His research interests are in air pollution, especially airborne particulate matter. In recognition of his government advisory work, he was appointed an Officer of the Order of the British Empire (OBE) in the 2004 New Year Honours List, and was elected a Fellow of the Royal Society (FRS) in 2017. He is author of over 600 papers in the peer-reviewed literature, has an h-index of 92, and is listed by ISI as a Highly Cited Researcher in the Cross-Field category (2020), and previously a Highly Cited Researcher in the Ecology/Environmental Science and Geosciences categories.