Harrison is an expert on air pollution, specialising in the area of airborne particulates, including nanoparticles.  His interests extend from source emissions, through atmospheric chemical and physical transformations, to human exposures and effects upon health.  His most significant work has been in the field of vehicle emitted particles, including their chemical composition and atmospheric processing.  This forms the basis of the current understanding of the relationship of emissions to roadside concentrations and size distributions.
In addition to leading a large project on diesel exhaust particles, he is also engaged in major collaborative studies of processes determining air quality in Beijing and Delhi.