Evolution of Ventilation Measures and Energy Performance in Buildings with High Ventilation Demands: A Critical Review

Abstract

Ventilation performance has historically been assessed using diverse metrics, ranging from air change rates and contaminant concentrations to occupant perception. This paper traces the evolution of these performance measures across research and practice, highlighting the progression from simple ventilation rate benchmarks to more sophisticated indicators like contaminant removal effectiveness (CRE), air exchange effectiveness (AEE), and age of air. The limitations of conventional metrics—particularly their inability to capture spatial variability, energy implications, and real-time contaminant removal—are critically examined. In addition, the historical evolution of these metrics and the rationale for their adoption is studied, specifically in the context of building codes and standards in the United States. A framework is proposed to categorize performance measures into ventilation rate-based, contaminant-based, air distribution-based, and perception-based groups, facilitating their comparison and selection. This critical review aims to support the development of more effective and context-sensitive ventilation assessment strategies, with implications for future research and building standards.

1. Introduction

Ventilation is found in nearly every building to achieve a variety of purposes for occupants and/or processes. There are many codes, standards, and guidelines generated over nearly a century now that inform the quantity and quality of air required to achieve intended ventilation goals. However, the scientific basis for many of these practices remains largely lost to history [1]. This manuscript intends to identify and interpret ventilation requirements for medium and high air change spaces based on published documents, looking backward from today to 1824.

Historical context is important throughout this paper in that “necessity is the mother of invention”. Global events and trends have had a significant influence on HVAC research over the last 150 years. From the public health perspective, the COVID-19 pandemic is the most recent driver for indoor air quality and health research [2,3], but previous health crises, including the 1918 Influenza pandemic [4], tuberculosis [5], measles, and others, all played a role at various times. Technological advances drove the design of cleanrooms where precision mattered in applications such as World War 2 bomb sight optics or the advent of computational technology requiring particle-free environments to reduce device failure rates [6]. Increased farm output based on lessons learned in animal laboratories was critical in the 1930s when most of the world’s population worked in agriculture, and their economic success was tied to the productivity of their animal herds. These events in their historical context are meant to provide an insight into what the authors of the reviewed publications were thinking at that time.

The practice that demands “the removal of dust and foreign matter from incoming air, whether it be for ensuring comfort or providing healthful conditions” broadly summarizes the concept of ventilation [7]. From a traditional perspective, ventilation is studied as the science of supplying an adequate quantity of air, with qualities conducive to good health and leading to a comfortable indoor environment [8]. The discussion about ventilation and subsequent research originated in the foundations of public health interests, to make it more effective in treating and healing patients. The concept of fresh air supply in British hospitals to maintain a sweet-smelling air by Florence Nightingale (1859) was one of the very first documents on air requirements for healthy environments [9]. Several years later (1879), Sir William Muir, director general of the British Army Medical Department, was tasked to collect, analyze, and report on the health of troops at all stations and the related parameters. He indicates that M. Tollet, a French civil engineer, had written in his memoir that space should be changed at least twice per hour. Neither provided any evidence to support this value, nor did they mention a basis (i.e., thermal comfort, cleanliness, etc.) for it. Reading between the lines of this document (and similar ones) suggests that these are, using today’s engineering terminology, design set points. Worthy of note, modern heating, ventilation, and air conditioning (HVAC) systems were first developed and introduced during the 1870–1910 timeframe [10]. A report by the New York State Factory Commission (1912) showed that out of 357 printing shops, only 25 had mechanical ventilation, and out of 151 of the observed manufacturing factories, only 16 had mechanical ventilation, and 112 had no kind of improved ventilation (e.g., fans, etc.) [11]. One of the earliest documented examples of the building mechanical system is the City Town Hall in Liverpool from 1867, where the ventilation system was set to provide 7 to 10 cubic feet per minute (CFM) of air per occupant. No reason or evidence was offered for this choice. However, from the rest of the document, one can deduce that the primary aim was to create a uniformly comfortable temperature [12].

While it seems that the initial intent of the HVAC system was to provide thermal comfort, especially in tall and large buildings with significant cooling and heating loads, its impact on occupant health, however, came to light shortly after. A good example is an early work by General Morin on the principles for providing clean air to buildings via the supplying of fresh air and the suction of old air from the space [13]. Though clearly stated by General Morin, these values emanate from his “…own observations and the consideration of those obtained by others”.

In 1910 and to urge the community to adopt a more systematic approach to ventilation’s impact on public health, Dr. Kimball writes “Few of us would care to put on underclothing immediately taken from another person or put into our mouths articles of food or drink taken from another’s mouth, yet we take into our lungs with but little or no hesitation air containing that which has but just come from other people’s mouths” [14]. Between 1900 and 1910 scientists, authorities, engineers, and architects began to realize that the quality of air we breathe is, perhaps, as important as the food we eat [15]. There were studies showing the undeniable effects of fresh air on health, productivity, and performance of occupants [15]. For example, the Germania Insurance Company of New York placed eighty clerks in one large room without ventilation and recorded a 10% absenteeism rate at all times. Introducing proper ventilation reduced this rate to practically zero [16]. However, the characteristics of proper ventilation were not fully documented or described [14]. Similarly, the U.S. Pension Bureau found that employee loss due to illness was nearly halved by introducing fresh air via the ventilation system [15]. A hat company was able to reduce its employees’ sick rate from 27% to 1.5% by bringing fresh air into the factory. In another work by Mauer (1918), the cause of a significant rise in death rates from tuberculosis in cities was associated with a lack of fresh air [17]. Another public space of interest was entertainment spaces, such as theatres, as occupants spend long durations indoors. Most theatre premises before 1900 were ill-ventilated, with a significant source of odor, mainly due to a lack of openings to the outside [18]. In 1884, Seddon stated that many people seldom go to a theatre due to the fear of getting sick [19]. He anecdotally alludes to a theater in Manchester where a new owner “thoroughly ventilated” and made a fortune. What is wanted, he continues, is “a plentiful supply of fresh air forced into every part of the building, not just the auditorium”.

