1. Introduction

1.1 Background

Problems of indoor air quality are recognized as important risk factors for human health in both low-income and middle- and high-income countries. Indoor air is important also because populations spend a substantial fraction of time within buildings. In residences, day-care centres, retirement homes and other special environments, indoor air pollution affects population groups that are particularly vulnerable due to their health status or age. Microbial pollution involves hundreds of species of bacteria and fungi that grow indoors when sufficient moisture is available. Exposure to microbial contaminants is clinically associated with respiratory symptoms, allergies, asthma and immunological reactions. Indoor air plays a special role as a health determinant, and management of indoor air quality requires approaches that differ from those used for outdoor air. For these reasons, the working group preparing the global update of the WHO air quality guidelines (WHO Regional Office for Europe, 2006a) recommended that WHO also prepare guidelines for indoor air quality. This is in line with the recommendations of an earlier WHO working group that formulated The right to healthy indoor air and, in particular, with its Principle 6, which states that “Under the principle of accountability, all relevant organizations should establish explicit criteria for evaluating and assessing building air quality and its impact on the health of the population and on the environment” (WHO Regional Office for Europe, 2000a). The subsequent working group meeting in Bonn in October 2006 acknowledged the applicability of the existing WHO guidelines for air quality (WHO Regional Office for Europe, 2000b, 2006a) to indoor air and identified a number of chemical substances for which specific indoor air guidelines was recommended. The working group also recommended guidelines for two additional categories of risk factors of particular importance to health in indoor environments: biological agents and indoor combustion (WHO Regional Office for Europe, 2006b). Biological agents of relevance to health are widely heterogeneous, ranging from pollen and spores of plants (mainly from outdoors), to bacteria, fungi, algae and some protozoa emitted outdoors or indoors. They also include a wide variety of microbes and allergens that spread from person to person. There is strong