Draft guidance on improving indoor air quality in office buildings: Ventilation

On this page

• For operators
• Defining ventilation
• HVAC systems
• Good operating practices
• Ventilation maintenance practices
• Standards, building codes and ventilation requirements
• Carbon dioxide as an indicator of ventilation
• HVAC safeguarding

For operators

Building operators and those responsible for IAQ should be knowledgeable about the ventilation system design and operation, which includes the outdoor air supply, outdoor air quality, filters and filtration efficiency, space planning, equipment maintenance, controlling other contaminant pathways such as unintended infiltration of pollutants, and when to engage an HVAC professional.

Defining ventilation

Effective ventilation helps improve IAQ as it reduces contaminant and moisture levels that may directly or indirectly result in poor occupant comfort, symptoms, or negative health effects (Health Canada 2018a). For a ventilation system to be effective, it must result in bringing in fresh outdoor air and exhausting indoor air. It is not sufficient to simply result in air movement (such as recirculation) or filtration.

Ventilation may occur naturally or mechanically.

Natural ventilation describes the air flow caused by pressure differences between the inside and the outside of a building, which may be through intentional or unintentional openings in the building envelope. Occupants generally have little control over natural ventilation, aside from opening and closing windows. Ventilation that relies only on opening the window can lead to excessive energy costs, particularly due to heat loss in the winter or loss of conditioned air in the summer. Window opening can also create challenges in managing relative humidity and will allow the entry of pests in the absence of window screens.

Mechanical ventilation refers to air flow intentionally created through the use of fans and ducting, which rely on designed openings in the building envelope. In the case of a smaller building or office, the mechanical ventilation may rely on a heat pump, a window unit, or a centralized forced-air system (such as a furnace). In most other office buildings, an HVAC system will be responsible for mechanically ventilating the building.

HVAC systems

Heating, ventilation and air-conditioning (HVAC) systems have a significant impact on how air contaminants move through a space, how fresh air is brought in and stale air is exhausted, and how contaminants are removed from the air. An HVAC system is designed to:

• provide thermal comfort (temperature and humidity control)
• distribute conditioned and filtered outdoor air to occupants
• remove, reduce or dilute odours and contaminant levels using system components such as supply and exhaust fans and filtration systems
• control the air pressure relationship between rooms.

The HVAC system typically has many interconnected parts throughout a building, such as intakes, filters, ducts and fans, that work together to move air into, around and out of rooms. A well-designed and properly functioning HVAC system will deliver the appropriate amount of air to each zone (such as a room or space) to achieve fresh air ventilation requirements and thermal comfort and maintain odour and contaminant control.

Understanding HVAC systems

In general, an HVAC system:

• brings outdoor air into the building
• cleans and filters the air
• heats or cools the air
• humidifies or dehumidifies the air
• distributes the air throughout the building
• removes stale air from the occupied space
• exhausts a portion of the indoor air to outside the building and recycles the remaining air.

These systems have a significant impact on how air contaminants move through a space or are removed from the indoor air. When functioning properly, the HVAC system will balance the different zones and maintain the desired pressures throughout the office. Blocking air vents or registers or interrupting air flow with furniture or boxes may unbalance the HVAC system, which may affect the ventilation in other areas of the office.

Filters are used to remove dust, pollen, mould, bacteria, viruses, and particulate matter (PM) from the air. They are rated using a minimum efficiency reporting value (MERV), which indicates a filter's ability to capture larger particles between 0.3 and 10 microns (µm) (EPA 2021b). The HVAC systems in many buildings will have MERV 8 filters installed as default, which are approximately 20% efficient in removing particles in the 0.3 to 1 µm size range. The current public health recommendation is to use a filter with a MERV rating of 13 or higher for recirculated air, when possible, in order to reduce viral transmission indoors (such as SARS-CoV-02 virus) along with improving IAQ overall (PHAC 2021b; CCIAQ 2021). A MERV 13 filter is at least 85% efficient at capturing particles in the 1 to 3 µm range, while a MERV 14 is at least 90% efficient. System manufacturers will recommend the appropriate MERV rating for the system in the building. Similarly, a HEPA filter is a type of pleated mechanical air filter used in air purifiers and is designed to remove at least 99.97% of dust, pollen, mould, bacteria and any airborne particle with a size of up to 0.3 microns (µm). Note that filters can help remove particulates but will not be effective against gases such as carbon monoxide or carbon dioxide.

