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Long time keeping hand in freeze or ice cold things, your blood vessels constrict to avoid heat to escape from the body. Continuously blood vessels constriction will not allow blood to reach to the hand tissues and nerves. So, numbness and pain comes. If you are diabetic or hypertensive, condition will be worse. So, in between your work just rinse your hand and fingers in warm water, not very hot. Use thick rubber gloves to avoid direct cold to your hand.