Cilia, tiny hair-like structures on the lung's lining, propel mucus up and out of the lung using a synchronised back-and-forth wave-like motion. Excess mucus is created when an irritant, like bacteria or dust, enters the lung. It is vital to a person's health that the cilia effectively move the irritant-carrying mucus out of the lung. Interestingly, some irritants are known to make the cilia beat faster and some make the cilia beat more slowly.
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