Which condition is characterized by oxygen saturation of less than 90% in room air?

Severe pneumonia is characterized by oxygen saturation of less than 90% in room air due to the significant impact of the infection on lung function. In severe pneumonia, the lungs are filled with fluid and inflammatory cells, which severely impairs gas exchange. This leads to a high risk of hypoxemia, where the blood oxygen levels drop below the normal range.

In contrast, conditions like mild asthma, the common cold, and acute bronchitis typically do not present with such low oxygen saturation levels. Mild asthma might lead to episodic wheezing and shortness of breath but usually maintains oxygen saturation above 90% in uncomplicated cases. The common cold primarily affects the upper respiratory tract and rarely leads to significant hypoxemia. Acute bronchitis, often a viral infection leading to inflammation of the bronchial tubes, may cause coughing and discomfort but does not typically result in severe drops in oxygen saturation unless complicated by other factors. Thus, when considering oxygen saturation levels, severe pneumonia is distinctively associated with significant respiratory compromise.