The overarching message from reviewing the early papers is that a lack of fresh air is commonly a major reason for adverse outcomes such as disease spread, lack of productivity and comfort, and feeling dryness in a variety of built environments. It was well known that a lack of fresh air is bad, yet how much fresh air is adequate still remains for future investigators to scrutinize. An outstanding case in point is a work on school ventilation by Winslow (1913), where 32 schools with ventilation configurations ranging from modern systems (using the 1913 terminology) to no mechanical systems were observed [20]. Dust concentrations, microbial load, temperature, and relative humidity were measured in every classroom. While the microbial load of well-ventilated classrooms was up to one-fifth of that in poorly ventilated ones, the author stated that more than 90% of these studied cases had a load below what is considered of ‘sanitary significance’. Comparison between natural and mechanical ventilation cases for bacterial concentrations showed similar behavior. For dust concentration, the weighted average was 600,000 particles per cubic foot. These values are much higher than what engineers and designers tolerate in our time, yet the author indicated that “There is no evidence to show that such particles as occur in ordinary school-room air have any sanitary significance”. Perhaps an interesting finding of this work, which also resonated as one of the major conclusions, is the role of mechanical ventilation in creating proper temperature and airflow distributions. Specifically, more than 90% of well-ventilated classrooms delivered 30 cubic feet of air per person, while the same ratio was less than 15% for poorly ventilated rooms. Despite the massive amount of data collected and analyzed, this work does not offer a comparative analysis of the performance of students against ventilation rates and, thus, does not recommend any rates.

As the concept of ‘air conditioning’, concerning air quality aspects, gained momentum (~1920–1930), the science of ventilation started evolving to include controlling temperature, relative humidity, and air purity. Scientists started to study the perfect quantity and quality of supply air to meet the demands and challenges of an occupied indoor environment. This line of research continues to date, among other reasons, owing to the fact that challenges associated with built environments are dynamic, evolve over time, and periodically focus on various human- and performance-centered aspects (e.g., comfort, productivity, health, contamination control) [21]. With that in mind, this paper aims to focus on the evolution of the analytical measurement of ventilation quantity and quality, and on the trends in ventilation, especially ventilation rates, over time. The evolution of ventilation through the decades, as the goal of ventilation transformed over time, led to the discussion regarding how ventilation performance has been assessed. The need for different metrics for measuring ventilation was dictated by the requirements of ventilation, which, in turn, followed the trend of the principal focus for ventilation. All these facets that controlled the rate of ventilation ultimately evolved to be specific for different space functions. As a result, codes and regulations emerged and were enforced. Undoubtedly, the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) is among the pioneer organizations that formulated prescribed values for ventilation (rates), starting in the 1910s, and a decade later, space-specific values were provided. These values were then adopted and enforced by U.S. building codes and regulatory bodies, which had a primary role in shaping the history and evolution of building mechanical system design and operations in the United States, and arguably the world. This article is a systematic review of the literature and publications by ASHRAE proceedings, handbooks, and standards, followed by a scrutiny of the history of how ventilation is accordingly measured and/or evaluated.

2. Materials and Methods

A one-year collaborative study between academia, industry, and regulatory bodies was performed with the objective of conducting a comprehensive literature review for evidence of the historical basis for specified air change rates (ACR) for cleanrooms, laboratories, animal facilities, and healthcare facilities. The ventilation rates are typically selected to comply with published guidelines with various levels of authority, from codes, standards, and publications of professional societies to design standards of the client’s institution. However, in many instances, the designers and engineers are not well aware of the reasons and justifications behind these published rates. They often design based on extreme conditions that may cause unnecessary energy usage or lead to system over-sizing.

The primary objective of this study is to provide a comprehensive document that identifies the basis for ventilation rate specification in different space functions. This requires knowledge about ventilation terminologies associated with the ventilation of spaces. Therefore, the first step was essentially identifying the ventilation terminologies used by scientific research articles, ventilation standards, and codes to describe ventilation. After careful investigation, the following terms were extracted from the RFP and ventilation standards: ventilation, ventilation rate, ventilation requirements, ventilation air, air change rate, and ventilation effectiveness. These terms were then used to identify relevant published scientific research articles using academic search engines. Search engines such as Engineering Village, Compendex (Elsevier), PubMed, Google Scholar, and Scopus enabled the research group to find articles that include at least one of the ventilation terms in their titles, abstracts, or conclusions. Considering the fact that ASHRAE, by nature, has a rich pool of articles related to ventilation, its online database was searched to extract publications including the aforementioned ventilation terms. As expected, ASHRAE has published lots of papers discussing ventilation within the scope of interest.

Among all articles gathered in the search process, only the ones that were published prior to the year 2000 were selected for the next step, which was reviewing the abstracts. This direction was made commensurate with the specific goals of the research project to focus on the history of ventilation rate instead of the recent advancements in science and technology. The initial search resulted in 451 research articles plus another 53 documents from the reference list of ASHRAE handbooks. The research team began reading the abstracts to collect the relevant research work and eliminate the irrelevant studies. Of the 504 papers, 17 were not in English, 33 were not found, and 134 were deemed not relevant to the scope of this project (Figure 1).

Once the database was complete and organized, the research team divided the publications among themselves to review. The next step was to read the papers one by one to identify and extract the type of study, method, goal(s), ventilation term(s), space function(s), and year of publication. The types of study indicate if the paper is a scientific research article, guideline, code, standard, or report. The method represents whether the author conducted experimental work or performed a numerical study. The goal(s) show the ventilation objective that the author pursued through the study. The spreadsheet dedicated a section to ventilation rates that studied values and supporting rationale for the recommendations that were later adopted by ASHRAE codes and standards. This process was implemented through weekly meetings and monthly check-ins to ensure that the search process would provide helpful results.

Next, the research team reviewed every ASHRAE handbook published from the very beginning. The first annual ‘handbook’, known as ASHVE Guide, was published in 1922 and every year until 1960. From 1961 to 1972, the handbooks were named the ASHRAE Guide and Data Book, with a separate publication for HVAC applications since 1964. In 1973, it became the ASHRAE Handbook, and in 1985 separate publications of inch-pound (I-P) and international system (SI) unit versions of the volumes began. The first Handbook of Applications was published in 1978 and has since been published every four years. The Interlibrary Loan System allowed the team to locate and request the publications from the early days of ASHRAE’s book publications. In a span of three months, a total of 56 books were collected and reviewed for pertinent information according to the scope of this work, i.e., to find relevant trends in the background theories that developed the current trends in HVAC design and application, with special emphasis on healthcare facilities, cleanrooms, laboratories, and animal facilities. All of the findings from these books were collected in a spreadsheet and coded to streamline the findings that correspond to the above-mentioned space functions or the theoretical background. Next, the reference list of each ASHRAE handbook was carefully studied to identify relevant documents that the team might have missed during the literature search.

