If breathed in, carbon monoxide damages the body's ability to carry oxygen in the blood and can cause serious injury or death. Warning signals for you to look out for are stains, soot or discolouring on your gas fires or an unusual smell when the appliance is on and an unusual amount of condensation on the windows and walls.
Physical symptoms are unexplained headaches, sleepiness, streaming eyes and general lethargy.
All landlords are responsible for making sure that any gas appliances are maintained and in good working order. You do not have to pay for these checks to be done.