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A common understanding of germ theory became prevalent in North America during the 19th century. As a result, a particular focus on the prevention of bacteria spread started a new effort in disease prevention and enhancing human well-being. The initiative to improve the quality of human health has led to the analysis of all aspects of exposure, including water and food consumption, the sterilization and protection of open flesh during surgery, and the focus of this article: the quality of air we breathe.
As we learned more about the effects of airborne contaminants, particularly that of carbon dioxide particulates and gaseous compounds, standards for replacing bad air with fresh outside air in buildings were starting to be established.
In 1829, Thomas Tredgold published his research on the “principles of warming and ventilating public buildings,” including recorded studies on the volumetric rate of outside air needed to purge carbon dioxide from a subject’s lungs. The works of researchers such as Tredgold provided a basis from which to fine-tune the minimum effective ventilation rates needed for building applications to ensure occupant health and quality of life.
By requiring minimum outside air ventilation rates in buildings, early considerations were needed to balance energy usage and mitigate costs. Buildings in the early 1900s provided large areas for outside air intake, primarily by opening windows during occupied and unoccupied hours, as air conditioning was not prevalent then.
With the additional presence of outside air came the oversizing of furnaces to provide enough heat to compensate for the increased infiltration of cold air. Older buildings also had more infiltration, allowing more air to pass through their building envelopes.
During the 1970s energy crisis, broad cutbacks to electricity and oil consumption occurred nationwide, and a natural cost-savings candidate for several building operators was to close off or minimize outside air ventilation rates. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 62-1973, Standard for Natural and Mechanical Ventilation Design, established minimum ventilation rates (generally 5 cubic feet/minute (cfm) a person instead of the previously established 10 cfm/person), but in hindsight these rates were inadequate.
Volatile organic compounds are commonly found in regularly used building materials, such as carpentry and insulation, and when not properly removed from the breathing air space, can cause a variety of health issues. Additionally, building construction was becoming tighter, reducing infiltration and exfiltration to mitigate heating/cooling losses.
During this period, it was not uncommon for large populations of workers and students to be located indoors for several hours a day, greatly increasing the potential rates of airborne illnesses. In 1983, the World Health Organization coined the term “Sick Building Syndrome” and was primarily attributed to a lack of adequate ventilation in buildings.
Ventilation rate standards
Further adjustments were made to ventilation rate standards in ASHRAE Standard 62.1-1989, Ventilation for Acceptable Indoor Air Quality. These updates were controversial as several criteria regarding acceptable ventilation rates increased significantly. One noteworthy update was the requirement to triple the general minimum ventilation rate from 5 cfm/person to 15 cfm/person.
Due to the backlash to these standards, ASHRAE committees were formed to develop solutions to best address a proper balance between what is needed to promote building occupant health while also taking the overall energy demand into account.
The committee stated two primary goals in its effort to improve the 62.1 standard:
1. To update the language to allow the standard to be more readily and easily adopted into building codes.
2. To improve the standard based on increased knowledge, verified research and practical experience.
As a result, this framework served as a catalyst for continuous improvements to the indoor air quality (IAQ) standards captured in ASHRAE 62.1 and the development of other standards.
ASHRAE 62.1-2022 is the most recent publication of the standard. The 2022 edition did something unusual: it changed the title of the standard. Long-time users of 62.1 will recall the title as “Ventilation for Acceptable Indoor Air Quality.” As of 2022, the title is “Ventilation and Acceptable Indoor Air Quality.”
This change reflects further developments of the standard that looks at IAQ as more than outdoor air ventilation. The current focus depends on the outside air ventilation rate and the outdoor air quality.
Another modern focus is addressing the quality of outside air in the building’s specific region and its effects on the air quality for the space. ASHRAE 62.1-2022 highlights requirements for documentation regarding the design criteria needed to comply with the standard.
Templates are then provided as a guide to properly document regional and local air quality results, which will be used as a basis for the ventilation system design strategy. The history of the study of IAQ has led us to this point, but many future innovations are also being considered.