3. Method Implementation and Early Findings

The timeline in Figure 2 describes the focal points of researchers working to provide effective ventilation over time. It is noted that the years mentioned here are sequences found in ASHVE/ASHRAE periodicals. Previous research reports, publications, and/or conference proceedings might have discussed these topics prior to being addressed by ASHVE/ASHRAE. The 1920s decade shaped the ventilation research, concentrating on providing a healthy and comfortable environment by eliminating dust and excessive CO₂ concentration, controlling exposure to smoke and odor, and meeting the heating and cooling needs for comfort. This supply of outside air to dilute indoor pollution was later termed as ‘dilution ventilation.’ This concept describes replacing indoor air with fresh outside air, i.e., air changes. Although initially air change was aimed at dilution, identifying factors that affect ventilation during this time led to an elaborate investigation, resulting in specific ventilation recommendations in subsequent decades (after the 1920s), so that the supplied outside air can be conditioned to make the environment healthy and comfortable. During the next decade (1930s), these factors were evaluated in broader contexts, and ventilation research revolved around occupant comfort—the focus of ventilation started to shift from merely air changes to conditioning of the air being replaced. That is, it was identified that comfort requirements varied for different buildings, as these host a wide spectrum of occupants. In the late 1930s, the ventilation for animal shelters first appeared in the ASHRAE literature, which apparently was not followed up in subsequent publications until the 1950s, when plenty of studies were done for animal safekeeping, their health, and well-being.

The US’s participation in the Second World War at the beginning of the 1940s and the related contagion outbreaks pushed the need to analyze healthcare systems’ ventilation practices. The research in this decade (the 1940s) provided insights into ideal ventilation for treating diseases and cleaning of air by sanitation ventilation. The evolution of better healthcare facilities necessitated the use of laboratories, and specific requirements for lab ventilation were introduced. The comfort of ventilation had drawn interest in the relation between the quantity of air circulated in an enclosed space, the number of occupants, their activity levels, and sensations for warmth or cold. Ventilation comfort parameters (i.e., temperature and relative humidity, and draft) were investigated in further detail, and the changes required for different climatic conditions were also included in the ventilation assessment.

During the same time, precision manufacturing and pharmaceutical advancement were hindered as contamination control was not as effective as required. The need for industrial ventilation and close control on the contamination of manufacturing goods led to discussions about contamination sources and removal strategies through the 1950s. As a result, ASHRAE literature published specific requirements for the ventilation of clean spaces.

The following decade (the 1960s) led to elaborate discussions and recommendations of ventilation parameters in specific spaces like hospitals, nursing homes, labs, and clean spaces. In this decade (the 1960s), air distribution received significant attention alongside ventilation rate (i.e., air change rate). In the ASHRAE handbooks, air distribution to prevent intra-ward infections led to recommendations regarding pressurization schemes or directional airflows requiring adequate differential pressures in different areas of indoor spaces. One such example of pressurization recommendations is shown in

Table 1. The recommended pressure differential for cleanrooms [14].

Application	Pressure Differential
General	0.05 in of water higher than surroundings
Between cleanroom and uncontaminated section	0.05 in of water, minimum
Between uncontaminated and semi-contaminated section	0.05 in of water
Between semi-contaminated section and locker area	0.01 in water

.

Ventilation Requirements

Although it was not specifically defined in the ASHRAE literature, analyzing the texts where ‘ventilation requirement’ was used indicated that the requirements are expressed as the quantity and quality of air required to achieve ‘proper’ ventilation. The term ‘proper’ has evolved over time and application range. Figure 3 depicts different facets considered to determine the requirements, leading to specific requirements for each space function.

The general need for ventilation evolved around comfort, freedom from substances, health, and cleanliness. As additional evidence was generated, more space-specific requirements were recommended. It is imperative to discuss the understanding of ventilation in facilities in general to analyze how certain needs for ventilation shaped the term “proper”, as is evident from the timeline of ventilation’s focus in Figure 3. As in the 1920s and a significant part of the 1930s, research-based conclusions were dependent on laboratory-based experiments, and general building ventilation was the primary focus. Ventilation was meant to serve three purposes: (1) supply necessary oxygen, (2) dilute objectionable substances to the proper level, and (3) maintain proper effective temperature. Maintenance of proper temperature brought forward the concept of thermal comfort. The literature published during that time suggests that several factors, like the vitality rate, age, sex, physiological conditions, and even geographic locations, contributed to determining comfort requirements [7,22,23]. The contemporary literature of the 1930s further suggests that supplied air quality was also under scrutiny to achieve the required conditions without causing discomfort [7,23]. For example, air motion had to be low enough not to cause drafts. A study showed that 15–30 feet per minute of air motion is indicative of satisfactory distribution [24]. During the same time, attention was also being paid to having an air supply dilute noxious substances to ensure the health of occupants [25].

With expanded research capabilities during the 1950s, scientists were able to establish ventilation requirements to establish ‘clean’ environments, spaces free from pathogens in addition to noxious elements, which were conducive to health and healing in sensitive spaces like operating rooms and nurseries. During the same time, the pharmaceutical and semiconductor industries were upgrading their facilities and applying decontamination methods using airflow. After this point, ventilation-related discussions were more fragmented, largely due to the specialization of industries leading to very specific requirements, and the space-specific requirements that dominated the published documents. The authors conclude that by the mid-1960s, the requirements for proper ventilation were broadly categorized into four major space functions—healthcare facilities, laboratories, animal shelters, and general, which included schools, offices, and other workplaces. Scientists started to differentiate between the principal motives of ventilation specific to space functions. For example, the primary goal of ventilation in a laboratory was to remove fumes, gases, and contaminants from the occupied space. In contrast, in hospitals, the principal goal was to provide the proper environment for treatments and control the airborne spread of pathogens. Whereas, in cleanrooms, there was a dire need for particulate-free space near the semiconductor pieces, as they could impair the functionality of the semiconductor pieces. By the 1970s, there was ample evidence that proper air conditioning is beneficial in preventing and treating many conditions.

