Urban rehabilitation is becoming more frequent nowadays. With cities full of falling buildings and no space to build new ones, the only solution is to rehabilitate the existing heritage in order to give new life to the vacant buildings and repopulate the cities. A large amount of the built heritage suffers from advanced degradation, making the recovery, increasing the complexity of the rehabilitation and restoration intervention and implying a highly specialized interdisciplinary component. In this type of works more dust is produced than in a new work. In addition, the origin and composition of existing materials is often unknown and can therefore pose many risks to the health of workers. Meaning, dust is breathable and can remain in the air for a long period of time, being invisible to the naked eye and very light. When a building is being demolishing, the production of dust increases significantly. Depending on the type of materials to demolish, the risk of occupational diseases, or risks, to the health of workers may be great. To avoid such risks, preventive measures must be taken in the execution of the tasks. In this study, the dust of two rehabilitation works in the district of Viana do Castelo was studied through experimental tests in order to verify the composition of the demolished materials. Particles characterization was done, using scanning electron microscopy (SEM). Then, with the tests results, the chemical composition of the existing materials was analyzed in detail. After that, the chemical components of demolished materials were carefully studied to identify which of these particles could represent a risk to the workers’ health. As a conclusion the preventive measures that workers must take when performing rehabilitation works are presented, depending on what type of materials they are potentially exposed.