Jan is an environmental epidemiologist with research interests in climate change and public health. He led the US CDC response to the 1995 heat wave in Chicago that claimed the lives of more than 700 people and elucidated the underlying environmental, societal, and behavioural risk factors for heat-related mortality.
He also worked internationally in Uzbekistan, Sudan, Egypt, Denmark, Brazil, and Haiti. He was a faculty member at UC Berkeley, UC Irvine, Oregon Health and Science University, and at Portland State University where he taught in the Oregon Master Program of Public Health.
Most recently he was the head of the Health Determinants Section at the European Centre for Disease Prevention and Control (ECDC), where he analysed and monitored determinants of infectious diseases, such as climate change. He was a lead author of the IPCC AR6 report and the co-lead of WG1 and WG2 of the Lancet Countdown in Europe. Currently, he is associated with the Department of Sustainable Health at Umeå University in Sweden and Heidelberg Institute of Global Health, at the University of Heidelberg in Germany.