BACKGROUND Evaluation of exposure incidents and laboratory studies with humans and animals have generated reasonable consensus on safe and unsafe workplace exposures for about 1000 chemicals and particles. Consequently@ many countries regulate exposures of workers to these agents. However@ chemical and particle concentrations that meet occupational health criteria usually exceed levels acceptable to occupants in nonindustrial spaces such as offices@ schools@ and residences@ where exposure times often last longer and exposures may involve mixtures of many contaminants and where those exposed comprise a less robust population (e.g.@ infants@ the elderly@ the infirm) (NAS 1981). The generally accepted broad definition of health is that in the constitution of the World Health Organization (WHO): ??Health is a state of complete physical@ mental@ and social well-being and not merely the absence of disease or infirmity.?? Another definition of health@ more narrowly focused on air pollution@ presented by the American Thoracic Society (ATS 1999) takes into account broader@ societal decision-making processes in defining what constitutes an adverse health effect of air pollution. Key points of the ATS definition of adverse effects include ? Biomarkers@ or biological indicators (e.g.@ in blood@ exhaled air@ sputum) of environmental effects. Because few markers have yet been sufficiently validated for use in defining thresholds@ not all changes in biomarkers related to air pollution should be considered adverse effects. ? Quality of life. Adverse effects of air pollution can range from watering@ stinging eyes to cardiopulmonary symptoms@ and even psychiatric conditions. ? Physiological impact. Physical effects of pollution can be transitory or permanent@ and appear alone or accompanied by other symptoms. The ATS minimum requirement for considering pollution to have an adverse effect is reversible damage accompanied by other symptoms (reversible damage alone is not sufficient). Also@ effects such as developmental damage to lungs@ or exacerbation of age-related decay in function@ must be considered. ? Symptoms. Not all increased occurrences of symptoms are considered adverse effects of air pollution: only those diminishing an individual??s quality of life or changing a patient??s clinical status should be considered adverse. ? Clinical outcomes. Detectable effects of air pollution on clinical tests should be considered adverse. ? Mortality. Any increase in mortality should be judged adverse. ? Population health versus individual risk. Any increase in the risk of an exposed population should be considered adverse@ even if there is no immediate@ outright illness. Definitions of comfort vary. Comfort encompasses perception of the environment (e.g.@ hot/cold@ humid/dry@ noisy/quiet@ bright/dark) and a value rating of affective implications (e.g.@ too hot@ too cold). Rohles et al. (1989) noted that acceptability may represent a more useful concept of evaluating occupant response@ because it allows progression toward a concrete goal. Acceptability is the foundation of a number of standards covering thermal comfort and acoustics@ as well as odor comfort. Nevertheless@ acceptability varies between climatic regions and cultures@ and may change over time as expectations change. Concern about the health effects associated with indoor air dates back several hundred years@ and has increased significantly in recent decades. During the 1970s and 1980s@ this attention was mainly a result of concerns about radon and lung cancer@ and about increased reporting by building occupants of complaints about poor health associated with exposure to indoor air or sick building syndrome (SBS). More recently@ interest has largely focused on asthma@ allergies@ and airway infections. SBS encompasses a number of adverse health symptoms related to occupancy in a ??sick?? building or room@ including mucosal irritation@ fatigue@ headache@ and@ occasionally@ lower respiratory symptoms@ and nausea. Large field studies (EPA 2012; Skov and Valbjorn 1987; Sundell et al. 1994) have shed light on the causes. Widespread occurrence of these symptoms prompted the World Health Organization to classify SBS symptoms (WHO 1983): ? General symptoms@ such as headache@ tiredness@ nausea ? Mucous membrane symptoms in the nose@ eyes@ or throat@ including coughing@ sensations of dryness ? Skin symptoms: redness@ itching@ on upper body parts Sick building syndrome is characterized by an absence of routine physical signs and clinical laboratory abnormalities with regard to sensory irritation and neurotoxic symptoms@ while skin symptoms often can be objectively verified. Some investigations have sought to correlate SBS symptoms with reduced neurological and physiological performance. In controlled studies@ SBS symptoms can reduce performance in susceptible individuals (M?lhave et al. 1986). Building-related illnesses (BRIs) have similar symptoms@ but include physical signs and abnormalities that can be more easily clinically identified (e.g.@ hypersensitivity illnesses@ including hypersensitivity pneumonitis@ humidifier fever@ asthma@ and allergic rhinitis).