What we know and should know about ventilation

Ventilation is an important component of any building, even if it is achieved by opening windows or infiltration. It is without a doubt recognized as an essential means of providing good indoor air quality. Although described widely in the scientific literature, there are still a few incompletely resolved questions concerning ventilation. They include, among others: (i) How much ventilation is needed in a given building?; (ii) Which criteria should be used to determine ventilation?; (iii) What is the absolute minimum ventilation rate in a given building?; (iv) Can we use epidemiological data for setting ventilation requirements?; and (v) Can ventilation be used as an indoor air quality metric? This short article attempts to discuss some of these questions.

Introduction

Ventilation with outdoor air has always been our “ally” in reducing the concentration of air pollutants indoors and thus in mitigating discomfort and health risks, including the risk of infection from airborne pathogens. In the past, when buildings were less airtight, ventilation was obtained by infiltration through the building envelope, shafts, or operable windows (adventitious ventilation); today, it is usually achieved by specially designed systems that not always but predominantly are mechanical. The positive effect of ventilating rooms with outdoor air is only obtained if the outdoor is clean and less polluted than indoors, which is the usual assumption when designing ventilation and is frequently the case. Simulation of indoor exposures to outdoor air pollutants brought in by ventilation imply, however, that increasing ventilation without proper filtration and air cleaning will considerably increase the burden of disease caused by these pollutants [1]. These results underline the importance of ensuring that clean outdoor air is supplied indoors instead of assuming that outdoor air is clean. The few published studies showing that increasing outdoor air resulted in an increased risk of health seem to have made this flawed assumption [2].

The knowledge

Increasing building ventilation, which should be understood in this article as an increased outdoor air supply rate, has otherwise been documented and summarized in many reviews [3,4,5,6] to improve indoor air quality and benefit health, comfort, office work, and schoolwork. The positive effect of ventilation was demonstrated in the classical studies of Yaglou et al. [7], where ventilation requirements to control body odour were determined; their results were verified later in experiments performed in Denmark [8], USA [9] and Japan [10]; in each of these experiments, increasing ventilation improved indoor air quality by reducing odour intensity or the percentage dissatisfied with the air quality as estimated from subjective ratings of the acceptability of air quality. Evaluations of the air quality were made upon entering the space. Other experiments, with occupants remaining indoors until they were adapted to the indoor air quality showed no effect of increased ventilation on ratings of air quality when the air pollution source was body odour [11], but did show an improvement in perceived air quality when the air was polluted by emissions from building materials [12]. Today, these results form the basis for the ventilation requirements prescribed by ASHRAE Standard 62.1 [13] and EN 16798-1 [38]. Increased ventilation was also shown in many studies to reduce both the prevalence and intensity of non-clinical acute health symptoms known either as Sick Building Syndrome (SBS) symptoms [14] or building-related (BRS) symptoms [15] independent of the recall time of these symptoms. The risk of increased symptoms, expressed as the probability (odds) of symptoms being reported as a consequence of reduced ventilation, was estimated, i.a., by Sundell et al. [16] and Fisk et al. [17]. Finally, increased ventilation was shown to improve the performance of office work by adults [18] and of schoolwork (and hence learning) by children [19]. Other studies showed that improved ventilation could be expected to reduce short-term absence rates for both adults [20] and children [21,22]. Ventilation can also reduce the transmission of infectious diseases [23]. Although some control of the risk associated with these pathogens can be obtained by filtering the air with high-efficiency filters, by disinfection of air using, for example, UV-C, to achieve virus-free air, good ventilation with clean outdoor air is also essential. However, the epidemiological data does not provide clear evidence on the required ventilation rates that should be provided in buildings to reduce the risk of infection with airborne pathogens r [24]. Still, estimations using the Well’s-Riley model document this effect [25], as well some studies analysing outbreaks related to SARS-CoV-2 [26].

In many of the above studies, carbon dioxide (CO₂) was used as a proxy for ventilation [27,28]: The ventilation rate was not measured directly but was calculated using measured concentrations of CO₂ and assuming the metabolic rate of building occupants and good mixing of air within a space. Relationships were also created between the concentration of CO₂ and different outcomes, including health [29] and cognitive performance [19]. Rudnick et al. [25] developed a CO₂-based risk equation to estimate the risk of indoor transmission of infection by the airborne route. These results do not imply that CO₂ is a causative factor, although they are sometimes interpreted this way. CO₂ is simply a marker of ventilation efficiency (when occupants are present). Some studies published recently have shown that pure CO₂ can reduce some aspects of cognitive performance; specifically, decision-making in complex and time-stressed tasks, at levels as low as 1,000 to 2,500 ppm [30,31], the performance of pilots at 1,500 ppm [32], and some have even claimed that pure CO₂ can affect office-type work such as proof-reading at levels as low as 3,000 ppm [33], although other studies were unable to confirm these results [34,35]. The comprehensive reviews by Fisk et al. [36] and Du et al. [37] show the inconsistency of the results concerning the effects of pure CO₂ on cognitive performance at levels typically occurring indoors, i.e., below its permissible occupational limit of 5,000 ppm. They concluded that it is more likely that no effects are to be expected. They also show that no effects on health or comfort are to be expected at these levels. Thus, attributing the negative effects on building occupants only to CO₂ levels is incorrect. Increasing ventilation will, of course, reduce the concentration of CO₂ that has been emitted by building occupants (if present) and also the concentrations of pollutants emitted from many other sources of pollution indoors, including humans and their activities, building, and furnishing, used filters as well as smoking (if occurring) and combustion. These emissions are mainly responsible for the effects observed in the studies described above and should be the main focus rather than CO₂.