Filters must also be the appropriate size for the HVAC system, as undersized filters will allow air to bypass the filter media. As filter efficiency increases, so does the amount of pressure required to force air through the filter. Consult the manufacturer's instructions or an HVAC specialist when upgrading filters to make sure that the mechanical system can handle the increased pressure drop across the filters.

In some cases, HVAC systems may be the source of the air contamination if air filters are dirty, the filter allows air to bypass it, there is stagnant water in drip pans, or there is moisture in air ducts that may encourage the growth of moulds or other microbial agents.

Components of an HVAC system

The HVAC systems used in office or commercial buildings have many components. It is important that all components of an HVAC system be inspected, cleaned, and maintained to ensure proper function of the equipment and delivery of quality air to the indoor environment. The following is a list of the major components and functions of an HVAC system.

1. Outdoor air intake: Where fresh outdoor air enters the building.
2. Outdoor air damper: Adjustable barrier that limits the amount of air being brought into the building.
3. Mixing chamber: Area where outdoor air is mixed with air returned from the occupied space and recirculated.
4. Filter bank: May consist of a pre-filter to remove large dust particles, bugs, feathers and leaves from air before passing through the primary filter. Following filtration, the air is distributed to the work areas.
5. Air handling unit: Includes the blower or fan, heating and/or cooling coils, and related equipment such as controls, condensate drip pans, and air filters.
6. Heating coil: Heats the air if needed.
7. Cooling coil and drip pan: Cools the air if needed; drip pan catches water that is produced during the air-cooling process and drains into the wastewater system.
8. Humidifier (or dehumidifier): Adds moisture to or removes moisture from the air as required to adjust the relative humidity of the building air.
9. Blowers (or fans): Pushes (supply) or pulls (exhaust) air through the system; controls flow to various parts of the building.
10. Damper: Adjustable barrier that limits air flowing into or out of a space.
11. Supply air diffuser: Distributes the vented air into the occupied space.
12. Occupied space: Where people are working or eating.
13. Return air grille: Removes air from the occupied space and returns it to the HVAC system for exhausting/recirculation.
14. Ceiling plenum: Space above the suspended ceiling that may be used as a part of the air return system.

Outside air enters through the outdoor air intake and travels through the outdoor air damper into the mixing chamber. The air then flows through the air handling unit, which is made up of a:

• filter bank
• heating coil
• cooling coil and drip pan
• humidifier

It then moves through a blower fan which moves the air through the supply air diffuser and into the occupied space. The air travels through the return air grille in the ceiling of the occupied space, past the return air fan and then is exhausted through the air exhaust.

A heat or energy recovery ventilation system may also be used. This equipment is designed to mechanically ventilate the building by replacing indoor air with fresh outdoor air while recovering the heat and/or moisture from the air stream to reduce energy costs. This system includes a sensible or latent heat exchanger core, which both the outdoor and exhaust air streams pass through in order to recover a percentage of the energy from the conditioned exhaust air within the building.

When inspecting the HVAC system of a building, consider the following items to confirm if the component is functioning properly or to identify if further action is required:

• location of outdoor air intake, to ensure no building air exhausts are nearby and their condition is clean
• position and condition of outside air dampers, to ensure they are open and functioning
• potential sources of contaminants near outdoor air intake (such as garages, loading areas)
• air supply and exhaust fans are in good repair
• proper fan belts and blades are in good repair
• fan blades are installed properly (correct orientation)
• duct work or intakes are free of dust and mould
• ducts are dry, clean and well maintained
• air handling units (fans, coil units, induction units) are in good repair
• air filters of appropriate filtration efficiency ratings and dry, clean, and well maintained
• drip pans are free of dirty water, slime, rust or mould
• humidifier reservoirs are free of mould, dirty water, foam or slime

This list can be found as a checklist in Appendix B. Include any other information that is pertinent to the specific HVAC unit being inspected. If you are unsure, it may be necessary to consult the operator's manual or a qualified HVAC professional for assistance. It is highly recommended to have the building operator/engineer or an HVAC specialist participate in this inspection.

