Mechanisms underlying nontoxic indoor air health problems: A review

Indoor air health problems of the type non-specific building-related symptoms (NBRS; formally called sick-building syndrome) and chemical intolerance (CI; e.g. multiple chemical sensitivity) can in severe cases lead to significant disability and poor quality of life. Apart from suffering of the afflicted individuals, the productivity loss ascribed to environmental intolerances of this kind is very costly for society. Preventive measures and appropriate treatment call for understanding of the mechanism underlying NBRS and CI. Considerable similarities between NBRS and CI suggest that the two conditions at large share mechanisms. Since typical cases of these conditions cannot be explained by toxic exposure, the present objective is to describe underlying mechanisms of psychobiological nature for which there is well-developed theoretical ground and empirical support. Focus lies on the mechanisms neurogenic inflammation and neural sensitization. Apart from describing its basic mechanisms, neurogenic inflammation is reviewed in relation to NBRS and CI regarding neurogenic switching, activation of the autonomic nervous system and axon reflex as well as interaction effects between chemical irritants, allergens, and psychosocial stressors. In addition to describing various types of sensitization, empirical support for their role in NBRS and CI is reviewed. The mechanism classical conditioning, symptom misattribution and somatosensory amplification, and nocebo are also addressed. The review rounds off with a discussion on why only a subset of individuals exposed to these indoor environments develop NBRS and CI, and a discussion on integration of the presented mechanisms, accompanied by proposed hypotheses for future research.