If an invading particle is big or especially irritating, you will almost certainly cough or sneeze it straight back out again (often in the process making it someone else’s problem). If it is too small to provoke such a vehement response, it will in all likelihood be trapped in the mucus that lines your nasal passages or caught by the bronchi, or tubules, in your lungs. These tiny airways are lined with millions and millions of hairlike cilia that act like paddles (but beating furiously at sixteen times a second), and they swat the invaders back into the throat, where they are diverted to the
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