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The factor that can increase susceptibility to hypoxia is carbon monoxide inhalation. Carbon monoxide is a colorless, odorless gas that can bind to hemoglobin in the blood more readily than oxygen, effectively reducing the amount of oxygen that can be transported throughout the body. This interference with oxygen delivery means that a person may experience symptoms of hypoxia even if they are in an environment with adequate levels of oxygen.

In contrast, the other factors listed have different implications for hypoxia susceptibility. Increased physical fitness typically enhances overall cardiovascular health and oxygen efficiency, which can help mitigate hypoxic effects rather than exacerbate them. High hydration levels generally promote better blood circulation and can help maintain oxygen levels in the body. Moderate caffeine intake may have complex effects on the body, but it is not directly linked to an increased risk of hypoxia like carbon monoxide inhalation is. Thus, inhaling carbon monoxide presents a direct threat by diminishing oxygen transport, leading to an increased risk of hypoxia.