The problem

Despite the above evidence, some central questions about ventilation are still not completely answered. Among others, which outcome shall be used to prescribe ventilation requirements in buildings, and consequently, how much ventilation is necessary? This problem is nicely exemplified in Figure 1 showing how ventilation rates changed over time. The difficulty to set ventilation requirements was also the case when one of the first ventilation guidelines were proposed by the Chicago Commission on Ventilation in 1914 and in 1923 re-confirmed by the New York State Commission on Ventilation. No ventilation requirements were proposed back then, although setting ventilation was the main purpose of these committees. There were only general recommendations regarding the methods on how to achieve ventilation. Window-ventilated rooms with natural draft were the preferred method of ventilation proposed in these documents, but ventilation was not recommended to avoid ill health, but to avoid over-heating. Temperatures of 15–19°C in window-ventilated rooms were observed to cause the lowest prevalence of respiratory illnesses, so the resulting guidelines recommended 20°C with proper control of relative humidity for living rooms. CO₂ was not recognized as a harmful agent. Ventilation was basically required to control humidity, and this would be impossible if 100% of the air was recirculated.

Figure 1. Ventilation requirements in buildings – historical perspective (adapted from Nielsen and Li). [64]

Today, most of the major standards define ventilation requirements based on the resulting air quality as building occupants perceive it. They consider pollutants emitted by both humans and building materials, and stipulate the air quality and ventilation requirements based on the percentage of visitors dissatisfied with the air quality upon entering the space [38] or on the acceptability for both visitors and occupants [13]. These standards implicitly assume a connection between fulfilling comfort requirements and fulfilling the requirements for health, assuming that if the former is reached, the latter will be ensured as well. Brelih [39] and Dimitroulopoulou [40] showed that ventilation in numerous buildings does not comply with the requirements in the standards, while Asikainen et al. [41,42] showed that substandard ventilation in homes will result in an increased burden of disease, which is providing some support to the above assumption on health effects. It is still pertinent to ask whether other criteria than sensory discomfort should be used to determine ventilation requirements in buildings or whether the current requirements are sufficiently able to reduce the risks related to exposures that are not affected by air quality as perceived by building occupants.

The reasons given for the need for ventilation changed over time (Boxes 1 and 2). Figure 1 shows that ventilation requirements changed too. It shows that the requirements are nearly "everywhere" on the figure, and they differ by an order of magnitude. Tredgold proposed the lowest ventilation in 1836; he suggested that the minimum ventilation rates in mines should satisfy the physiological needs of a miner. It was set at 1.7 ℓ/s per person, of which 0.2 ℓ/s was for purging CO₂ from the lungs, 1.4 ℓ/s for removing the moisture produced by the body, and 0.1 ℓ/s for keeping a candle burning. One of the highest rates was proposed 170 years later using the results of two reviews of peer-reviewed literature on ventilation and its effect on man showing that at 25 ℓ/s per person, no negative effects on health symptoms and the performance of office work are to be expected [4,43]. Billings proposed the highest ventilation rate in buildings in 1893 to reduce the risk of spread of tuberculosis and recommended between 30 and 60 cfm/person (14–28.5 ℓ/s per person).

Figure 1 sends several messages. One of them is that it is difficult to define one ventilation requirement that will satisfy all demands and conditions. The reason is quite simple: different approaches were used to define ventilation requirements, different sources were controlled, and different outcomes were managed. If comfort (sensory perception of indoor air quality) is a design parameter and the target is 20% dissatisfied (80% acceptability) and humans are the major source of pollution, the ventilation rate would be about 10 ℓ/s per person as reflected in the current ventilation standards; this supports the widely accepted CO₂ concentration of 0.1% (1,000 ppm) proposed by Pettenkofer [44]. However, if health is considered, the rates required can be much higher and as high as 28.5 ℓ/s per person, as proposed by Billings. The question then is whether we can propose a ventilation rate that will satisfy all needs and criteria? In other words, can we develop a consistent framework for setting ventilation requirements instead of changing these requirements depending on the current contextual needs, assumptions made, and criteria defined? The answer seems to be affirmative, considering that an example of such a framework for ventilation requirements based on health criteria was proposed by the HealthVent project [45].

