The crew and passengers of a flying aircraft suffer generally from chronic obstructive pulmonary disease due to the effect of—

The crew and passengers of flying aircraft are frequently exposed to elevated levels of ozone (O3), which is found in high concentrations at typical cruising altitudes in the stratosphere. Ozone enters the cabin through the aircraft's ventilation system (bleed air) if not properly neutralized by catalytic converters. Exposure to ozone is well-documented to cause respiratory distress, including upper and lower airway inflammation and significant decrements in pulmonary function [3]. Chronic exposure to such irritants among aircrew is linked to higher rates of respiratory diseases, including chronic obstructive pulmonary disease (COPD) and chronic bronchitis. While other pollutants like nitrogen oxides and particulates exist in cabin air due to engine emissions or 'fume events', ozone is the primary high-altitude ambient pollutant specifically associated with the unique atmospheric environment of flight that leads to chronic respiratory impairment [2].

Sources

1. [3] https://www.ncbi.nlm.nih.gov/books/NBK219004/
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 29: Environment Issues and Health Effects > Irritants and Toxic Chemicals > p. 440