4. Results and Discussion

In this section, and based on the early findings presented in the previous section, we aim to discuss how ventilation has been assessed, measured, and expressed in a historical context, and what the theoretical backing and justification were for such decisions.

4.1. Assessment of Ventilation

The quantification and assessment of the process contemplated a wide array of factors over time, as depicted in Figure 4. One of the very first assessments of ventilation was carried out by measuring CO₂ concentrations, and ventilation was deemed effective when less than 1000 ppm was measured. A number of factors regulate proper ventilation, such as supply airflow rate, air temperature, air cleanliness from dust and suspended matter, air sanitation related to freedom from bioaerosols, relative humidity, air distribution, air motion, freedom from odors, freedom from injurious substances, etc. These factors were combined to quantify ventilation through an indirect index known as ventilation perfection, a method to assess the indoor environment in order to maintain an acceptable level of these factors continually [7].

In addition to CO₂ concentration, temperature, humidity, and air motion are also determinants of comfort conditions. These parameters that conform to the environmental requirements for public health and comfort were congregated into a single index known as the ‘Synthetic Air Chart’, introduced by Dr. E. Vernon, O.W. Armspach; adopted by ASHVE in 1920, it provided ‘ventilation perfection’ in terms of percentage. A hundred percent perfect ventilation had all the parameters within the required ranges. These comfort parameters were summarized in an indirect index called the effective temperature (ET) index, an arbitrary index of the degree of warmth or cold felt by the human body in response to temperature, humidity, and air movement [7]. This index was first published by F. C. Houghten and C. P. Yagloglou in 1923 [14] but was adopted into the ventilation discussion by ASHVE in 1935. According to the ASHVE guide (1935), “Effective temperature is an index of warmth or cold. It is not in itself an index of comfort, as it is often assumed to be, nor are the effective temperature lines necessarily lines of equal comfort. This is true because, in determining this index, the subjects compared not the relative comfort but rather the relative warmth or cold of various air conditions. Moist air at a comparatively low temperature and dry air at a higher temperature may each feel as warm as air of an intermediate temperature and humidity, but the comfort experienced in the three air conditions would be different, although the effective temperature is the same”. Depending on the occupants, these comfort conditions dictated heating or cooling requirements to ensure optimum comfort that was acceptable to the majority. Local climate conditions had direct impacts on temperature and humidity regulations of ventilated air, and research by ASHVE showed that the ET index differed significantly in places that are geographically dispersed [7]. Depending on the weather, the sizing of the designed systems was optimized based on the heating/cooling requirement.

Satisfactory ventilation conditions in an occupied space depends upon two parameters: (a) the occupancy level, and (b) the air conditioning requirement, i.e., heating or cooling condition. The occupancy level is dictated by the number of persons gathering in a space, the average available space per person, their physiological factors such as age, gender, acclimatization capabilities, etc., and the level of activity. Research supported by ASHVE tested the requirements of supply air quantity for different activity levels and space availability adopted from ASHVE Guide, 1953 (

Table 2. Minimum outdoor air requirement to remove objectionable body odors under laboratory conditions [7].

Type of Occupants	Air Space per Person ft3	Outdoor Air Supply CFM per Person
Heating Season with/without recirculation, air not conditioned
Sedentary adults of average socio-economic status	100	25
	200	16
	300	12
Laborers	200	23
Grade school children of average socio-economic status	100	29
	200	21
	300	17
Grade school children of lower socio-economic status	200	38
Children attending private school	100	22
Heating Season, total air circulation 30 CFM per person
Sedentary adults	200	12
Summer season, air-cooled and dehumidified by spray dehumidifier, spray water changed daily. Total air circulation 30 CFM per person
Sedentary adults	200	<4

) [7]. It is evident that the outdoor air supply requirement decreases for larger spaces but increases with activity level. The rate of air change in Table 2 is from 10–30 cfm per person. This level of air change was achievable by leakage through gaps around windows and doors in winter and by opening windows in summer, when only a few people occupied the room. With 400 ft³ of space allotted per person, only a 1.5 air change rate (ACH) was necessary to attain a 10 cfm per person ventilation rate. Therefore, in ordinary dwellings, with enough volumetric space allotted, special provisions for maintaining the chemical purity of air were not necessary, and controlling the temperature was the main focus. Whereas, in more crowded spaces (offices, auditoriums, etc.), the volumetric space per person is lower, and it is usually not possible to admit untempered outside air, so mechanical ventilation was required.

Another aspect of ventilation assessment originated from a general interest in public health. The timeline of ventilation suggests that the challenges related to public health drove the assessment of ventilation to ensure healthy buildings. The studies related to ventilation in the broad interest of healthcare had a significant impetus from the highly contagious Spanish Flu outbreak in the late 1910s, which ravaged both hospitals and general buildings. The research conducted during this time led the way to using dilution ventilation to help control the spread of viruses, even though this event has never been explicitly discussed in ASHRAE periodicals. Even though the need to supply adequate quantities of air to patients had been studied since the late 19th century, research-based evidence was not published and incorporated by ASHVE until the mid-1920s. In the late 1930s and early 1940s, the explosion hazards from anesthetic gases in operating rooms demanded dilution ventilation and removal, where the exchange of air eliminated the accumulation of explosive gases. Additionally, the need for adequate ventilation in operating rooms (OR) stemmed from both the surgeon’s and the patient’s physiological properties. Anesthetized patients displayed dilation of blood vessels in the skin, resulting in profuse sweating and inability to regulate body temperature, i.e., all anesthetized patients suffered from heat loss. Physiological effects, such as excessive sweating and rapid pulse, from high OR temperature on attendants and patients during the hot months signify the need for cooling and, thus, ventilation to deliver that cooling. Doctors who performed surgery in air-conditioned spaces and non-conditioned ones reported that the conditioned spaces introduce less fatigue and offer greater recuperative power to the patient [7].

