Rose Edyvean, of Noosa, asks :-
Why do our eyes weep when running in cold air?
Gordon Sanderson, an opthalmologist at the University of Otago's School of Medicine, responded.
We need tears to maintain our eyes in a healthy state, because technically our eyes are still at the “fish phase” of evolution. Tears provide oxygen to the cornea and contain small quantities of anti-microbial compounds. They pool along the lower lid and every time we blink the top lid dips into this reservoir and pulls a new layer of tears across our eye, they keep our eyes moist and protected.
To prevent them evaporating too quickly, tears are covered with a thin film of oil, our eyelids keep them covered when we are asleep. Tears are important not just to protect the eye, but also to wash out foreign bodies. This means that they must be replenished regularly. Having been secreted by the lacrimal glands they then drain away through tiny ducts on the inside edge of the eyelids. If you pull the lower lid down slightly and look very carefully at the inner corner of your eyelid you will see the ducts called punctae, they look like pinholes; from here the tears drain away to the back of your nose.
Eyes water for a number of reasons: one is irritation causing overproduction of tears, for instance chopping onions; onion juice is an irritant just like teargas. Small particles such as smoke can also make your eyes run. If the punctae become blocked, this makes the tears overflow. Occasionally temporary blockage is caused by obstruction of the drainage duct. This can be due to a number of factors; one is allergy when the ducts become inflamed, another is when we have a cold and our naso-lacrimal canals become filled with mucous. Cold weather, in particular cold wind, can cause the ducts to constrict. This is the most likely explanation for the weeping eyes that you have experienced on cold days.