Is negative air pressure bad for you?

Negative air pressure itself in small amounts is not directly bad for your body, as humans regularly experience pressure changes, but excessive unintentional negative pressure in a house can be bad for your health indirectly through several mechanisms. In hospitals, intentional negative pressure in isolation rooms is good because it protects others from infection. In homes, however, strong negative pressure can cause backdrafting of fuel-burning appliances like furnaces, water heaters, and fireplaces, pulling carbon monoxide, a colorless odorless deadly gas, back into living spaces instead of venting outside, which can cause headaches, dizziness, nausea, and death. Negative pressure also sucks in unfiltered outside air through cracks, bringing in dust, pollen, mold spores, radon, and humid air that can trigger allergies, asthma, and mold growth inside walls, leading to respiratory symptoms. It can cause cold drafts and dryness that irritate sinuses. Very high negative pressure can make doors hard to open and create uncomfortable whistling noises. On the other hand, mild negative pressure of a few pascals is normal and not harmful. The key is whether it is controlled and filtered, intentional negative pressure with HEPA filtration for mold containment is protective, while uncontrolled negative pressure that pulls in garage fumes or combustion gases is hazardous. If you suspect strong negative pressure, install carbon monoxide detectors, ensure combustion appliances have dedicated outside air, provide makeup air for large exhaust fans, and have an HVAC professional test pressure and venting to prevent health risks.