Abstract IntroductionIn March 2014, NHS Lothian setup an Incident Management Team (IMT) to investigate carbon dioxide (CO2) migration into houses in a local council estate. The estate was built above abandoned mine workings with unstable and insufficient rock in places. Migrating mine gas presented a risk to public health that required intervention. MethodsThe IMT investigated the source of carbon dioxide using the source-pathway-receptor model, did a health risk assessment and proposed risk management options for action by the respective member agencies. Results22 of the 174 residents, in the affected area contacted healthcare services between September 2013 and September 2014 with complaints of headaches, dry coughs, dizziness and anxiety. Radiocarbon analysis identified geological CO2 in the houses that was similar to that originating from the mines. The most likely source of the gas was abandoned coal workings with oxidising coal deposits. Atmospheric pressure changes amongst other factors caused surface migration of CO2. Potential pathways included ungrouted disused mine shafts, natural ground cracks, investigation drill holes, grouting work for the Borders rail link, deep drainage installations and, ground stabilisation grouting and vibro stone. ConclusionsThis was a rare, complex, large-scale and costly environmental public health incident. Midlothian Council demolished all 64 properties to safeguard the health of the residents. Inadequate remedial work prior to construction and the lack of gas-proof membrane resulted in CO2 seeping into the houses. Mine gases are a potential hazard across similar sites in Scotland and other parts of the world with old mines. External funding details Not Applicable