Everything above that level is positively pressurized and air will leak out of any open windows or holes, while below the neutral pressure level the house is negatively pressurized and air will leak into the house through any openings. In old leaky houses the negative pressure rarely exceeds about eight pascals, which is about the difference in pressure in ten feet of altitude - not much. Furnaces and fireplaces usually do all right pulling against a negative pressure of up to eight pascals. Chimney height and location matter. When possible, locate chimneys inside the exterior walls of the house and build them taller than the highest part of the house.
Modern houses tend to be tightly built, wrapped, sealed and caulked.
They also tend to be full of powerful kitchen and bathroom exhaust fans. So the negative pressure in the lower areas of a new house can sometimes far exceed eight pascals. For a fireplace to draw well, especially as the fire dies down and little heat is being produced to overcome negative pressure, you may have to let in some makeup air near the fireplace or somewhere low in the house. Usually the six square inch combustion air kits designed to be built into the firebox are not big enough. And they can lead to other problems like creosote or ash smells and enough turbulence to cause the fireplace to smoke.