Good operating practices

The HVAC system is critical to the overall IAQ; therefore, building operators and those responsible for IAQ should follow good operating practices. A well-implemented preventive maintenance program improves the functioning of the mechanical systems and can save money over the long term by proactively maintaining the system (CCIAQ 2013d).

Good operating practices will:

• limit indoor sources of contaminants
• limit outdoor contaminants from entering the space
• maintain the HVAC system to ensure the system is operating correctly
• provide adequate outdoor air to occupants (such as amount of fresh air, temperature, velocity, relative humidity)
• control temperature, velocity, and relative humidity in the space and within the HVAC system
• dilute, capture, and remove contaminants though ventilation, filtration, and air cleaning
• The following elements should be considered as part of good operating practices.

Outdoor air

When considering outdoor air:

• make sure that the outdoor air intake is free from exterior contaminant sources (Table 3), including:
  • exhaust from vehicles (such as parking lots. Relocate pick up and drop off locations, avoid idling, relocate air intake locations.)
  • birds, bird droppings, animals, animal droppings
  • contaminants from other sources (such as other businesses, garbage bins, industries, local air quality issues)
• use an air intake screen with less than 13-mm sieve openings to help trap and reduce entry of debris (ASHRAE 2019)
• maintain operational requirements such as dampers that open and close freely, linkages, damper set-points, drainage if snow or rain may enter, and clear drains
• keep the building under a slight positive pressure, if possible (that is drawing slightly more outdoor air into the building than air that escapes or exhausts). Positive pressure limits the infiltration of cold air during heating seasons or intake of contaminants
• ensure that sufficient outdoor air is flowing into the space. There are guidelines with formulas to determine these requirements (expressed as airflow rates of outdoor air). Airflow rates (Table 2) of outdoor air consider the number of occupants, the area/size, and the activities performed in that space (ASHRAE 2019)

Operating schedules

Operating schedules help conserve energy during off-peak hours when the building is at a reduced occupancy overnight and on weekends. It is important to make sure that enough air exchanges occur before regular work hours to remove indoor air contaminants that may have accumulated overnight, and that temperature and humidity are brought to a desired level. The number of air changes depends on the length of the shutdown period and other factors. These parameters can be determined through consultation with an HVAC specialist. In situations where respiratory infectious diseases are circulating in the community, it is recommended to run the system for 2 hours at maximum outside airflow before and after the building is occupied (PHAC 2021c).

Air movement

Excessive air movement may cause people to feel drafts or chills. In most cases, maintain air velocity below 0.2 m/s (40 fpm) for comfort (ASHRAE 2020b).

Circulating air will also help ensure the temperature within a room is consistent (no hot or cold spots). Without room air mixing, individuals will feel and notice if their feet and head are different temperatures due to thermal stratification. The difference between floor and ceiling temperatures should not be more than 3 °C or 4 °C to avoid thermal discomfort (ASHRAE 2020b).

Special areas

Special areas include photocopy rooms, bathrooms, kitchens, parking garages, loading docks, print shops, janitorial closets and some storage areas (such as for paint, cleaning chemicals, or other hazardous products). These special areas require additional consideration for exhausting air, as there may be higher contaminant levels present. Consultation of Section 8 (Exposure controls/Personal protection) of the safety data sheet (SDS) for any hazardous products may provide additional suggestions for exposure controls and personal protection. To avoid recirculating contaminants into the main air supply, these areas should be designed to exhaust air directly outdoors.

