A major obstacle for integrating energy and indoor air quality (IAQ) strategies in the design and optimization of buildings is the non-existence of an agreed measure, which can quantitatively describes the IAQ and will allow the assessment of measures to improve energy performance. A complication to develop such an IAQindex is that hundreds of chemical compounds are present in indoor air, including residential environments, at concentrations much lower than occurring during occupational exposures. There is a lack of clear consensus on which pollutant or group of pollutants should be used to form such an index as well as on how they should be integrated into one index. IEA EBC Annex 68 was formed with the objective to discuss Indoor Air Quality Design and Control in Low Energy Residential Buildings. The objective of Subtask 1 of this Annex described in this paper was to review, discuss and propose methods and approaches to define an IAQindex and to develop such an index so that it can be used as a key performance indicator to examine the methods to control IAQ which are integral parts of other subtasks in this IEA EBC project. To meet this objective, the pollutants measured in low-energy houses were compared with pollutants measured in traditional houses. Pollutants measured and known to be harmful for health were identified and selected to be represented on a list of pollutants that should be considered when the IAQindex is defined. The selected pollutants were: acetaldehyde, acrolein, a-pinene, benzene, carbon dioxide, formaldehyde, naphthalene, nitrogen dioxide, PM10, PM2.5, radon, styrene, toluene, trichloroethylene, TVOC and mould. Both short-term and long-term effects of these pollutants were considered. The compound with the highest ratio of concentration to its exposure limit value was proposed as the IAQ index for the short term effects so that the existing exposure limits could be referred to and to avoid problems associated with the aggregation of many air quality indices. The same approach was proposed for the IAQindex for the long-term effects together with the calculation of the burden of disease caused by the compounds of concern expressed as the sum of disability-adjusted life years. It is recognized that the proposed IAQ indices are crude, and the list of compounds is incomplete. It should be progressively updated once new data on exposure limits and pollutants of concern become available. (C) 2017 Elsevier B.V. All rights reserved.