The exchange of indoor air with outdoor air reduces the risk of infection by diluting the concentration of airborne pathogens. Theoretically, the risk of transmission should decrease with increasing fresh-air ventilation.
To achieve a balanced ventilation system, the amount of supply air (air mechanically pushed into a room) and the amount of exhaust air (air mechanically pulled from a room) must be set to ensure room conditions are stable. Factors such as infiltration (e.g., space around doors, windows and curtains), doors and conditions of an adjacent room need to be considered when balancing a ventilation system.
Ventilation recommendations for airborne infection isolation rooms and other select areas are of critical importance because of their positive impact on reducing the risk for healthcare-associated transmission of M. tuberculosis. The supply and exhaust air system need to be properly designed to achieve effective air changes within a space. The location of the supply and exhaust air diffusers, the speed of the air, furniture in the room and other items that affect air flow patterns will affect the effectiveness of the air changes per hour (ACH). Increasing the number of ACH from 1 to 6 will result in more rapid clearing of infectious airborne microorganisms from the room air. However, further increases above 6 ACH will have progressively less effect, and increases above 12 ACH may provide minimal additional benefit.47,48 In general, as ACH rates are increased, there are increased costs for building and maintaining the ventilation system. Pressurization, which prevents particulates from leaving the room through infiltration, is equally, if not more, important than ACH. Pressurization is also critical for when doors are opened and closed, to ensure that air keeps flowing into the room and not out through the door. Opening the window may cause reversal of the direction of air flow, depending upon the prevailing wind direction and outdoor temperature.
A number of organizations and agencies, such as American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), Canadian Standards Association (CSA) and Centers for Disease Control and Prevention (CDC) have published recommendations regarding ventilation levels to reduce the risk of healthcare-associated transmission of airborne pathogens, such as M. tuberculosis.49–51 Differences among their recommendations (Table 1) are not based on differing evidence but, rather, on the risk-benefit assessment of each organization. In deciding which recommendations to implement, hospital administrators may take into account factors such as facility design, latest scientific evidence, facility risk assessment, financial resources, environmental conditions and, at a minimum, provincial and territorial building regulations.
Table 1. Ventilation recommendations for selected areas in healthcare facilities.
|Area/Function||Minimum total ACH a||Relative pressurization b (CSA 2019)||Exhaust b , c (CSA 2019)|
|CDC 2019 49||CSA 2019 50||ASHRAE 2021 51|
|Ambulatory care clinic||NA||9||4||Negative e||Dedicated d|
|Autopsy (general zone)||12||20||12||Negative e||Dedicated d|
|ED waiting rooms||12||12||12||Negative||Dedicated d|
|ED trauma room, life support||15||15||15||Positive||NR|
|General diagnostic imaging suites (includes CT, MRI, X-ray, ultrasound)||6 g||9||6||Equal||NR|
|Microbiology laboratory h||6||NA||6||NA||NA|
|Anteroom for operating rooms used for patients with airborne infection||NA||9 i||NA||Negative to operating rooms and contiguous space j||Air removed from anteroom and from the operating rooms shall be exhausted to the outdoors|
|Patient room in an acute care facility||6||4-6||4 k||Equal||NR|
|Resident room in a LTCH||2||4||2||Equal||NR|
|Resident gathering areas in a LTCH||4||NA||4||NA||NR|
|Sputum induction/ pentamidine aerosol administration||NA||NA||12||NA||Local l|
This table provides key information related to ventilation and does not replace information found in the relevant standards and guidelines. For further details, consult the complete guidelines and/or standards. Adapted from tables in the CDC, 49 CSA 50 and ASHRAE 51 documents.
Abbreviations: ACH, air changes per hour; CDC, Centers for Disease Control and Prevention; CSA, Canadian Standards Association; ASHRAE, American Society of Heating, Refrigerating and Air-Conditioning Engineers; AIIR, airborne infection isolation room; NA, not addressed; ED, emergency department; NR, no requirement; CT, computed tomography; MRI, magnetic resonance imaging; LTCH, long-term care home; HVAC, heating, ventilation, and air conditioning; HEPA, high-efficiency particulate air.
a As per CSA, “minimum air change rate can be defined by either the supply or the exhaust depending on the situation. If defined by the exhaust flow rate, supply shall be chosen/configured to ensure consistent air quality and to meet heating and cooling needs. In all cases, the minimum outdoor air change rate shall be maintained.” 50
b Recommendations from CSA (2019) 50 only. See full versions of ASHRAE (2021) 51 standard or CDC (2019) 49 guideline for further detail on relative pressurization (or pressure relationship) and exhaust (or direction of air movement).
C Clinics/facilities that have rooms where airborne disease is of concern and/or aerosol-producing medical procedures are performed should have ventilation for negative directional airflow for airborne disease prevention (e.g., dedicated exhaust or HEPA-filtered exhaust). 50
d An exhaust system that extracts air from spaces or through equipment of similar function that cannot be recirculated or transferred. 50
e For patient care areas or an area that is intended for the provision of services that directly supports patient care areas. 50
f High-level air separation requirements shall apply. 50
g X-ray (diagnostic and treatment). 49
h The Canadian Biosafety Standard (Second Edition 2015) describes the HVAC requirements associated with specific laboratory activities (ie, diagnostic or research). 52
i Value extrapolated from “Anterooms for special precautions rooms”. 50
j As per CSA, anterooms shall be provided for operating rooms used for patients with airborne infectious disease (e.g., TB). The anteroom pressure shall be maintained negative to both the operating room and the contiguous space; air shall flow from the operating room into the anteroom and from the corridor into the anteroom. 50
k For single-bed patient rooms using Group D diffusers, a minimum of 6 total ACH shall be provided and calculated based on the volume from finished floor to 6 feet (1.83 metres) above the floor. 51
l Rooms or booths used for these procedures shall be provided with local exhaust ventilation designed to capture contaminants as close to the source of emission as possible. 50
3.1.1. General hospital areas
Adequate ventilation in general areas such as inpatient, examination and treatment rooms is important because people with unsuspected respiratory TB may be placed in them, posing a risk of transmission to other patients and HCWs.44,53 Ventilation in these areas may be poor and further disrupted by opening and closing doors, installation of ceiling fans post-design and use of items such as ground circulation fans and electric heaters.
