Soot is in the air: the physiological impacts of air pollution in birds and bumblebees

Air pollution has become a widespread consequence of human activities, with increasing urbanization and a warming climate shaping its distribution and effects. Organisms with distinct respiratory systems and physiological traits are exposed to multiple air pollutants across human-modified environments, particularly in cities. However, much of our current understanding of how air pollutants affect physiology and health is derived from studies on mammalian systems, raising questions about whether, and how, these insights apply to other taxa. This thesis addresses this knowledge gap by investigating the physiological responses of birds and bumblebees to traffic-related air pollutants, with a focus on particulate pollution. By integrating tools and concepts from aerosol science and ecophysiology, I first quantified particle deposition and clearance in an avian lung model (Paper I), and subsequently examined both short- and longer-term physiological outcomes under controlled exposures (Paper II, III and IV) as well as real urban environments (Paper V). Results show that deposition of ultrafine particles in the avian lungs increases with smaller particle size and higher ventilation rates, but particle retention is efficiently reduced within weeks after exposure. In birds, particle exposure induces multilayered physiological responses, with effects on oxidative balance, immune function and genomic integrity occurring at different rates and timescales, and depending on particle properties to varying degrees. Specifically, oxidative and immune responses seem to gradually build up over time, while genomic instability happens much more rapidly and is either resolved or sustained. Complementing these experimental findings, field data in free-living birds also show that innate immune responses can be associated with fine-scale pollution levels (in this case, nitrogen dioxide and particulate matter), but the responses are pollutant-dependent and differ between species and life stages. In bumblebees, oxidative balance is affected by particle exposure, but the oxidative challenge is successfully resolved in most colonies, at least immediately after exposure. In contrast, ozone exposure does not seem to affect oxidative balance in these insects. Taken together, the present findings demonstrate that the impacts of air pollution in organisms are nuanced and dynamic across physiological systems and taxa, unfolding over time and being shaped by pollutant properties and organismal traits. By moving beyond traditional mammalian models and incorporating an ecological perspective, this thesis contributes to the mechanistic understanding of how air pollutants influence animal health in human-modified environments.