In addition to the advantages of proper ventilation in operating rooms (OR), other types of spaces also benefit from conditioned air ventilation. Burn victims, patients who have arthritis, and premature babies have responded significantly better to treatments when stationed in a controlled environment [7]. Ventilation was measured from a cleanliness perspective to protect susceptible patients from intra-ward infections [26]. It is the understanding of the research team, after going through the published literature, that air motion and distribution methods took precedence over temperature and humidity levels. Multiple publications, as cited by the ASHRAE Guide and Data Book, 1968, concluded that fast air streams caused the reemergence of particles already deposited and led to turbulent air mixing (Airflow around buildings, by J.H. Clarke [27]; Air Conditioning of research and test facilities; contamination control, by J. Peterson [28]. These resulted in non-homogeneous air distribution, which, in turn, ensued incongruent particle concentration in different parts of the space. According to ASHVE guide 1960, good ventilation was categorized when the airspeed was between 20 and 50 fpm in hospitals to control these aspects. The detailed range of recommended airspeed corresponding to proper ventilation in different parts of a healthcare facility has been discussed in detail in the ASHVE/ASHRAE literature [29,30,31].

The concept of air cleanliness for evaluating ventilation in clean spaces was adopted from healthcare facilities. The measures of cleanliness were indirect (presence of CO₂, other gases, odor, etc.), not measured in real time, and mostly came from laboratory-based experiments until actual particulate measurement was possible and used to design cleanrooms. Particulate control by filtration, air velocity, and differential pressure was recommended to restrict the contamination of integrated circuits, sterile drugs, and other sensitive manufactured goods from airborne contamination [32,33]. By the introduction of air quality as one of the main goals for space ventilation, discussions on the requirements for ventilation, and consequently, ventilation assessment, evolved to be space-specific. Every space function has separate occupancy and usage conditions. For example, in a laboratory, temperature and humidity of ventilated air for comfort requirements were secondary, and the quantity of abundant fresh air took precedence to dilute and carry away the fumes and chemicals. Whereas, in a cleanroom, the requirement is different, as the control of flow patterns, the quality of incoming air, and the thermal properties of air are stringently controlled. Hence, the ventilation evaluation, which was formed around the idea of a generic comfort and health requirement, started to develop specific assessment criteria for specific space functions.

4.2. Standardization of Ventilation Requirement

The importance of proper ventilation in specific places raised concerns for documenting the space-specific requirements to standardize ventilation system design. Even though standardization is a process that first appeared in the ASHVE Guide 1925–1926 edition, the standards have evolved to be robust and comprehensive in the 21st century. The evolution of ventilation standards was evaluated by reviewing the ASHVE/ASHRAE literature, and the findings are summarized in the process diagram shown in Figure 5.

Scrutiny of the first-ever ASHRAE ventilation code, named “A code of Minimum Requirements for the Heating and Ventilation of Buildings” published by ASHVE in June 1925, reveals that the recommended values could now be published because the different factors influencing proper ventilation (air temperature, humidity, air velocity, distribution) could be measured accurately with the advent of new measuring instruments. Primarily, the surrogate measurements of air distribution in terms of CO₂ distribution provided a satisfactory indication of ventilation effectiveness. The need for such a code was emphasized by the fact that designers were using varied design parameters, losing homogeneity among the indoor environments. For rigorous scientific research and evidence generation, buildings were to have a similar ventilation system—hence, a code from a society of professional HVAC designers made it very popular among engineers. As multiple sponsored research projects were being conducted in the 1920–1930s in laboratories to consolidate the ‘desirable’ properties, field experiments brought new insights about the physiological considerations of comfort in real-life buildings.

Newly available experimental data from a wide range of sources prepared the engineers to examine them and come up with ‘minimum requirements’ that resulted in satisfactory ventilation performance for a significant proportion of the buildings. Published research from ASHVE and other societies and organizations corroborated the minimum requirements, consolidating the recommended values. Then, from the beginning of the 1940s, different ventilation requirements for various indoor environments were the avenue that scientists started to pursue to gain knowledge focused on spaces with special requirements (e.g., operating rooms and laboratories). As industrial demand increased for controlled environments like cleanrooms, new information focusing on such critical spaces was published. Numerical simulations supplemented experimental data and provided crucial perspectives of the flow that affect ventilation performance. As evident from the first mention of a ventilation standard in the ASHRAE handbook, 1978, from the late 1970s onwards, ASHRAE started standardizing for specific space functions, which are being evaluated and revised based on newly unearthed scientific evidence at regular intervals. The concerns related to energy effectiveness and the quantity and quality of air have been deemed a critical factor for ventilation performance assessment and thus are a significant part of the standardization process, especially in the last two decades of the 20th century (1980s and 1990s). When searching the ASHRAE literature, the research team could not establish a correlation between changes in ACR recommendations and experimental or numerical evidence that led to the change. In general, citations to external references were not sufficient to make such correlations. Therefore, the space-specific investigations below are conducted independently of the ASHRAE periodical review while the team made an attempt to interpret and retrospectively correlate these findings to ASHRAE recommendations and possible changes in time.

4.3. Expression of Ventilation Rate

Ventilation rates, i.e., the quantifiers of ventilation, have been expressed using two principal indices: CFM per person and air changes per hour (ACH). Additionally, the usage of these expressions was often occupancy-dependent. Refer to

Table 3. Ventilation Requirement: Amount of New Air to be Supplied [22].

Buildings	Without Humidification or Recirculation (CFM/Person)—Outside Air	With Humidification but Without Recirculation (CFM/Person)—Outside Air	With Humidification and Recirculation (CFM/Person)—Outside Air	No. of Air Changes per Hour
Schools
Classroom	30	20	5 to 10	-
Assembly Room	15 to 20	10 to 15	5 to 10	-
Gymnasiums	30	25	15 to 20	-
Toilets	-	-	-	10 to 20
Locker Rooms	-	-	-	5 to 10
Kitchens	-	-	-	20 to 60
Theatre
Seating Space	30 to 50	20 to 30	10 to 15	-
Hospitals
Wards	30 to 40	20 to 30	-	-
Kitchen	-	-	-	20 to 60
Dining Rooms	-	-	-	10 to 20
Toilets	-	-	-	10 to 20
Hotels
Dining Rooms	-	-	-	10 to 15
Kitchens	-	-	-	20 to 60
Ballrooms	-	-	-	5 to 10
Work Space	-	-	-	5 to 10
Assembly Rooms	20 to 30	15 to 20	10 to 15	-

, adopted from ASHVE Guide, 1925–1926, which demonstrates the requirement of fresh outside air to be supplied in certain areas of different buildings.

