Toxics are everywhere, both natural and human induced.  Some toxics, like formaldehyde have both natural and man made sources, while others are strictly man made.  Added to this is a long list of allergens, that although not strictly toxics can have a large negative impact on many people's immune system.   While it would seem that most people have no negative health consequence to the generally low level of naturally occurring toxins, there is currently no scientific proof of this, and some believe that for some toxics, the only safe level is zero.  This idea that there is a safe level, is often called the "threshold" theory, and for some substances (especially radioactive ones) this theory is hotly debated.  At issue is not whether a person will see an immediate negative consequence, but whether there is long term implications (like cancer) many years later.

Eliminating contaminants completely is very difficult, and in most cases not necessary, unless the house is located in a area with highly contaminated outside air.  Instead the usual goal is to avoid adding any extra contaminants beyond those already present in outside air.  Assuming this is done well, a filtration unit will help reduce the contaminants further to where the indoor air can be cleaner than outdoors.

Contaminants can be broken into four broad categories: chemicals (VOCs), particulates, microbes (bacteria, viruses & molds), and radiation.  Medically there are a wide variety of conditions and diseases that can result from exposure to these contaminants, with the respiratory system showing the most frequent symptoms. Toxicants can also be ingested, be absorbed through the skin, or be absorbed into the bloodstream through the lungs.  The effect of toxicant could be fatal to exposed cells, cause the cell to fail over time, affect cell division, cause the immune system to overreact or cause mutations that result in cancer.  Although many contaminates would produce rapid death in large quantities, the amount in a home is low enough that symptoms often don't appear for weeks, months or years, depending on the quantity of contaminants.  Typical symptoms of contaminant exposure include itchy and watery eyes, sneezing, difficulty in breathing, asthma, coughing, itching, rashes, headaches, dizziness, anxiety, inability to concentrate and other changes in mood. 

VOCs - Volatile Organic Compounds are gases that come from a variety of sources, typically via the process of off-gassing, which is the evaporation of chemicals from such materials as paints, glues, varnishes, caulks, plastics and fabrics.  Some materials release their VOCs quite rapidly (in weeks), while others continue to do so for many years.  Just because a chemical is a VOC doesn't necessarily mean its toxic, although many of them are.  The following is a partial list of common materials that emit VOCs:

• Particle Board, Plywood or other wood products with "interior" glue:  The most common interior glue is Urea-Formaldehyde (apparently because its nearly invisible), and is often found in cabinetry, shelving, interior trim and furniture.  Because of the strict European standard for formaldehyde emissions, new versions of Urea-Formaldehyde glue have appeared that very low emissions.  Some particle board manufacturers have turned to alternative glues with low VOCs (eg products like Medex and Medite).  The current situation is buyer beware.

• Paint: VOCs in paint is the part that evaporates when the paint dries on the wall.  Both latex (water based) and oil paint typically contain VOCs, although many low-VOC paints are now appearing on the market.  Just because a paint has low VOCs doesn't mean it isn't toxic, as paints typically have many additives: preservatives, biocides, fungicides, heavy metals, and various binders that give the paint its finish.  Currently the only low-VOC products are latex, while oil paints not only don't come in low-VOC versions, they much higher VOCs than any latex. Unfortunately the latex enamels on the market don't produce the smooth, hard finish of oil.

• Combustion byproducts: gas stoves, dryers and water heaters, furnaces and fireplaces all produce some level of toxic byproducts due to incomplete combustion.  Natural gas is the easiest fuel to burn cleanly, but no mater what the fuel, 100% combustion is rarely obtained.  The easiest way to eliminate these sources of pollution in the house is to use a sealed combustion unit, which uses outside air to burn the fuel, then exhausts the waste back outside, often by the use of a power vent, which is a fan that forces the combustion gases out of the house.

• Varnishes & Floor finishes: these products come in both water based and solvent based versions, with the water based version having lower VOCs, but potentially still many toxins.

• Caulks & glues:  these also come in a variety of forms, and low toxic versions are available.  Many caulks and construction adhesives ("liquid nails"), are highly toxic when applied, but off-gas fairly rapidly. The most common "exterior" grade glue, phenol formaldehyde, off-gasses relatively rapidly, and so is not usually considered a problem. Common carpenters glue (white or yellow), is also considered non-toxic, but isn't water resistant.

• Carpets: it is usually not the carpet fiber itself that off-gasses, but the backing, various additives (eg stain guard), and the padding under the carpet.  Carpets off-gas so much they have a easily detectable odor which lasts for a number of years.  Rugs that can be removed for cleaning are preferable to carpets, and if wall-to-wall is used, keep it out of high traffic areas and consider removing shoes before walking on it.  Natural fiber carpets, such as wool have less VOCs, but some people are allergic to wool, and some wool carpets contain toxic mothproofing additives.  Some synthetic fiber carpets (including recycled "pop bottle" ones) are naturally stain resistant and have little or no added chemicals, but  (what backing and pad??) 

• Synthetic fabrics: most fabrics on furniture and curtains are synthetic and furniture often has a "stain guard" added to it.  While these items can off gas toxins, the question is whether the amount is significant for anyone who isn't chemically sensitive, since alternatives are often difficult to find.

• Cleaning products: many of these are toxic and should be avoided, other than the situation where nothing else works.

• Photocopiers and office supplies: only problems for the chemically sensitive??

