Polyurethane insulation foam, which is used as insulation on district heating steel and copper pipes, is made from polymerisation of polyol and diisocyanates monomers. Installation of new district heating pipes or replacement of old pipes involves masses of welding activities. Welding processes of pre-insulated district heating pipes can cause exposure of polyurethane insulation to high heat generated during the welding process. Temperatures of more than 150 °C can lead to thermal degradation of polyurethane insulation and isocyanates formation. Exposure of isocyanates to welders can cause allergy, irritation of eyes, nose, throat, skin, and lung problems, and isocyanate asthma. In this thesis project welding processes of varying nominal sizes of district heating pipes, varying welding method and consequential formation of isocyanates have been studied. This project has found that selection of welding method plays a key role in the formation of isocyanates. It was identified that although the oxy-fuel welding method is considered as easy method for welding, there is a generation of high heat. This since oxy-fuel welding warm up the district heating pipes to high temperatures, which lead to thermal exposure of polyurethane insulation and ultimate formation of hazardous isocyanates. Other aspect such as varying nominal sizes of the pipes and varying length of insulation-free ends effect the exposure of polyurethane insulation to high heat and eventual formation of isocyanates. This study found that small nominal sizes of district heating pipes DN20, DN25, DN32, DN50, DN65, DN80 of insulation-free ends from 20 to 22 cm, welded by oxy-fuel welding, have a low risk to generate critical high heat 150 °C. But if the length of the insulation-free ends is reduced to 16 cm it can lead to thermal degradation of polyurethane insulation and isocyanates formation. For air samples of isocyanates collected during welding of nominal pipe sizes DN100, DN150, DN200 having insulation-free ends 22-20 cm welded by oxy-fuel welding, the analysis showed notable types of monoisocyanates MIC, PHI, ICA and diisocyanates 4.4´-MDI. The concentration of the detected monoisocyanates ICA and diisocyanates 4.4´-MDI in some samples were higher than the acceptable short-term exposure limit STEL (μg / m3) determined by AFS 2018:1. The presence of large fractions of isocyanates in samples collected from surroundings of high-temperature-exposed polyurethane insulation showed high risks of isocyanates exposure to welders.