Whether you are a smoker or non-smoker, you should take measures to protect your lungs. You can test your lung capacity by holding a lighted match about six inches from your face and trying to blow it out with your mouth wide open. If you cannot blow out the flame, arrange for a medical examination. Using a simple machine called a spirometer, a doctor can measure the lung's capacity and strength. In other words, he can test how efficiently you breathe. When combined with a complete physical examination, the spirometer test may detect or discover early signs of respiratory problems. Which sentence expresses the main idea?