Look over this FAQ. Find the material most related to your circumstances. In most cases, there are suggestions as to things that can be checked into. Some of these things can be checked by anyone; others require an expert (medical doctor, HVAC service company, IAQ investigator, etc.).

The usual clue is that people feel ill while inside the building in question, but their symptoms dissipate shortly after leaving the building. Or, in some cases, the symptoms will cease after they are away from the building for a period of time (as on weekends, or a vacation). In most cases, medical testing and investigation of the building is necessary to determine whether the building is the source of the problem.

One of the most important IAQ subjects will receive very little attention in this FAQ: tobacco smoke. Tobacco smoke is recognized as carcinogenic and a cause of increased heart disease to those who inhale it - whether the smokers themselves or those subjected to the second-hand smoke. 

If necessary, hire an IAQ consultant to investigate the building. Be forewarned: There is currently no license, degree, certification, or training that guarantees the consultant will competently perform the investigation. Check the consultant's references, and ensure the person has done similar work before.

You may remember I said the state of the research is incomplete? So don't expect a total answer on this one as it is not known. Some air contaminants are toxic; they affect the body as a poison. Pesticides and lead fall into this category.

Air quality is affected by the presence of various types of contaminants in the air. Some contaminants are in the form of a gas. These would generally be classified as toxic chemicals. The contaminant types of interest are combustion products (carbon monoxide, nitrogen dioxide), volatile organic compounds (formaldehyde, solvents, perfumes, fragrances, etc.), and semi-volatile organic compounds (pesticides). Other pollutants can be in the form of particles. This type of comtanimant includes bioaerosols (mold spores, pollen, viruses, bacteria, insect parts, animal dander, etc.),;soot; particles from buildings, furnishings, and occupants, such as fiberglass, gypsum powder, paper dust, lint from clothing and carpet fibers, etc.; and dirt (sandy and earthy material), etc.

IAQ is a new field in many ways. Much of the current knowledge is less than 20 years old, and there is a great deal that is not known about it. In fact, it is safe to say that far more is unknown than is known about the field of IAQ at this time. Even experts do not have definitive answers in every situation.

The vast majority of IAQ problems are relatively minor. This means that occupants may suffer health effects such as cold or flu-like symptoms.  In the case of workplaces, some individuals may miss some work days, and productivity could be affected. Serious and/or permanent health effects are much rarer;however, they do occur, as in the case of Legionnaire's disease outbreaks.

According to the World Health Organization, approximately 30% of all commercial buildings have significant IAQ problems. Similar estimates have been made by other reputable organizations. I am unaware of any similar estimates in regards to residences.

No building is perfectly sealed so the air inside ultimately originates from outside. Air pollution present in the outdoor environment will therefore enter into the building as well. 

IAQ refers to the effect, good or bad, of the contents of the air inside a structure, on its occupants. Usually, temperature (too hot or too cold), humidity (too dry or too damp), and air velocity (draftiness or motionlessness) are considered "comfort" issues rather than IAQ issues.