When to not use positive pressure ventilation?

You should not use positive pressure ventilation in situations where you need to contain contaminants and prevent them from spreading to clean areas, or when it could cause harm by pushing harmful air outward. In hospitals, you should not use positive pressure for airborne infection isolation rooms for tuberculosis, measles, COVID during aerosol procedures, because positive pressure would push infectious air out into hallways and infect others, negative pressure is required instead. In mold remediation, asbestos removal, and lead abatement, you should not use positive pressure inside containment, because it would blow mold spores, asbestos fibers, and lead dust out of containment into the rest of the building, spreading contamination, negative air with HEPA filtration is required by OSHA and IICRC. In homes with attached garages, you should not pressurize the house if it could push garage fumes and carbon monoxide into living spaces or vice versa depending on layout, balanced ventilation is better. In cold climates, excessive positive pressure can push warm moist indoor air into wall and attic cavities where it condenses and causes mold and rot, so balanced or slightly negative pressure may be better in winter. You should not use positive pressure ventilation if you have a fireplace or fuel-burning appliance without dedicated combustion air, as it can interfere with drafting. In clean rooms for electronics manufacturing, positive pressure is good, but for biohazard labs, negative pressure is needed. Always match pressure strategy to containment goal.