• Pesticides:  these products often off gas for a very long time.  They should be stored outside the building envelope or otherwise sealed away.  Of course its best to avoid them completely.

• Dry cleaned clothes: the dry cleaning fluid is toxic, and comes back on your clothes.  A few infrequently cleaned clothes are probably not a problem, but an entire closet of them can add up to a significant problem.  An alternative is wet-cleaning (look on www.greenpeace.org for more info)

• Tobacco smoke: this goes without saying! 

Particulates - The air in most houses is filled with dust, even when the air is what we'd consider clean.  This dust is composed of myriad of different substances including: pieces of skin and hair from both humans and pets, pieces of paper, wood, paint and clothing, dirt, heavy metals, tire fragments, pesticides and other industrial chemicals, dust mites, duct mite feces, pollens, bacteria, viruses and  mold spores.  Virtually anything we come in contact with wears and produces fine particles that end up airborne, and anything that is in the outside air ends up in the inside air.

Particulates tend to settle out of the air over the course of time, staying in the air anywhere from a few seconds, to many hours or day, and some are so light they never settle out at all.  The mucous membranes of the body are good at filtering out the larger particles (greater than 10 microns), but are not at the finer ones, which can reach deep in the lungs and be an irritant, allergen and carcinogen. 

Since many of these particulates are the result of the process of life, they are difficult to avoid, so the best solution is to dilute them with outside air and filter them.  Carpets are a very large source of particulates, because they act like a sponge for them (just take and rug and bang it out outside, and an amazing amount of dust will come out of it).  They are also the perfect habitat for dust mites, whose feces is an allergen to some.  Keeping the house clean is obviously a benefit, but most vacuum cleaners only remove the large duct particles while spiting the smaller particles that cause most of the health problems back into the air.  Central vacuum systems that put all the dust outside are one solution, or a vacuum with a high efficiency filter and a powerful motor is another good option.

Among the particulates are a few worth special mention:

Allergens: pollens, pet dander, dust mites.
While some allergens are present in the outside air, others come as the result of using the house.  Dust mites and molds need water to survive, and by keeping the relative humidity of the house mostly below 50% will inhibit their growth severely.  Installing a high efficiency or HEPA filter in the ventilation will help filter out most allergens.

Lead/Heavy metals from paint chips
Until it was banned in the late sixties, lead was a major component of many paints since it was the pigment of choice to make white paint.  In addition to lead, other heavy metals were used (and some still are) to make other colors.  Paints also can contain a variety of preservatives and fungicides. Since none of these chemicals will off gas from the paint, the only significant hazard is when the paint is loosened from surface, either by aging or scraping and sanding.

Chemicals from outdoor dirt
The outdoor soil can contain a wide variety of toxics, from metals to molds, to chemicals like PCBs, all of which are tracked in the house on shoes.  Removing shoes when you enter the house is the best solution, but "track off" mats can be quite effective at removing dirt from shoes.

Asbestos
Common used as a furnace liner, as the fiber in asbestos/cement siding, and as a component of some gypsum plasters (especially "popcorn" ceilings).   Asbestos has been removed from most products, and the simplest way to deal with it is to contain it.  Because siding is outside, and paint typically lasts longer on it than on wood, it is easily contained by an occasional coat of paint.  Walls and ceiling that have asbestos and aren't flaking can also be contained with paint, or covered over with 1/4" sheetrock. (How to deal with furnaces??)  If you do have to have asbestos removed, you will need a special contractor and it will likely be quite expensive.

Microbes - Molds - while some microbes are parasitic in nature, many have vital functions in the ecosystem.  In particular, molds break down cellulose containing materials and recycle those nutrients into the ecosystem: it just turns out that some of them produce toxic byproducts (penicillin is a case where the toxic byproducts of a mold are used as an antibiotic).  Molds will grow almost anywhere there is water and cellulose available: on wood, paper, food and fabric.  It will also grow on surfaces like stone and tile, consuming the various cellulose containing particles of dust.  The only way to control mold is to keep it from getting water, by avoiding standing water, minimizing the possibility of condensation and keeping the relative humidity below 60%.  For more in depth information, see the section on Moisture Control.

Radiation - Radon & EMFs - Of all the contaminants, radiation is probably the once that induces the most fear in people, possibly because our bodies have no ability to sense its presence.  It is impossible to avoid all radiation, since there is a level of background radiation everywhere, including our own bodies.  Background radiation is composed largely of cosmic radiation and naturally occurring radioactive materials in the soil, and varies by latitude, elevation and soil type, and can vary by up to a factor of four within the United States, and by much more in other locations in the world.

The most significant problem with radiation is due to Radon, which can accumulate in a house at fairly high levels by slowly leaking in from the ground.  Not all soils have Radon have radon in them, and the only sure way to tell is to test for it (there are regional maps available).  If radon is present, the solution is to dilute it with fresh air.  In an unheated basement a passive radon vent from the basement to the roof, which takes advantage natural ventilation is usually good enough.  Alternatively an exhaust fan could be used, or any other type of mechanical ventilation. 

Resources:

A Guide to Planning, Building & Maintaining a Healthier Home, Dan Morris,
Columbia Design Group, 1999

Prescriptions for a Healthy House, Paula Baker, Erica Elliot & John Banta, Inword Press, 1998

Healthy by Design, David Rousseau & James Wasley, Hartley & Marks, 1997

WA Toxics Coalition

EPA?