Temperature and humidity

When considering temperature and humidity:

• maintain the indoor temperature of the building between 20 °C and 23.5 °C in the winter and between 23 °C and 26 °C in the summer (CSA Group 2017)
• maintain the relative humidity of indoor spaces between 30% and 50% (Health Canada 2016a), depending on the maximum tolerances of the building envelope, to avoid condensation on interior surfaces
• confirm that the HVAC equipment and thermostats are functioning properly, including humidity control
• correctly operate and maintain water systems associated with ventilation systems, such as cooling towers, steam systems, and humidifiers
• adjust ventilation to better match the persons, activities, or equipment in the building
• confirm grills, vents or other ventilation components are not restricted or blocked
• use portable humidifiers with caution, as they may become a source of mould and bacteria if not maintained
• use space heaters with caution, as they are not connected to the building-wide ventilation system

Thermostats

Thermostats must be functioning, calibrated, located correctly, and not obstructed or enclosed in a way that limits air flow. To be correctly located, the thermostat should be placed on an interior wall that is in the centre of the building and away from direct sunlight or other heat sources. Be aware that personal heaters or humidifiers in work areas will confuse the HVAC system's sensors and may lead to inaccurate adjustments of the temperature or humidity in an area.

Balanced system

A balanced ventilation system introduces and exhausts equal quantities of outside air and inside air, respectively (Health Canada 2018a). The filtered outdoor air is supplied to offices and meeting rooms where occupants spend most of their time, and air is specifically exhausted from areas where there may be moisture and contaminants, such as in lunchrooms and washrooms. Office areas and meeting rooms will also have return air grills to ensure proper air movement.

Blocking or removing air supply diffusers may cause the system to overcompensate, leading to balancing issues. Make sure the correct volume of air is delivered to all locations in a building to provide adequate air quality. It may be beneficial to use zone control to help manage changing temperature and humidity needs. For example, a south-facing sunny location may need more cool air, or a north-facing location may require more heated air. Induction heating or cooling units on the building perimeter can also be used for this purpose.

Ventilation maintenance practices

All HVAC systems should be inspected and maintained by a qualified professional as per the manufacturer's recommendations. If none are available, the ASHRAE 62.1 (2019) standard has a recommended activity table that building operators and employers can reference (see Table 8-1 of ASHRAE 62.1-2019), along with an inspection frequency. Alternatively, the following sample checklist may be used to help inspect and maintain the HVAC system regularly. It is a good practice to document and record each inspection.

The following is a sample checklist (Table 1), which is also available in Appendix C. This checklist may be adapted to align with the needs of the workplace.

Standards, building codes, and ventilation requirements

Standards

Standards are produced by voluntary organizations, such as the Canadian Standards Association (CSA), the American National Standards Institute (ANSI), ASHRAE and the International Organization for Standardization (ISO). Specific standards are often and incorporated into building codes, regulations and certifications. Examples of standards relevant to IAQ include the following:

• ASHRAE 62.1: Ventilation for Acceptable Indoor Air Quality
• ASHRAE 62.2: Ventilation for Acceptable Indoor Air Quality in Residential Buildings
• ASHRAE 180: Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems
• ASHRAE 55: Thermal Environmental Conditions for Human Occupancy
• MD-15161: Control of Legionella in Mechanical Systems - Public Services and Procurement Canada standard to provide the minimum requirements for design, operation, maintenance and testing to prevent legionellosis associated with building water systems in federal buildings

Building codes

Building codes outline the requirements for air exchanges, thermal comfort, and occupancy limits in a building, structure or facility. Heating, ventilation, storage facilities, renovations, carbon monoxide alarms, smoke alarms, and maintenance are typically covered and are enforced by the provinces, territories and municipalities.

A building code's objective is to ensure that:

• the building is structurally adequate for its intended use
• the structure and materials can withstand temperature extremes, wind, seismic activity, and ice and snow loads
• the building design meets the fire code, electrical safety code, and other regulatory requirements
• the building is suitable for the planned occupancy

Ventilation requirements

Building codes often define ventilation requirements. Most building codes reference the ASHRAE Standard 62.1-2019 "Ventilation for Acceptable Indoor Air Quality" or previous versions (Table 2). Airflow rates for various zones in a commercial building can be compared to these rates as guidance for assessing and adjusting settings. Always check with the jurisdiction for requirements that apply.

