These are tests to see how well your lungs are getting oxygen into your blood and taking carbon dioxide (a waste gas) out.

The simplest test measures how much oxygen is in your blood using an oximeter (pictured) – a little gadget that clips on your fingertip or earlobe. Sometimes it is attached to a monitor that you wear on your wrist, like a watch.

The oximeter doesn’t take blood. It shines a light though your fingertip or earlobe, and measures how red your blood is.

NB: If you wear nail varnish, this might block the light and affect the reading, so you’ll be asked to take it off (on one finger only). False and acrylic nails can also cause a problem and are not so easily removed in an emergency situation.