Carbon monoxide (CO) is a colourless, odourless, tasteless gas produced by incomplete combustion of carbon-containing fuels. It is acutely lethal at high concentrations and causes chronic health effects — including cardiovascular damage and neurocognitive impairment — at lower levels experienced near traffic, in poorly ventilated homes with combustion appliances, or in vehicles. In the UK, CO poisoning causes approximately 60 deaths and 4,000 hospital admissions per year, predominantly from faulty gas boilers, blocked flues, and portable appliances used in enclosed spaces. At sub-lethal concentrations, CO is a cardiovascular toxin and neurological stressor.
Where it's found
Faulty or poorly maintained gas boilers, gas fires, and central heating systems — the leading cause of fatal domestic CO poisoning in the UK. Wood-burning stoves and log burners with blocked flues or poor draught. Petrol-powered generators, barbecues, and camping stoves used indoors or in enclosed garages — a cause of particularly severe acute poisoning. Vehicle exhaust in enclosed spaces: garages, underground car parks, tunnels. Car interiors in heavy traffic with windows down or poor cabin ventilation. Paraffin heaters and portable propane or butane heaters. Tobacco smoke is a significant CO source for smokers — carboxyhaemoglobin levels in smokers are chronically elevated.
Routes of exposure
Inhalation is the sole route. CO binds to haemoglobin with an affinity approximately 250 times greater than oxygen, forming carboxyhaemoglobin (COHb) which cannot carry oxygen. At low concentrations (COHb 10–20%) headache, dizziness, and fatigue occur. At moderate concentrations (COHb 30–40%) confusion, chest pain, and loss of consciousness. At high concentrations (COHb above 50%) rapidly fatal. Sleeping victims of acute CO poisoning may not wake before losing consciousness — making overnight boiler faults and bedroom portable heater use particularly deadly. Chronic low-level exposure from traffic and smoking causes persistent COHb elevation with cardiovascular and neurological consequences.
Health concerns
Acute CO poisoning at high concentrations is rapidly fatal and well understood. At sub-acute concentrations causing COHb of 10–20%, neurological injury including memory impairment, cognitive slowing, and personality change can occur — sometimes without the victim attributing symptoms to CO. Chronic low-level CO from traffic increases carboxyhaemoglobin, reducing oxygen-carrying capacity, contributing to cardiovascular disease (CO accelerates atherosclerosis) and worsening cardiac ischaemia. CO exposure during pregnancy is associated with foetal growth restriction and preterm birth. Smokers have chronically elevated COHb from tobacco CO, contributing significantly to smoking-related cardiovascular disease.
Evidence
Acute CO toxicity is unambiguously established — the mechanism (haemoglobin binding, cellular hypoxia) is completely characterised. Sub-lethal neurological effects are well documented in survivors of acute poisoning and in occupational studies. Cardiovascular effects of chronic low-level CO are supported by epidemiological data, including traffic-related studies. The UK government's own estimates of CO deaths and hospital admissions are well-evidenced. This is one of the best-characterised indoor air pollutants.
Who's most at risk
People sleeping in homes with faulty or poorly maintained gas appliances face the most lethal risk — CO poisoning during sleep is often fatal because symptoms do not wake the victim. Pregnant women face fetal risk at concentrations that are non-lethal for the mother. People with pre-existing cardiovascular disease are at increased risk from sub-acute CO exposure. Smokers carry chronically elevated COHb from tobacco. Vehicle mechanics and drivers who spend significant time in traffic or underground car parks. Campers and outdoor enthusiasts who use portable stoves or generators in tents or vans.
Regulatory status
RegulationUK Building Regulations require CO alarms in any room with a solid-fuel burning appliance (log burners, open fires) in new builds and renovations (Part J). Gas Safety (Installation and Use) Regulations require annual servicing of gas appliances by a Gas Safe registered engineer. UK Carbon Monoxide and Smoke Detector Act 2022 extended the CO alarm requirement in England to all rooms with gas appliances. CO alarm requirements in Scotland, Wales, and Northern Ireland are separately regulated. Outdoor ambient CO concentrations are regulated under the EU/UK Air Quality Directive (hourly mean limit: 10 mg/m³).
How to reduce your exposure
Install audible CO alarms in every room with a combustion appliance — they cost around £15–20 and last 5–7 years. Have your boiler, gas fires, and flues serviced annually by a Gas Safe registered engineer. Never use barbecues, generators, or camping stoves indoors or in enclosed spaces. Keep log burner and fireplace flues clear and have chimney swept annually. Ensure log burner doors seal correctly and there is adequate ventilation in the room. In heavy traffic, use vehicle air recirculation mode and keep windows closed. If your CO alarm sounds, get everyone out of the building immediately and call 999.
The nutrition connection
Carbon monoxide is one of the starkest examples of an invisible environmental threat where a simple, inexpensive intervention — a £15 alarm — provides near-complete protection against the most severe outcomes. This principle of targeted, proportionate action against specific hazards aligns with Nutriofia's approach: the most impactful changes in chemical risk reduction are often not the most complex or expensive. As with diet, the fundamentals matter most — CO alarm installed, boiler serviced, no barbecue indoors — before more nuanced risk reduction strategies become relevant.