Make sure that the quality of the outdoor air being captured for indoor use is of good quality. The ASHRAE Standard 62.1 (2019) specifies the following minimum distances between the air intake and sources of contamination (Table 3):

Carbon dioxide as an indicator of ventilation

The primary source of carbon dioxide (CO₂) in indoor air is occupant respiration, along with other sources such as poorly vented combustion appliances and cigarette smoking. Increased CO₂ levels indoors have been associated with health effects (Carbon dioxide), and the concept of using indoor CO₂ levels as an indicator of ventilation has been discussed for decades (ASHRAE 2022). With an increase in public awareness of the importance of ventilation, alongside the availability of inexpensive CO₂ monitors in the marketplace, there is a renewed interest in using CO₂ monitoring as a method for quantifying ventilation. Guidance provided by ASHRAE contends that indoor CO₂ levels do not provide an overall indication of IAQ and that the evaluation of ventilation rates requires technical knowledge beyond the use of a simple sensor. Sensory accuracy, location, frequent monitoring, and calibration, among other issues, are all critical for drawing meaningful inferences from measured indoor CO₂ concentrations (ASHRAE 2022).

HVAC safeguarding

Building owners and operators should consider and plan for the possibility of deliberate attempts to tamper with or damage HVAC systems and/or introduce chemical, biological or radiological contaminants into the building through outdoor air intakes or from within the building. Additionally, depending on the HVAC system, cybersecurity incidents such as malware and ransomware attacks on the controls could disable and damage system components. Prevention of tampering or damage requires an assessment of the access points, operating conditions and controls of the HVAC system.

Conducting a risk assessment of the building's mechanical ventilation system is an important component of protecting the building and its occupants. Using up-to-date drawings (such as mechanical, electrical) and the written operational procedures for the existing HVAC system, owners and operators should do the following:

• Determine the location of the outdoor air intakes and outdoor mechanical components. Design and/or modify the location of air intakes and mechanical components so they are not readily accessible and are less noticeable. If this is not possible, incorporate barriers such as fences and locked gates and/or surveillance and monitoring to limit access as much as possible.
• Determine the location of the filtration and distribution systems, as well as the ventilation system controls. Secure areas where equipment is located, restrict access to these areas to authorized personnel, and screen and supervise personnel and contractors that have access to the systems.
• Establish a procedure for rapidly and safely shutting down the system should it be required. Ensure staff are trained and available.
• Determine if the system has the capacity to zone off, isolate or independently control ventilation to specific sections of the building. If the HVAC system has this capacity and is damaged, operators may be able to continue to provide ventilation to portions of the building while the damaged section(s) is/are undergoing repairs. In the event that contaminants are deliberately introduced into the system (from inside or outside), having this capability and being able to respond quickly, operators may be able to prevent/limit the exposure of inhabitants. This capability may also aid in evacuation planning.
• Determine the location and types of filters used in the system. This will aid operators in regular maintenance and help inform whether filters are able to mitigate the entrance of certain contaminants.
• Assess whether the system has adequate cybersecurity protection. Software used to automate and/or control these systems may be subject to cyber attacks that may disable or harm system components, which can in turn cause IAQ, heating and cooling issues. This may impact business operations through disruption of occupant activity and could cause damage to other equipment or materials stored in the building. Additionally, if the system controls are not adequately protected from intrusion, they may provide a gateway to access other networks and information, including financial and personal data.

These are examples of mitigation measures that may be implemented to protect the HVAC system and the IAQ of an office building; however, they do not represent a comprehensive list. A building-specific risk assessment is necessary to determine specific risks and help identify additional mitigation measures that may be implemented.

While undertaking a comprehensive risk assessment and implementing changes to help protect the HVAC system will help reduce intentional disruption of the building operations, these actions may not be sufficient to prevent these activities. Monitoring and surveillance, comprehensive operational procedures for which staff are adequately trained, and communication with building occupants are all necessary to lessen the adverse impact to the mechanical ventilation system, the IAQ and the health of the building occupants should an incident occur.

Additional information is available in the links below. All design, maintenance and risk-assessment activities should be conducted with the assistance of an HVAC professional.

• Guidance for Protecting Building Environments from Airborne Chemical, Biological or Radiological Attacks (NIOSH, 2002)
• Protecting HVAC Systems from Cybersecurity Threats (ASHRAE, 2021)