It’s normal for nails to have some imperfections, but the variations below may indicate health problems. Beau’s lines. These horizontal indentations across the nail beds can be caused by an injury, but they may also form in the presence of an illness, such as peripheral vascular disease, uncontrolled diabetes, high fever, or zinc deficiency. Blue nails. Very cold temperatures can temporarily slow the flow of blood through the skin leading to a bluish nail color; this typically goes away when you warm up. In Raynaud’s disease, the fingers and toes blanch, then turn blue and may become numb or painful on exposure to cold. Blue fingernails can also be due to low levels of oxygen in the blood, which is a symptom of many different respiratory problems, including asthma, emphysema, chronic obstructive pulmonary disease, and chronic bronchitis.