Pressure relationships are the single most consequential infection-control parameter an HVAC system delivers: they decide which way air moves between a room and everything around it, and therefore whether contaminants are kept out of a clean space or kept inside a dirty one. This Article covers the physics of room pressurization, the directional logic of positive and negative spaces, the design and control means that establish and hold those relationships, and the verification that proves they exist. The full space-by-space pressure table belongs to the ASHRAE 170 space-requirements Article; air-change rates and filtration belong to their own Article; this article concentrates on direction, differential, and containment.

Why Pressure Relationships Govern Air Movement

Air flows from higher pressure to lower pressure. In a healthcare building, a designer exploits that simple fact to steer airborne contaminants. By engineering a room to sit at a slightly higher or lower static pressure than the adjacent corridor or anteroom, the design forces air to move consistently across the door gap in one chosen direction — regardless of who walks through, what they are carrying, or what is happening inside the room.

The pressure differential involved is tiny. Healthcare room pressurization is measured in fractions of an inch of water column (in. w.c. or "water gauge"), or in pascals (Pa); a single inch of water column equals roughly 249 Pa, so these are very low pressures relative to the building's structural or mechanical pressures. What matters is not the magnitude but the consistency and direction of the differential, because even a small, steady pressure bias produces a steady directional airflow through every leakage path in the room envelope — door undercuts, gaps, outlets, and penetrations.

Two design quantities define a pressure relationship:

• Differential pressure — the static-pressure difference between the room and its reference space, held above a minimum threshold so the relationship is robust against door movement, stack effect, and wind.
• Directional airflow — the net volumetric airflow (cubic feet per minute, cfm) moving across the room boundary in the intended direction, produced by an intentional imbalance between supply air and the combined exhaust/return air.

A room holds a pressure relationship only when both are present: a measurable differential and an offset airflow large enough to maintain that differential as conditions change.

Positive, Negative & Neutral Pressure: The Directional Logic

Healthcare spaces fall into three pressurization categories, each chosen for what the room is trying to protect:

• Positive pressure (protect the patient/clean contents from the surroundings). The room is held at higher pressure than adjacent spaces, so air flows out of the room across the boundary and contaminated air cannot infiltrate inward. Positive rooms protect immunocompromised patients and sterile or clean processes. Classic examples: operating rooms, protective-environment (PE) rooms for severely immunosuppressed patients, sterile processing clean sides, and compounding cleanrooms for sterile (USP 797) preparations. The protective-environment and OR room mechanics are detailed in the specialized-spaces Article; the principle is the same — clean air sweeps outward.
• Negative pressure (protect the surroundings from the room). The room is held at lower pressure than adjacent spaces, so air flows in across the boundary and airborne contaminants generated inside cannot escape into corridors or neighboring rooms. Negative rooms contain infectious or hazardous airborne agents. Classic examples: airborne infection isolation (AII) rooms, bronchoscopy and sputum-induction rooms, autopsy suites, soiled holding and decontamination rooms, triage and waiting areas designated for potentially infectious patients, and hazardous-drug (USP 800) compounding rooms.
• Neutral / no specific requirement. Many spaces have no mandated directional relationship; their HVAC is balanced for comfort and air quality without a designed pressure bias. These rooms still behave predictably only if the building as a whole is kept slightly positive to the outdoors to limit unconditioned infiltration.

A crucial point that trips up owners and operators: the pressurization category is a property of the room's function, not of the patient in it on a given day. An AII room is engineered negative and must stay negative; it cannot be "flipped" positive to serve as a PE room for an immunocompromised patient. Rooms that must serve both protective and isolation roles require a dedicated anteroom and a documented, carefully controlled switchable design — a configuration that ASHRAE 170 and FGI treat cautiously and that most facilities avoid in favor of separate, single-purpose rooms.

The Cascade: Engineering a Pressure Gradient Across a Suite

Pressure relationships are rarely about one isolated room. The power of the technique comes from chaining rooms into a pressure cascade — a deliberate, monotonic gradient that steps cleanliness up or down across a sequence of spaces, so air always migrates from the cleanest zone toward the dirtiest and never the reverse.

A surgical suite is the textbook positive cascade. The operating room sits at the top of the gradient (most positive, cleanest); the sterile core and clean corridors step down from it; the unrestricted perimeter corridors are lower still. Air sweeps continuously from OR → clean support → corridor, so particulates and microbes are flushed away from the surgical field toward the periphery and out.

A negative cascade runs the other way. For airborne isolation, the corridor is positive relative to an anteroom, which is positive relative to the patient room; air flows corridor → anteroom → room → exhaust. The anteroom is the engineered "air-lock" that buffers the door-opening transient and preserves the gradient even as staff enter and exit. Hazardous-drug pharmacy suites use the same buffered, stepped-down logic toward the negative-pressure compounding room.

Designing a cascade well requires:

• A single, consistent reference for each step — each room's differential is defined against a named adjacent space, and the chain of references must not contradict itself.