In this study we carry out receptor modelling via Positive Matrix Factorisation (PMF) to identify and quantify the main natural and anthropogenic sources of indoor PM_2.5 at an urban background site in the island of Malta. Quartz and PTFE filters were collected, analysed gravimetrically and chemically, using ICP-MS, IC and an OC-EC aerosol analyser to determine the concentrations of PM_2.5, 18 elements, 5 ions, organic carbon (OC) and elemental carbon (EC). The EPA PMF was used to identify 8 factors that were affecting the receptor site. Seven outdoor sources were identified: ammonium sulfate (31%), traffic (10%), shipping (10%), sea salt (9%), fireworks (4%), Saharan dust (2%) and industrial (2%). An indoor factor was also identified, which contributed 26% to the indoor PM_2.5. Cooking and e-cigarette smoking were identified as the main contributors to the indoor factor. The mean indoor PM_2.5 concentration (5.7 μg m⁻³) at the receptor site was slightly higher than the WHO AQG limit of 5 μg m⁻³. Uniquely for Malta, we have isolated a fireworks factor for indoor PM_2.5. Fireworks have been identified as being responsible for most of the Sb and Ba and hence are of great concern due to the health implications of these toxic elements.