by James Montgomery
The continued drive towards more sustainable building design, construction, and operation has begun to expand its focus from improvements in the building energy efficiency towards making buildings better for the people inside them. A key aspect of this is providing improved indoor air quality (IAQ) to ensure we have a healthy environment. The LEED rating system contains credits focusing on indoor air quality such as minimum ventilation and filtration requirements, and the WELL building standard is an emerging rating system that focuses on quantitative measurements of indoor air quality (among other health aspects) to promote healthy buildings. Achieving these ratings helps to distinguishe buildings that have been designed with the occupant in mind.
Air quality is measured based on the concentration of pollutants such as fine particulate matter (PM2.5), carbon dioxide (CO2), and other gas pollutants like formaldehyde and benzene. These pollutants are introduced to a building in a number of ways such as human activity, inflow of dirty outdoor air, release from mechanical equipment, off-gassing from building materials, or even from cleaning activities.
Guidelines for acceptable concentrations for many pollutants are outlined by the World Health Organization and Health Canada. The concentrations of these pollutants is important for building occupants as they can cause adverse health effects that range from eye irritation to increased risk of cardiovascular disease, and can impact worker productivity. The Lawrence Berkeley National Laboratory (LBNL) has estimated that changes in commercial and institutional building characteristics can reduce the rate of infectious respiratory disease in the general population by 10 to 30 per cent corresponding to an annual economic benefit of $6 to $19 billion. Additional benefits can also be found for reductions in asthma and other acute health symptoms. A recent report from the World Green Building Council indicates that improvements to air quality in office buildings can result in an 8-11 per cent increase in worker productivity. The potential benefits of improved indoor air quality significantly outweigh the costs to implement improvements in building design and operation.
The IAQ in a building is largely controlled through the design and operation of the building mechanical systems. Two main methods of controlling pollutant levels with the mechanical system include dilution through effective ventilation and removal through filtration. The ASHRAE Indoor Air Quality Guide provides guidance on methods to provide good IAQ throughout all phases of construction and operation.
Dilution through Effective Ventilation
Standards for ventilation are provided in ASHRAE Standard 62.1 for commercial and high-rise residential buildings and Standard 62.2 for low-rise residential buildings. The ventilation flowrates in the prescriptive path to meeting these standards are based on controlling the build-up of CO2 within the space. The ventilation system should be designed to provide the minimum airflow required by these standards. Additional guidance is also provided for designing ventilation rates based on control of pollutants to specific desired indoor concentration limits.
System maintenance is another important factor in ensuring that all spaces of a building have the required ventilation rates. Maintenance at all levels of the mechanical system can contribute to ensuring proper ventilation levels. Coils and filters become contaminated with dust during operation and impose increased static pressure on the system that can result in reductions in airflow if not cleaned or replaced. Drift of CO2 sensors in demand controlled ventilation systems can cause reduced provision of airflow rate if recalibration is not performed. Testing, adjusting and balancing of the airflow within the building system can potentially identify changes in ventilation rates to spaces that can be fixed with modified damper and motor settings.
Active Removal of Contaminants
IAQ can also be maintained by controlling the contaminant levels directly through preventing their ingress to or actively removing them from the indoor environment.
A large source of pollutants to the indoor environment is the outdoor air. Air pollution is generated from natural processes such as wind storms or forest fires as well as anthropogenic sources such as industrial and transportation emissions. These pollutants can be kept out of the building by reducing the infiltration through the building envelope and by proper placement of outdoor air vents to limit exposure to concentrated pollutants from vehicle exhaust.
Pollutants that are generated indoors or have found their way into the building can be removed directly to improve IAQ. Examples are effective placement of exhaust air sources such as range hoods to control particle emissions from cooking, exhaust grilles in printer rooms, or dust control in mechanical shop facilities. Air filtration can also be provided to remove particles (particle air filters) or gas phase pollutants (carbon filters) either as a component of the central HVAC system or as portable units placed in specific rooms. Better IAQ can be achieved through installation of more efficient filters but often comes at the cost of higher flow resistance and the potential for reduced ventilation.
As the industry moves towards an emphasis on healthy building performance the need to provide high quality indoor environments will become more prevalent. An understanding of how system components impact air quality can help building designers and mechanical contractors improve the health of building occupants through the construction of new buildings as well as retrofits to improve environments in the existing building stock.
James Montgomery is a building science research engineer at RDH Building Engineering. He holds a PhD in Mechanical Engineering from the University of British Columbia.