The emergency department is one of the most demanding HVAC environments in a hospital: it mixes high-occupancy public waiting with acute clinical care, undifferentiated and potentially infectious patients, and decontamination needs — all of which must be reconciled with the ventilation, pressurization, and filtration requirements of ANSI/ASHRAE/ASHE Standard 170 and the FGI Guidelines.

Governing standards and design basis

ED ventilation is governed primarily by ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, which the FGI Guidelines for Design and Construction of Hospitals** adopt by reference and which most state health departments enforce. Standard 170 sets, per space type, the minimum requirements for:

• Pressure relationship to adjacent spaces (positive, negative, or no requirement).
• Outdoor air changes per hour (ACH) and total ACH.
• Whether recirculation by room units is permitted.
• Required filtration efficiency (MERV) levels.
• Minimum temperature and relative-humidity ranges.

These are minimums; the design of record must meet or exceed them, and the authority having jurisdiction (AHJ) may impose stricter local rules. Commissioning and Test-and-Balance (TAB) verify that as-built airflow and pressure relationships match the design intent before occupancy.

Waiting, triage, and the general ED air strategy

The post-2009 SARS/influenza era reshaped how designers treat the ED front-of-house. Standard 170 classifies the ED waiting room as a space requiring a negative pressure relationship to adjacent areas, minimum 12 total ACH (2 outdoor), with MERV-rated filtration — recognizing that waiting rooms collect undiagnosed, potentially contagious patients.

Practical strategies for waiting and triage include:

• Negative-pressure waiting areas so air migrates into the waiting room and is exhausted, not pushed toward staff or clean corridors.
• Generous outdoor-air rates and high ACH to dilute airborne pathogens and odors during peak census.
• Upper-room or in-duct UVGI and high-MERV filtration as supplemental control layers.
• Sizing diffuser/return placement for directional airflow that sweeps from staff toward patient toward exhaust.

General treatment/exam and trauma/resuscitation rooms carry their own Standard 170 lines — exam rooms typically no pressure requirement, while trauma rooms function as a category of operating/procedure space with elevated ACH and tighter humidity control.

Airborne Infection Isolation (AII) rooms

Every ED should include one or more Airborne Infection Isolation (AII) rooms for patients with suspected airborne disease (TB, measles, novel respiratory pathogens). Standard 170 requirements for AII rooms: