### What you need to know A 23 year old man with no medical history presents to the emergency department with a three day history of headache, transient visual disturbance, dizziness, and hypertension. On clinical examination the patient is flushed and drowsy with redness in the sclera, with no further visual or systemic symptoms. A venous blood gas on air shows a carboxyhaemoglobin level (COHb) grossly elevated at 26.0 (0-3), which leads to the diagnosis of carbon monoxide poisoning. Carbon monoxide (CO) is a poisonous gas produced through burning fuel. Sources of CO in the home include boilers and central heating systems, cookers and barbecues, and fireplaces and chimneys (figure).1 Owners of such appliances are required to have them serviced by registered technicians as per recommended individual guidance (usually once per year).1 If outlets from these appliances (such as flues and chimneys) become blocked or if they are operated as normal in a closed environment with no ventilation, dangerous levels of CO can build up in living spaces.1 When inhaled in high quantities, CO enters the bloodstream and binds to haemoglobin molecules with a much greater affinity (230 times higher) than oxygen, creating carboxyhaemoglobin.2 The binding of CO results in reduced delivery of oxygen to tissues, which leads to tissue ischaemia.2 Figure Common domestic sources of CO Presentation of CO poisoning is not frequent enough for the condition to prioritised in medical training, diagnostic decision making, initial patient testing, and publicity and public …