3.1.2. Airborne infection isolation rooms
Measures to ensure that adequate ventilation is in place for airborne infection isolation rooms are outlined in the following sections and discussed in more detail in other guidelines.45,54
Air should be exhausted to the outdoors through a dedicated exhaust system, ideally exiting from the roof of the building. It is important that the exhausted air does not re-enter the building or an adjacent occupied building.
Good practice statements
In the absence of provincial/territorial building standards for healthcare facilities, healthcare organizations should follow either the Canadian Standards Association, American Society of Heating, Refrigerating and Air-Conditioning Engineers, or the Centers for Disease Control and Prevention ventilation recommendations for healthcare facilities.
With the exception of rooms in which operative procedures are performed, the direction of air flow should be inward from the hall into the room (negative pressure), and then the air should be exhausted outdoors, or high-efficiency particulate air filtered.
If an anteroom is used, the air from both the anteroom and patient room should be exhausted outdoors; if an airborne infection isolation room does not have an anteroom, there shall be an adequate pressure differential between it and the corridor and adjacent rooms.
For positive pressure rooms in which sterile operative procedures are performed, there should be an anteroom for which the direction of air flow should be inward from the corridor and also from the operating room into the anteroom. In this way, clean air enters the operating room, but air does not go from the operating room into the corridor or beyond. The air should be exhausted outdoors from the anteroom.
Windows and doors should be kept closed, both during and after aerosol-generating medical procedures (long enough for air clearance in the room), and in the state that the heating, ventilation, and air conditioning system was designed to function.
The rate of air changes and pressurization should be verified at least every 6 months when the room is not being used as an airborne infection isolation room; rooms that are balanced with a Building Automation System should be under constant verification and have integrated alarms that notify maintenance staff of operational issues. Older buildings may require alternative air flow indicator devices.
When the airborne infection isolation room is in use, the direction of air flow should be verified daily and recorded, and the facility’s infection prevention and control team should review these ventilation system monitoring results.
Airborne infection isolation rooms, as well as other areas routinely used for care of patients with respiratory tuberculosis, should have at least 12 air changes per hour.
The number of airborne infection isolation rooms in hospitals should be based on the number of patients admitted each year with suspected respiratory tuberculosis; in organizations with very few admissions for tuberculosis, the number of such rooms should be decided by the organizational authorities according to an analysis of airborne infection isolation room utilization in the previous 2-to-3 years. One or two more rooms than needed in the past at peak times could be considered as the optimal number, recognizing that they may be used for airborne infections other than respiratory tuberculosis. Appropriate resources should be made available to hospitals that will have such rooms and therefore receive patients with respiratory tuberculosis from other healthcare facilities.
3.1.3. Sputum induction and administration of aerosolized pentamidine
The smaller the room where these procedures are performed, the easier and more practical it is to achieve required ventilation levels. Sputum collection booths, which are a type of enclosed local exhaust ventilation device, are commercially available for these purposes.
Good practice statements
Acute care hospitals should have an airborne infection isolation room or sputum collection booth for sputum induction; a sputum collection booth is preferred.
A sputum collection booth should meet the ventilation requirements of an airborne infection isolation room, be properly functioning, used and maintained according to manufacturer’s instructions, and adequate time allowed for air clearance between users.
If a room is used for sputum induction, it should meet the ventilation requirements of an airborne infection isolation room.
3.1.4. Bronchoscopy and autopsy
Areas where these procedures are performed tend to be much larger than inpatient rooms, making it difficult to achieve consistently high levels of ventilation with an inward direction of air flow.
Good practice statement
All bronchoscopies should occur in an airborne infection isolation room unless tuberculosis has been excluded as a diagnosis; if tuberculosis is confirmed, the necessity of bronchoscopy for optimal patient management should be carefully considered, in view of the considerable risk of transmission.
3.1.5. Entering a room after generation of infectious aerosols has ended or patient with respiratory TB has been discharged
Table 2 provides guidance on when it is safe to enter a room previously occupied by a patient with respiratory TB without needing to wear a respirator or when a procedure room can be used for another patient after generation of infectious aerosols has ceased. The time required to remove airborne particles from an enclosed space depends on the number of ACH, which is a function of the volume (cubic feet of air) in the room or booth; the rate at which air is exiting the room or booth; the location of the ventilation inlet and outlet; and the configuration of the room or booth.49 The minutes required to allow at least 99% removal of airborne microorganisms is considered sufficient to allow room entry.
Table 2. Time needed (by number of air changes per hour) to remove airborne microorganisms after generation of infectious droplet nuclei has ceased. a
|Air changes per hour||Minutes required for removal of airborne microorganisms|
|99% removal||99.9% removal|
a This table was adapted from the CDC recommendations. 25
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