Do humans inhale by negative pressure?

Yes, humans do inhale by negative pressure, which is the fundamental mechanism of normal breathing. Inhalation is an active process driven by creating a lower pressure inside the chest compared to outside air. When you inhale, your diaphragm contracts and moves downward and your intercostal muscles between ribs contract and lift the rib cage outward, which expands the volume of the thoracic cavity and lungs. According to Boyle's law, when volume increases, pressure decreases, so pressure inside the alveoli drops below atmospheric pressure, typically by 1 to 3 millimeters of mercury, creating a negative pressure relative to outside. This pressure difference causes air to flow from higher pressure outside into lower pressure inside the lungs, just like air being sucked into a syringe when you pull the plunger. Exhalation is usually passive, where muscles relax, thoracic volume decreases, pressure inside rises above atmospheric, and air flows out. This negative pressure breathing is different from positive pressure breathing used in ventilators that push air in. Conditions that interfere with negative pressure breathing, such as pneumothorax where air enters the chest cavity and prevents pressure drop, or paralysis of diaphragm, make it difficult to inhale. So yes, every breath you take is powered by generating slight negative pressure in your chest to draw air in, a principle that also underlies how negative pressure rooms work to draw air inward.