Normal functioning meibomian glands secrete clear oil.  Clogged meibomian glands produce crusty, toothpaste-like or pus-looking meibum.
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Clogged meibomian glands will make your eye easily sensitive to environmental factors such as wind, dust, air conditioning etc...  Your vision won't be as crisp because the tear film is unstable, which does not allow the light to reflect correctly and consistently onto your retina.
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