Air pollution is a major global health threat. There is growing evidence for a negative effect of air pollution on health and well-being. Relationships between air pollution and health are mediated by health risk perceptions and play a crucial role in public response to it. Air pollution in the public’s mind is often different from air pollution defined by the scientific community. Therefore, in order to develop successful prevention and alleviation strategies, an understanding of public risk perceptions is key. The central question of this paper is: ‘How does “the public” (in Brussels) perceive air pollution?’ This research is an attempt to enrich the limited body of qualitative research in the field, approaching the topic of perception from 4 different, complementary angles: definition, association, categorisation and problematisation. About 51 interviews were conducted in the Brussels-Capital Region. Consistent with earlier research, this research illustrates that perceptions of air pollution are diverse, subjective, context-dependent and often deviate from conceptualisations and definitions in the scientific community. Respondents underestimate the potential harm and problematisation depends on comparative strategies and perceived avoidability. The novel aspect of this paper is the identification of 5 mental schemes by which specific elements are categorised as being air pollution: (1) the source of the element, (2) its health impact, (3) its climate impact, (4) its functionality and (5) sensory perceptions. The insights gained from this research contribute to the field of environmental epidemiology through a better understanding of how ‘the public’ perceives air pollution and in what way this may deviate from how it is perceived by experts. We hope to raise the awareness among experts and policy makers that air pollution perceptions are far from universal and consensual but on the contrary individual and contested.