There is a lack of precision in the definition of health, which leads to confusion in the assessment of adverse effects arising from indoor air pollution. Adverse health effects range from annoyance to life-threatening conditions. Survey responses suggest that males and females differ in their perception of a healthy person, but both sexes envisage a male in terms of positive fitness, strength, energy and the possession of an athletic body, rather than how long one was likely to live. Psychological fitness was relatively unimportant in describing the health of others, but was rated as very important with respect to one’s own health. Mortality statistics tend to obscure the proportion of the population who suffer chronic illness that is not life threatening. Although health is largely determined by genetic constitution, lifestyle and environmental factors, the morale of an individual is also important. A new classification of the adverse effects on health of indoor air pollution is proposed: this includes ‘comfort’ responses, such as sick building syndrome (category 1); acute biological effects, including allergies and microbiological infections (category 2A); acute chemical effects, the nature of which depends upon the specific intoxicant (category 2B), and perceived chronic grave risk, including cancer causation (category 3). The magnitude of risk in this latter category is imprecise, because its measurement involves the technique of quantitative risk assessment.