The term "building occupants" is generally used to describe people who spend extended time periods (e.g., a full workday) in the building. Groups of occupants that may be particularly susceptible to the effects of indoor air contaminants include, but are not limited to the following:

• allergic or asthmatic individuals
• people with respiratory disease
• people whose immune systems are suppressed due to chemotherapy, radiation therapy, disease, or other causes
• contact lens wearers

Because of varying sensitivity among people, one individual may react to a particular indoor air quality problem while surrounding occupants have no ill effects. In addition, a single indoor air pollutant or problem can trigger different reactions in different people. Some may not be affected at all.

Types of Symptoms and Complaints
The effects of indoor air quality problems are often non-specific symptoms rather than clearly defined illnesses. Symptoms commonly attributed to indoor air quality problems include:

• headache
• fatigue
• shortness of breath
• sinus congestion
• cough
• sneezing
• eye, nose, and throat irritation
• skin irritation
• dizziness
• nausea

All of these symptoms, however, may also be caused by other factors, and are not necessarily due to air quality deficiencies.One of the most common indoor air quality complaints is that "there's a funny smell in here." Odors are often associated with a perception of poor indoor air quality, whether or not they cause symptoms. Environmental stressors such as improper lighting, noise, vibration, overcrowding, ergonomic stressors, and job-related psychosocial problems (such as job stress) can produce symptoms that are similar to those associated with poor air quality.

SBS, BRI, and MCS
The term sick building syndrome (SBS) is sometimes used to describe cases in which building occupants experience acute health and comfort effects that are apparently linked to the time they spend in the building, but in which no specific illness or cause can be identified. The complaints may be localized in a particular room or zone or may be widespread throughout the building. Many different indoor air quality symptoms have been associated with SBS, including respiratory complaints, irritation, and fatigue.
Any or all of the following issues may cause SBS problems:

• the combined effects of multiple pollutants at low concentrations
• other environmental stressors (overheating, poor lighting, noise)
• ergonomic stressors
• job-related stressors (overcrowding, heavy workload)
• unknown factors

Building Related Illness (BRI) is a term referring to illness brought on by exposure to the building air, where symptoms of diagnosable illness are identified (certain allergies or infections) and can be directly attributed to environmental agents in the air. Legionnaire's disease and hypersensitivity pneumonitis are examples of BRI that can have serious, even life-threatening consequences. A small percentage of the population may be sensitive to a number of chemicals in indoor air, each of which may occur at very low concentrations. The existence of this condition, which is known as multiple chemical sensitivity (MCS) is a matter of considerable controversy. MCS is not currently recognized by the major medical organizations, but medical opinion is divided and further research is needed. Sometimes several building occupants experience rare or serious health problems (cancer, miscarriages, Lou Gehrig's disease) over a relatively short time period. These clusters of health problems are occasionally blamed on indoor air quality, and can produce tremendous anxiety among building occupants. State or local Health Departments can provide advice and assistance if clusters are suspected. They may be able to help answer key questions such as whether the apparent cluster is actually unusual and whether the underlying cause could be related to IAQ.