In the 1965–1966 edition of the ASHRAE Guide and Data Book, the recommended air supply used another index that utilizes a spatial correlation with the quantity of air, similar to ACH. However, unlike ACH, the ventilation rate (in CFM) was normalized by the space floor area (SF). It supports the hypothesis presented in Figure 6 that ventilation is measured in CFM per person for spaces with high occupancy variance and measured in CFM per SF for spaces with low occupancy variance and/or known sources of odor/fumes/smoke. It is noteworthy that ventilation rate is expressed both by CFM per person and CFM per SF of floor area for some spaces. Admittedly, as the space floor area increases, the ventilation recommendation based on the per person value results in lower rates. Though not explicitly indicated in the ASHRAE handbook, it seems that recommending two parallel approaches allowed for the lowering of ventilation rates for large spaces. The airflow in unoccupied spaces was to ensure a stable indoor environment and prevent contaminations generated from sources other than people (e.g., building materials) [32].

In summary, after analyzing healthcare facilities, laboratories, animal facilities, and cleanrooms, it has been recognized that ventilation rate has been expressed in a variety of forms depending on the occupancy level and the source of contamination. The recommended ACH values provided enough ventilation where the space’s occupancy level was variable and rather low, with a negligible need to account for occupancy level when recommending a ventilation rate. Conversely, CFM/person was used in large spaces like industrial shop floors, where specifying a single ACH value for the entire floor was not prudent.

5. Theoretical Framework

Reviewing the ASHRAE literature reveals frequent use of a group of parameters (e.g., ACH, CFM, etc.) to quantify and prescribe ventilation rates. This observation triggers a fundamental question: Why are these parameters used to define and quantify ventilation, and why do some parameters seem to be more frequently used for particular space functions? Hence, a holistic review of the theoretical understanding of the problem is instrumental. This section reviews the evolution in the theory of airflow, air quality, and contaminant distribution in the built environment. Alluding to two notes is necessary: (1) This section is entirely devoted to discussing theoretical (mathematical) approaches. Details on how these approaches have been used and the reasons why they were used in particular settings are not discussed. (2) Undoubtedly, there has been a magnificent advancement in knowledge and practice through the use of Computational Fluid Dynamics (CFD) approaches. These approaches, and for that matter, the diffusion–advection equation, solve a series of partial differential equations that do not have analytical solutions, and therefore, must be solved numerically. As a result, the use of CFD was severely limited by earlier computational capability and only gained real attention in the 1990s and onward. Since this research focuses on the history of ventilation, the research team did not include CFD techniques in this section. Instead, in this section, we discuss the five most prevalent theoretical frameworks with analytical solutions. These models were later found to play an outstanding role in regulating, measuring, and prescribing ventilation rates. We begin with the first and most widely used model (i.e., well-mixed) and follow on with more sophisticated spin-offs.

5.1. Well-Mixed Condition

One of the main goals of building scientists and engineers was to study building ventilation through the laws of physics. The knowledge of some relationship between contamination and ventilation dates back nearly two centuries, where Tredgold (1824) [34] discussed CO₂ expiration by the introduction of fresh air. Since that date, scientists have used CO₂ and other sources (e.g., water vapor) as tracers to quantify ventilation in buildings [34]. However, this knowledge stemmed particularly from chemical and industrial engineering on the mixing behavior of gaseous species, which triggered the idea of using the same notion for building ventilation. Based on our search, the well-mixed assumption was first introduced mathematically in 1946 by Lidwell and Lovelock, where the rate of contaminant ($c$) disappearance was equal to the volume of air entering and leaving the room ($v$) normalized by the volume of the room ($V$) [35]:

$$-{\displaystyle \frac{dc}{dt}}={\displaystyle \frac{v}{V}} c$$

(1)

This equation has three main simplifying assumptions: (a) the room is perfectly well-mixed, meaning that the concentration $c$ is spatially uniformly distributed, (b) the air leaving and entering the room are equal, meaning that the room is in full balance, and (c) the air entering the room is 100% fresh and is void of contamination. Later, Renbourn and colleagues (1949) took the same notion by formally defining the room as a control volume. The rate of concentration equals what enters the room minus what leaves it +/− any internal source or sink [36]. Their formulation made it possible to release two of the above assumptions, i.e., room pressure balance and 100% freshness. Yet, the well-mixed assumption was still held:

$$V{\displaystyle \frac{dc}{dt}}=v_s{C}_0-v_{e}C+S(t)$$

(2)

where $v_s$ and $v_e$ are supply and exhaust flow rates, respectively, $S\left(t\right)$ is an arbitrary function that defines an internal source or sink, and $C_0$ is the outside contaminant concentration. Depending on the functional form of $S\left(t\right)$, the above equation could have an analytical form. For example, if there is a sudden burst of contamination, meaning that $S\left(t\right)=\left\{\begin{array}{c}{S}_0 for t=0 \\ 0 for \forall t>0\end{array}\right.$, the above equation will take the below form:

$$C\left(t\right)=C_i e^{-\lambda t}$$

(3)

It can be shown that $\lambda$ = $v/V$ is the number of air changes per unit of time, also known as ACH. It is worth noting that, historically, these discussions were developed from the experimental/measurement standpoint. That is, at the early stages (1940 and before), scientists and engineers used $\lambda$ as an indirect approach to measure ventilation rates. However, it was recognized later (1945 and onward) that the same approach (i.e., well-mixed condition) could determine air change values, particularly in spaces with known sources of contamination. In the absence of rigorous scientific evidence, many of the ACH values recommended by ASHRAE could have emanated from this theory of the well-mixed space. Although the above theoretical framework is an extremely powerful tool for ventilation design and measurements, the assumption of a well-mixed space is unrealistic. While previous studies mainly approached the problem from the room control-volume perspective, Chen and colleagues (1969) took a different approach [37]. Instead of investigating the room as a control volume, Chen et al. (1969) [37] defined the age of air and proposed to measure it using the exit age of air probability distribution ($E(t)$). They showed that under a perfectly mixed condition, the exit age of air probability distribution takes the following form:

$$E\left(t\right)=\lambda e^{-\lambda t}$$

(4)

where $\lambda$ is equal to the room exit flow rate normalized by the room volume (i.e., ACH). One should note that since the room is perfectly mixed, the probability of contamination at the exit is equal to that in the space. It was further shown that the internal age of air distribution ($I\left(t\right)$) has the following relationship with the exit age distribution [38]:

$$E\left(t\right)=-{\displaystyle \frac{1}{\lambda }}{\displaystyle \frac{dI\left(t\right)}{dt}}$$

(5)

This problem formulation enabled the researchers to develop some theoretical understanding of a non-well-mixed (nWM) condition. To that end, the nWM space could be divided into $n$ perfectly mixed subspace where the volume of each subspace is $V/n$. Let us further assume that each subspace performs independently of the others, and the exit air of a subspace is passed along to the next until finally being discharged outdoors. Chen et al. then showed that the overall exit age of air distribution follows the below formulation:

$$E\left(\theta \right)=-n^n{e}^{-n\theta }{\displaystyle \frac{{\theta }^{n-1}}{\left(n-1\right)!}}$$

(6)

where $\theta$ is the dimensionless time, and it is equal to time ($t$) multiplied by air changes per unit time (e.g., ACH). Figure 7 shows the exit age of air distribution for several $n$ values.

