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1. Respiratory Health Issues. Downward curved fingernails can indicate a problem with respiratory health, and are often present in people who have medical conditions such as COPD, chronic bronchitis, asthma and lung cancer. Fingernails often curve downwards as a result of lack of oxygen.
Causes of Fingernail Ridges, Pitting and Curved Nails. 1  Longitudinal ridges (running from the cuticle to the free edge of the nail) may be due to aging and not a sign of any disease.  Nail beading, where the nail loses its smooth curvature, is also a normal part of aging.