Fluidized Catalytic Cracker Units (FCCU) are used in the oil refining industry to convert long chained hydrocarbons into shorter ones. At normal operations there is little concern regarding catalyst particle emission into the atmosphere from these devices. However, when the unit is started, higher than normal emission rates are common. FCCU stack emissions were tested and categorized in terms of metal composition and were compared with the operational conditions experienced under start up conditions. It was found that excessive emissions can be explained due to fluctuations in cyclone performance, the production of fine particles via attrition processes and the thermal fracture of particles. In addition to particle emissions, levels of emitted metals also fluctuate during the start up period, with the highest levels being experienced at the moment oil is re-fed into the unit. The current paper summarises stack testing results as well as the operational conditions in order to identify the key causes of start up emissions and provide recommendations to reduce and manage particle emissions from refineries.