5.2. Effect of Air Distribution

Starting with cleanrooms, practitioners focused on specific air distribution configurations aiming to maximize ventilation effectiveness by delivering clean air to where it is most needed. Therefore, the spatial uniformity of contaminant distribution was no longer a valid assumption. Moreover, the well-mixed assumption only works with one ventilation parameter to describe contamination level indoors, and that is air changes per time (e.g., ACH). As shown in the above formulation, the greater the ACH, the higher the rate of contamination decay in the space. This log–linear correlation was a limiting factor if one desired to study indoor air quality vs. energy consumption of the building. Plus, the system cannot provide an ultimate ventilation rate for temperature and contamination control since an excessive air velocity may cause a feeling of draft, which bothers the occupants. Thus, the challenge was to ensure that the supply air reaches the areas where it is needed with a reasonable airflow rate and minimum cost. Hence, attempts were invested in defining and formalizing other approaches that enable the designer to conceptualize, set, and measure the effectiveness of the ventilation system. In response, ventilation indices were defined to quantitatively represent the ventilation system’s effectiveness in delivering the ventilated air to the areas of interest to remove the contaminants originated from an internal source. We identified three distinct approaches to define ventilation effectiveness: (1) based on age distribution, (2) based on trace gas concentrations, and (3) based on local air properties.

5.3. Age of Air Effectiveness

The age of air was utilized to propose an early understanding of ventilation effectiveness. It must be noted that the two terms of ventilation effectiveness and ventilation efficiency, while having different literary definitions, were used arbitrarily in the literature, where they both carried the same meaning. One could define $F\left(t\right)$ as the fraction of the air elements with age less than or equal to $t$ [39]. Admittedly, $F\left(0\right)=0$ and $F\left(\mathrm{\infty }\right)=1$. Therefore, the mean age of air can be found by the following equation:

$$\mu ={\int }_0^{\infty }\left(1-F\left(t\right)\right)dt$$

(7)

With a little bit of algebra, one can show that the mean age of air ($\mu$) is equal to $\lambda$ (air change rate) for a perfectly mixed space, and it is $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right. \lambda$ for a perfect displacement (i.e., stratified piston motion). $\mu$ can be measured experimentally by releasing a trace gas inside the supply air duct and estimating $F\left(t\right)$. Another definition for the local age of air proposes injecting a pulse of trace gas in the inlet duct and monitoring its concentration, $C_j\left(t\right)$, over time at the point of interest,$j$. The age distribution at the point of interest, $F_j\left(t\right)$, becomes

$$F_j(t)={\displaystyle \frac{C_j\left(t\right)}{{\int }_0^{\infty }{C}_j\left(t\right)dt}}$$

(8)

where the term $t$ denotes the elapsed time since the particle is injected. Similarly, the average age of the air at an arbitrary point, ${\theta }_j,$ may be defined as follows:

$${\theta }_j={\int }_0^{\infty }{tF}_j\left(t\right)dt={\displaystyle \frac{{\int }_0^{\infty }{C}_j\left(t\right)tdt}{{\int }_0^{\infty }{C}_j\left(t\right)dt}}$$

(9)

While these two definitions seem to be different, they both lead to the same types of results. Additionally, the residence time was defined as another measure for ventilation effectiveness as the inverse of air change rate (i.e., $1/\lambda$) [37]. Sandberg (1981) offered the exact same definition and called it the mean recycle time for the system [8]. Skaaret (1986) defined air exchange effectiveness, $\beta$, as the ventilation system’s mean age of air divided by that for ideal displacement [40]. With this definition, a perfect displacement system has an air exchange effectiveness of β = 1, for a well-mixed room β = 0.5, and it is below 0.5 for stagnant flow due to poor air distributions [40].

5.4. Concentration-Based Effectiveness

Another way to define the mean age of air is the average time that a particle takes from the point entering the room to the measurement point. Sandberg (1981) defined the effectiveness of the ventilation system via (i) relative ventilation effectiveness; and (ii) absolute ventilation effectiveness [8]. Relative ventilation effectiveness represents how the ventilation varies across the room, and it is expressed as follows:

$${{\epsilon }^r}_j={\displaystyle \frac{{C^s}_f-C_t}{{C^s}_j-C_t}}$$

(10)

where ${C^s}_f$ represents the concentration in the exhaust, ${C^s}_j$ denotes concentration at the point $j$, and $C_t$ is the supply air concentration. Two points shall be made about this definition: (1) while $\epsilon$ can technically take any values, with a reasonable assumption that the contamination at the supply air is less than that in the room, then $\epsilon \in [0, \infty ]$. Higher values show greater ventilation performance. (2) Others further simplified this equation by setting the supply air concentration to zero and defining relative effectiveness as the ratio between the exhaust and local concentrations [41]. (3) If ${C^s}_j$ is replaced by the (spatial) average concentration of the room, the overall relative effectiveness is achieved. Later studies showed that the practical upper bound for the overall $\epsilon$ is about two for perfect displacement systems [42]. The above discussion on the mean age of air further substantiates these findings.

Relative ventilation effectiveness, as defined, does not shed light on the actual magnitude of peak concentration. In many applications, the peak concentration could be independent of the average concentration. Therefore, a second parameter was defined as absolute ventilation effectiveness, which determines the ability of the ventilation system to reduce the pollution concentration relative to the maximum theoretical value, and it may be written as follows:

$${{\epsilon }^a}_j={\displaystyle \frac{C\left(0\right)-{C^s}_j}{C\left(0\right)-C_t}}$$

(11)

where $C\left(0\right)$ denotes the initial concentration of the pollution in the room. Assuming that the room concentration is always greater than that in the supply air, one could establish that ${{\epsilon }^a}_j$ takes a value between zero and one, where a larger value means higher effectiveness in removing contamination from the space. Lastly, the same definitions have been offered for thermal comfort effectiveness by replacing concentration values with local and global temperature magnitudes [43].

