Professor Harrison has made a significant contribution to the understanding of urban chemistry, environmental pollution, particulate matter and human health, both nationally and worldwide. He has advised and served on a number of committees including the Department of Health Committee on the Medical Effects of Air Pollutants, the DEFRA Air Quality Expert Group, the Natural Environment Research Council’s Atmospheric Science and Technology Board, the HEFCE RAE panel, the Medical Research Council Advisory Board, and the Natural Environment Research Council's Science & Innovation Strategy Board. He notably advised the World Health Organisation on their 2005 global air quality guidelines, and more recently on their 2011 indoor air quality guidelines.