The monitor that is commonly used in hospitals to estimate oxygen saturation in the blood is the pulse oximeter. It usually clips onto your fingertip and uses LEDs and photodiodes to measure oxygenation in the blood. Here's a good explanation pulse-oximetry-basic-principles-and-interpretation. This can give you a quick estimate of lung and blood circulation performance.
If you are referring to monitoring lung capacity, then you are likely looking for a spirometry sensor. These are typically differential pressure sensors.
