Draft guidance on improving indoor air quality in office buildings: Ventilation

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.

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.

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:

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.

In general, an HVAC system:

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.

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.

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:

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:

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.

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:

When considering outdoor air:

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).

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 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.

When considering temperature and humidity:

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.

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.

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 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:

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:

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):

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).

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:

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.

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