Final answer:
In obstructive lung disease, the FEV1% decreases due to increased resistance in the lung airways, which hinders efficient air exhalation.
Step-by-step explanation:
When obstructive lung disease develops, the FEV1% (Forced Expiratory Volume in 1 second / Forced Vital Capacity) actually decreases. This is due to increased resistance in the lung airways which makes it harder for an individual to exhale air out of the lungs. As a result, a person with obstructive lung disease has a slower and less efficient exhalation process, leading to a lower FEV1%.
In contrast, restrictive lung disease would show a reduced FVC, but since the airways are not obstructed the FEV1% might not decrease as dramatically. Thus, it's in obstructive lung diseases such as chronic obstructive pulmonary disease (COPD) or asthma where the FEV1% shows a notable decline, which aids in their diagnosis. This decreased ratio can be detected during pulmonary function tests.