Indoor air quality (IAQ) in residential buildings is an issue of increasing importance. IAQ has a serious impact on human wellbeing, performance and health. In order to minimize negative consequences, considerable efforts are made to assure high quality of indoor air. Yet, attention should be payed to the fact that occupants, in particular their activities, heavily influence this component of environment. Therefore, the aim of this work was to develop a method, which allows to characterise the impact of occupant activity on indoor air. The method is based on a semi-quantitative measure called occupant activity impact (OAI) profile. It is individually created for each activity. The profile is prepared using indoor air parameters, which are significantly affected by the activity of occupants. The profile displays how these measurable parameters change due to the occurrence of the activity. The main elements of the method of occupant activity impact characterization are: 1) measurements of multiple indoor air parameters, 2) recording the occurrence of indoor activities, and 3) evaluation of the relation between indoor air parameters and occupant activities. The first task is realized using sensor technique. The second goal is attained by observation. The third aim is achieved using Fisher's discriminant ratio. The method presented in this work allows to evaluate the character of impact, its range, direction and magnitude. Using an experimental data collected in an exemplary household, we demonstrated that OAI profile is effective in quantifying and analysing the impact of human activity on indoor microclimate. (C) 2017 Elsevier B.V. All rights reserved.