The medical term for the bluish hue of the skin, including underneath the fingernails, is cyanosis. Blue fingernails can be the result of hypoxemia, an abnormally low amount of oxygen-carrying hemoglobin within red blood cells. Normally, arterial blood is flush with these oxygenated cells, making it appear bright red, but when oxygen levels decline, blood appears bluish red. Cyanosis is an indicator of this deficiency, often first noticed on the lips, gums, around the eyes, and under the nails.
Very cold temperatures can temporarily slow the flow of blood through the skin, also leading to the bluish color, but this typically goes away when you warm up. Blue fingernails or purple nail beds can be a sign of various disorders and should be investigated. In the case of Raynaud’s disease, the fingers and toes blanch on exposure to cold, then turn blue and may become numb or painful. This problem represents oversensitivity of nerves controlling blood flow through small arteries in peripheral areas of the body (fingers, toes, nose, and ear lobes); it may be associated with autoimmunity.
Chronic cyanosis and blue fingernails can also be signs of many different lung and breathing problems, including a pulmonary embolism, asthma, emphysema, chronic obstructive pulmonary disease (COPD), or chronic bronchitis. They also can result from heart problems, exposure to high altitudes, and reactions to certain drugs, such as minocycline (an antibiotic), narcotics, benzodiazepines, and some sedatives. Be aware that taking colloidal silver can also cause the appearance of a slate-blue color just above the lunula (half-moon in the nail bed) and on the face, an irreversible skin condition called argyria.
If blue nails aren’t merely a temporary reaction to cold temperatures, I suggest having a medical checkup to identify the underlying cause. Your physician will probably want to do a simple rapid-blood gas analysis to measure the oxygen in your blood and perhaps some tests to check your heart and lungs.
Andrew Weil, M.D.
Adebayo Adeyinka and Noah P. Kondamudi, “Cyanosis,” StatPearls, March 24, 2019, ncbi.nlm.nih.gov/books/NBK482247/#__NBK482247_ai__