How much air a healthcare space receives, how much of it is fresh outdoor air, and how thoroughly that air is filtered are the three levers that—together with pressure relationships—define whether a room is safe for its intended clinical use. This article covers the air-quantity and air-cleaning requirements that ASHRAE Standard 170 and the FGI Guidelines impose space-by-space, why those numbers exist, and what they mean for design, construction, commissioning, and ongoing operation.

Air-change rate, outdoor-air rate, and filtration are the dilution-and-removal half of healthcare ventilation; the directional-airflow and pressurization half is covered in the sibling Article on pressure relationships and airflow direction, and the full space-by-space requirement matrix lives in the ASHRAE 170 space-requirements Article. Temperature and relative-humidity control are addressed separately. This Article stays on quantity of air, source of air, and cleanliness of air.

The governing framework: ASHRAE 170 adopted through the FGI Guidelines

In the United States, the binding source of healthcare ventilation parameters is ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities. ASHRAE 170 is written as a standalone consensus standard, but it becomes enforceable because it is incorporated by reference into the FGI Guidelines (the Guidelines for Design and Construction of Hospitals and the Guidelines for Design and Construction of Outpatient Facilities). Most U.S. states adopt the FGI Guidelines—and with them ASHRAE 170—as the design code for licensed healthcare construction, either directly or through the state department of health. The Authority Having Jurisdiction (AHJ) for the project (state agency, local building official, and accreditation surveyor) determines the exact edition in force, which is why the applicable edition must be confirmed at project start rather than assumed.

The heart of the standard is the Design Parameters table (the space-by-space ventilation table). For every recognized healthcare space type, it prescribes:

• Pressure relationship to adjacent areas (positive, negative, or no requirement).
• Minimum outdoor air changes per hour (ACH) — the fresh-air component.
• Minimum total air changes per hour (ACH) — outdoor plus recirculated supply.
• Whether air may be recirculated by room units (e.g., fan-coil units, induction units).
• Whether all room air must be exhausted directly to the outdoors.
• Design relative-humidity and temperature ranges.
• Filtration requirements, expressed by referencing a filter-efficiency table.

This Article concerns the outdoor-ACH, total-ACH, recirculation/exhaust, and filtration columns. The companion Articles handle pressure and humidity/temperature.

What "air changes per hour" actually means

An air change per hour (ACH) is the number of times the full volume of air in a room is replaced in one hour. Numerically:

ACH = (supply airflow in CFM × 60) ÷ (room volume in cubic feet)

A room of 1,000 ft³ supplied with 100 CFM receives 6 ACH (100 × 60 ÷ 1,000). The same CFM in a higher-ceiling room yields fewer ACH, which is why ceiling height and room volume matter to compliance—a design that meets ACH on a standard 9-foot ceiling can fall short under a 10- or 12-foot clinical ceiling at the same supply CFM.

Two distinct ACH numbers are specified for most spaces:

• Total ACH is the total conditioned supply delivered to the room, including recirculated air. It governs thermal load capacity and overall dilution—the room must move enough air to remove heat from people, lights, and equipment and to dilute airborne contaminants generally.
• Outdoor ACH is the fresh-air subset. It governs dilution of CO₂, odors, and human bio-effluents, and it provides the makeup air needed to maintain pressure relationships when air is exhausted. Outdoor ACH is always less than or equal to total ACH.