5.5. Local Measures of Effectiveness

Sandberg (1981) first defined the local air exchange rate by taking an infinitesimally small, well-mixed parcel of air, for which we have the following [8]:

$${\displaystyle \frac{dC}{dt}}=-C{\displaystyle \frac{dQ}{dV}}$$

(12)

where $C$ is the local concentration, and $V$ is the volume of the air parcel. The definition of $Q$ needs additional explanation. First, $Q$ is an artificial quantity that does not exist, aiming to quantify the flow rate of contamination-less air into the parcel. Worthy of note, the incoming flow rate into any element is a mix of fresh and old air [8]. Assuming that $dQ/dV$ is independent of time, one could integrate both sides:

$${\int }_{C_0}^{C_t}d{C}_j=-{\displaystyle \frac{dQ}{dV}} {\int }_{t=0}^t{C}_{j}dt\to {\displaystyle \frac{dQ}{dV}}={\displaystyle \frac{C_t-C_0}{{\int }_{t=0}^t{C}_{j}dt}}$$

(13)

In this formulation, ${\lambda }_j={\displaystyle \frac{dQ}{dV}}$ is the local air exchange. It can be calculated for every point in the domain if the time series of local concentrations is available. Offermann (1983) used this notion and defined local effectiveness as the ratio between local and global air change rates (${\epsilon }_j={\displaystyle \frac{{\lambda }_j}{\lambda }})$ [44]. One could further define the overall system effectiveness if the concentration data is available for N different points within the room:

$$\widehat{{\epsilon }_j}={\displaystyle \frac{\sum _1^N{\lambda }_j}{N\times \lambda }}$$

(14)

In summary, three major points must be discussed. First, from a historical perspective, almost all of the above theoretical parameters were initially developed to help (passively) measure ventilation rates in a space. However, later studies (mainly in the 1980s and onward) utilized these approaches to recommend design setpoints (e.g., ACH) and rationalize their recommendations. Second, one can clearly see that ventilation parameters, such as ACR and flow rate, consistently appear in almost all of the above formulations. Although none of the documents reviewed explicitly mentioned the use of theoretical frameworks for recommending design parameters, it is fair to interpret that the scientists and standards bodies, such as ASHRAE, were familiar with this science and used it, at a minimum, to support or rationalize a change in their recommendations. Third, during the literature search and review, it was observed that these theoretical approaches and parameters were not equally used in different space functions. For example, the approaches for cleanroom ventilation design and measurement could use a different set of parameters than those used for healthcare facilities.

6. Conclusions

Ventilation has been studied for more than 100 years, as many people have observed a relationship between ventilation and desirable outcomes, such as human health, animal productivity, and equipment failure rates, to name a few. The intent of ventilation, the removal of contaminant(s) of concern, has largely stayed the same since the concept was introduced more than 100 years ago. However, the list of contaminants has changed, and the precision with which they are measured has significantly improved over that time. Each time new contaminants are introduced, or new instrumentation becomes available, engineers and scientists review established guidance related to achieving indoor environment goals.

The drivers of ventilation research have largely been to support societal and/or economic needs, i.e., “Necessity is the mother of invention”. As occupants observed results in infections, product quality defects, lack of animal productivity, etc., as they related to air, the motivation was set to enhance performance. Similarly, after the 1970s energy crises, energy performance and current environmental impacts drove much of the research. There were three identified outbreaks (1918 Influenza, tuberculosis, and measles) that drove ventilation research, all of which preceded the COVID-19 pandemic. Economic cycles, wars, and climate events will rise and fall in criticality, but they will return. The generation rate of ventilation knowledge is relatively slow, as seen from 198 referenced documents since 1867. A strong research agenda and steady funding are needed to solve ventilation-related challenges in a timely manner. Major mobilizations of studies may not provide timely results when history repeats itself again.

The ability to define contaminants of concern is of paramount importance in any environment where ventilation is required to achieve stringent indoor environmental quality goals. Most papers defined contaminants of concern and their approaches to addressing them even if no quantitative analysis was included that prescribed a solution. As early as 1884, Seddon, J.P. described a theatre as “…thoroughly ventilated” [19]. This is meaningless from a quantitative perspective, but it implies that engineers should provide as much air as possible to the space without being specific, as specificity was not easy to achieve at the time. Furthermore, an author at that time could not assume that an engineer had access to the staff or knowledge to measure flow accurately so a prescribed value would do little good. There seem to be two common themes across all investigated building types. First, the research and subsequent evidence seem to be problem-oriented. That is, a problem was encountered, and the researcher aimed to merely study that issue without regard to its potential desired or undesired consequences. For example, one of the very first problems in operating rooms was that the environment was too hot and humid and thermally uncomfortable for the doctors to perform surgery; hence, there is research on how ventilation can provide thermal comfort. This line of research initially ignores the potential impact that ventilation can have on infection transmission, and it focuses on thermal comfort only. Later on, explosive anesthetic gases become the problem, and the same research approach is observed. From a broader perspective, we discovered a significant body of research about the OR where there is a problem: regarding open wounds and post-surgery infections. Many examples of the like can be found in other space functions. Second is that research is almost always limited to the availability of a physical facility. That is, once the problem is identified, depending on what is available to the research team, a line of experiments is performed. This approach severely limits variation in the parameter of interest, and other confounding parameters, for that matter. The combination of these two issues has made the existing scientific evidence very sparse. The discovered evidence is analogous to a number of black dots in an n-dimensional white space, which makes hard conclusions extremely hard, if at all possible. Lastly, there have been remarkable advances in HVAC control mechanisms and the use of data-driven approaches to control building systems. However, and especially for buildings with sensitive requirements (such as hospitals and cleanrooms), there seems to be resistance towards such new technologies. The primary reason for this is a lack of proper knowledge and understanding of how we got to this point. That is why a critical review of past systems and approaches is valuable in convincing building code and standard developers to stay on board.