A 48-year-old male, living in an industrial urban area, presents to his primary care physician with a persistent cough, shortness of breath, and wheezing over the past year. He has no history of smoking, occupational exposure to irritants, or significant family history of respiratory illnesses. Upon further questioning, it is revealed that he has been exposed to high levels of air pollution due to residing near a factory emitting fine particulate matter (PM2.5). The physician hypothesizes that chronic exposure to air pollution may be contributing to his respiratory symptoms. To establish a causal relationship between air pollution and respiratory disease, the Bradford Hill criteria are applied. Which of the following Bradford Hill criteria is considered essential for establishing causality?
