Causes of nail cap defects in the thumb: 1. Heavy physical labor. Among the people who develop nail cap defects, there are more heavy workers than light workers. The size of the labor intensity has a certain impact on the nail cap defects. 2, nutritional status. The nail defects may be caused by iron deficiency anemia and conditions that cause ischemia of the fingertips. 3, prolonged exposure to cold water. Frequent use of cold water to wash hands, or regular exposure to cold water, is also a cause of nail defects. 4, hypoxemia. Hypoxia may cause a decrease in the partial pressure of oxygen in the body’s arteries, resulting in a lack of oxygen in the blood circulation of the end tissues, causing an impairment in the nutritional metabolism of the nail, resulting in a defective nail.