Contaminated aerosol control
New standards such as ASHRAE Standard 241, Control of Infectious Aerosols, published in 2023, cover a scope gap in the ASHRAE indoor air standards. Previous standards lacked requirements for managing airborne infectious particulates in nonhealthcare or laboratory spaces. With its introduction, Standard 241 covers the clean air needed in indoor spaces to mitigate the spread of contaminated aerosols.
Using ASHRAE Standard 62.1 and ASHRAE Standard 170, Ventilation of Health Care Facilities, as a basis, Standard 241 defines the amount of clean air by space type and occupancy counts. Additional aspects of the standard include requirements for the measurement-accepted levels of certain gaseous chemicals in the air and the development of a Building Readiness Plan for the building’s operation during an infectious aerosol event such as high flu season or recurrence of the COVID-19 virus.
ASHRAE Standard 241 is also one of the first standards to call for a minimum efficiency rating on filters. While ASHRAE 62.1 has long called for a minimum MERV 8 rating, the MERV-A rating is a variation based on Appendix J of ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.
MERV (Minimum Efficiency Reporting Value) is a performance scale for filters from ASHRAE Standard 52.2 regularly used in the industry. Starting at a rating of 1 and going up to 16 and across three ranges of particulate size capture efficiency, the rating system categorizes filters used in air-handling systems. However, a filter may not stay in its rating category for its full life.
The production process for an air filter often generates a resultant electrostatic charge that increases the capture efficiency of a filter and can skew its rating. Once the filter loads with dust and particulates, the electrostatic charge no longer attracts as many particles and the rating drops.
MERV-A ratings establish the MERV rating of the filters without any electrostatic charge. This form of the MERV rating is often called the “actual” rating of the filter, as the filter will not drop a rating with loading. MERV-A rating requirements are starting to crop up in building requirements put forth by clients and the Authority Having Jurisdiction.
Dealing with particulate matter in smoke
Infectious aerosols are not the only contributor to creating dirty air. According to the National Centers for Environmental Information, an average of 3,623 wildfires occur in the United States each year. Wildfire smoke can spread hundreds of miles and impact communities outside of the burn region.
While there are many components to wildfire smoke, the main concern for health organizations is particulate matter smaller than 2.5 µm (PM2.5), smaller than the thickness of a single hair, and can travel and deposit into the deeper regions of the lungs.
In the strive for improved IAQ, ASHRAE and other organizations have begun developing a framework for how building designers and owners can ready the built environment for handling smoke.
ASHRAE released the “Planning Framework for Protecting Commercial Building Occupants from Smoke During Wildfire Events” in 2021 (https://bit.ly/4gVH2Wb). It outlines a decision template and steps for building managers to prepare a building for a smoke event. ASHRAE Guideline 44P, “Protecting Building Occupants from Smoke During Wildfire and Prescribed Burn Events,” is in development and will formalize recommendations for the design, installation, operation, commissioning and maintenance of building envelopes and air systems for multifamily buildings.
The Environmental Protection Agency (EPA) partnered with the Missoula City-County Health Department in Montana, the University of Montana, the Hoopa Valley Tribe in California, and other communities to conduct research called the Wildfire Advancing Science Partnerships for Indoor Reductions of Smoke Exposures (ASPIRE) Study.
The ASPIRE Study began in 2019 and compared indoor and outdoor PM2.5 concentrations to create methods of reducing indoor pollutants during wildfire smoke events. The study also considered the effectiveness of air filtration systems during smoke events and whether using portable air cleaners effectively reduced PM2.5 levels. Test results have not been officially released at this time and are being evaluated by the EPA and its partners.
Humans spend most of their time indoors, with some estimates as high as 90%. This value was first reported in 2001 in “The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants,” published in the Journal of Exposure Science & Environmental Epidemiology (www.nature.com/articles/7500165). In more than two decades, it has not changed.
Humanity will continue living and working indoors, so the IAQ of the built environment will continue to need improvement. The evolution of IAQ continues to develop, moving from simple concepts of ventilation to examining the numerous facets of controlling and limiting contaminants within indoor spaces.
Steven Urosevic, PE, is a mechanical engineer at the SmithGroup Phoenix office. He has more than seven years of experience in the industry, including HVAC and plumbing design.
Megan Kittredge, PE, is a mechanical engineer at SmithGroup’s Phoenix office. She has more than six years designing mechanical systems for various building types.