A spirometer is an instrument that doctors use to measure how well a person's lungs are working. To use one, a person breathes into a mouthpiece. The doctor may ask the person to breathe normally or to take a deep breath and blow the air out quickly. A recording device measures the amount of air inhaled or exhaled and how much time each breath took. These measurements can help the doctor find out if a person has a lung problem, such as asthma.