Due to the wide variety of maintenance operations that take place in very different sectors, maintenance workers may come into contact with a huge variety of dangerous substances. Because maintenance is carried out in all sectors and workplaces and involves a wide range of tasks, it is associated with many different hazards and risks, including chemical hazards. Maintenance workers come into close contact with a broad variety of often hazardous chemicals. Depending on the specific type, these chemicals may not only cause diseases like skin sores or cancer, but many of them are highly flammable and explosive. This part focuses on the fact that many accidents have occurred because equipment, though isolated correctly, was not completely freed from hazardous materials or because the pressure inside it was not completely blown off and the workers carrying out the repair were not made aware of this.