The air pressure in your lungs has to be less than the air outside your lungs, to get your lungs to inflate. This is because air moves from high pressure areas to low pressure areas. During bad weather and at high altitudes the air pressure is lower, making it harder for us to breathe.
When you inhale, muscles increase the size of your thoracic (chest) cavity and expand your lungs. This increases their volume, so pressure inside the lungs decreases. As a result, outside air rushes into the lungs. That's because a gas always flows from an area of higher to lower pressure.
