Carbon monoxide (CO)

A poisonous gas with no colour or smell, made when gas and other fuels do not burn properly.

Carbon monoxide forms when combustion is incomplete: too little air, a chilled flame or a blocked burner. Breathed in, it binds to haemoglobin and cuts the oxygen the blood can carry.

Headaches, dizziness, nausea and flu-like feelings can be signs. Unexpected yellow flames, soot and staining can be clues on the appliance.

If you suspect carbon monoxide, get everyone into fresh air, get urgent medical advice and call the gas emergency service. Call 999 if someone collapses or struggles to breathe.