The main difference between a tension pneumothorax and an open pneumothorax lies in the relative pressures of the pleural cavity compared to the atmospheric pressure. An open pneumothorax occurs when the pleural cavity is in an open state due to some reason. During inhalation, gas can enter the pleural cavity, and during exhalation, the gas can be expelled through this opening or another opening, resulting in pleural cavity pressure that is roughly equal to the atmospheric pressure. In a tension pneumothorax, due to some reason, a one-way valve is formed. The gas enters the pleural cavity easily but encounters resistance during expulsion, causing the pressure in the patient's pleural cavity to increase progressively. The gas can only enter the pleural cavity and cannot be effectively expelled. Therefore, the main difference between a tension pneumothorax and an open pneumothorax is the level of pleural cavity pressure compared to atmospheric pressure; in a tension pneumothorax, the pressure in the pleural cavity is significantly higher than the atmospheric pressure, while in an open pneumothorax, the pressures are roughly equal.