood fan isn't operating).

If the duct termination is located on the windward (as opposed to the leeward) side of the house, the wind forces will be pushing air into the the duct. This will be partially but not completely counteracted by the backdraft damper, which in no cases will be airtight.

The fact that outdoor air is cold does not prevent it from entering a house in winter. The driving force for this type of infiltration, as I said, is usually the stack effect or wind, but it can also be due to depressurization caused by a clothes dryer, a water heater, or a bathroom exhaust fan.

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