A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg. Normally, the A–a gradient increases with age. For every decade a person has lived, their A–a gradient is expected to increase by 1 mmHg. A conservative estimate of normal A–a gradient is "in years + 10/ 4". Thus, a 40-year-old should have an A–a gradient around 12.5 mmHg. The value calculated for a patient's A-a gradient can assess if their hypoxia is due to the dysfunction of the alveolar-capillary unit, for which it will elevate, or due to another reason, in which the A-a gradient will be at or lower than the calculated value using the above equation.