Setting ventilation requirements is only part of the problem. Scientific and technical literature shows that the design, operation, and maintenance of systems providing the air for ventilation have not always been adequate, resulting in the ventilation systems themselves becoming a strong source of pollution that can increase exposures and consequently increase health risks (e.g., [15,46,65]). The list of reported problems is long. The most common faults include insufficient air inlet size causing a loss of pressure, missing condensate drains for in-ground air heat exchangers and/or ventilation devices, no insulation of ducts conveying cold air (so condensate is formed), poor maintenance of filters, low class of filtration, inaccessible and dirty filters seldom changed for new ones, missing sound attenuators, improper cross-sections of ducts causing the air velocity to be too high or too low, inappropriate material of pipes (flexible tubes), improper location of main air intakes and exhausts, too short distances between air intakes and exhausts, supply and exhaust air openings that cause ventilation to short-circuit, partially or fully blocked (covered) air terminals. These problems have been observed irrespective of building type, and guidelines to deal with them are needed; an example of such a guideline that was developed by the HealthVent project is shown in Box 3.

The solution (?)

When discussing the criteria that should be used to define ventilation, we have to keep in mind the purpose and role of ventilation in buildings. This is presented in Figure 2. It shows that ventilation is used to reduce exposure to air pollutants, that it is exposure to pollutants that affects human response (not the ventilation), and that ventilation can only be a part of any solution to reduce the exposure. We learn that exposure can also be reduced by other means, including source control, i.e., reducing emissions from products used in buildings or capturing the pollutants at their source, filtration, and air cleaning. Ventilation can be used instead of these solutions, together with them, or once the other solutions are in place and ventilation is the last way of improving indoor air quality, to reduce the risks related to exposures that could not be reduced by other means; in each of these cases, it is, of course, assumed (as pointed out earlier) that the air supplied indoors is clean and this precondition must always be satisfied.

Figure 2. Ventilation is a mediating factor, not a cause.

When ventilation is used as the primary measure to reduce exposure, different literature reviews of ventilation indicate that a ventilation rate of 10 ℓ/s per person [15], 15 ℓ/s per person [29] or 25 ℓ/s per person [4,43] would be necessary to keep indoor air quality high and ensure that health risks and the risk of reduced cognitive performance for building occupants are both kept low; a single study by Federspiel et al. [47] suggests even higher rates might be necessary.

When ventilation is used as a secondary and complementary measure for controlling exposures once other means have been exploited, we need to know the minimum (base) ventilation requirement that must be supplied in buildings when people are present. Different approaches can be used to determine the base ventilation requirement. We have already mentioned the 1.6 ℓ/s per person that Tredgold proposed for miners. Viessman [48] proposed that the minimum ventilation rate that should be ensured to provide oxygen (O₂) for breathing is about 1 cfm/person (ca. 0.5 ℓ/s per person). Following occupational hygiene standards and keeping CO₂ below 5,000 ppm, Sundell [49] proposed 4 ℓ/s per person (with a safety factor of 5); 3 ℓ/s per person was proposed to keep humidity below 45% when only moisture emitted from humans was considered, a rate similar to what was proposed by Viessman [48]. A different approach was used by Carrer et al. [45]. They reviewed the peer-reviewed epidemiological literature and reported the minimum ventilation rates at which no effects for different outcomes were seen. For example, the lowest ventilation rate at which no elevated risk of asthma and allergic symptoms was observed in the published literature was 7 ℓ/s per person [50], while 8 ℓ/s per person was the lowest in the case of acute non-clinical health symptoms in homes [51] and 9 ℓ/s per person in offices [52]. These rates can be considered as the best available tentative and empirically determined estimates of the lowest rates with no observable adverse health effects. For the sake of comparison, if the performance of office work and schoolwork are considered, these rates were higher, about 16-24 ℓ/s per person, and if short-term sick leave is considered, they were 24 ℓ/s per person. Numerous limitations of these rates were indicated by Carrer et al. [6], restricting their generalization: (i) the data were incomparable or difficult to compare; (ii) exposures were improperly characterized; (iii) there were no data on indoor pollution sources, including the maintenance of ventilation systems; (iv) it was assumed that outdoor air was clean (unpolluted); (v) health outcomes were insufficiently characterized and included mainly self-estimated acute symptoms with no data on chronic health effects; (vi) ventilation was inadequately characterized and based often on crude measurements of the ventilation rate; (vii) exposed populations and their sensibility were poorly characterized; and (viii) experimental designs were weak. Considering these limitations, Carrer et al. [45] proposed another ventilation rate as minimum base: 4 ℓ/s person. This rate was assumed to be sufficient to keep the risk for acute health symptoms low, but only if the World Health Organization’s air quality guidelines [53,54,55,56] are met, and the major source of pollution are the occupants.

The risks of infectious diseases were not considered by Carrer et al. One of the reasons was insufficient data. A single study by Sun et al. [57] performed in student dormitory rooms in China that were occupied by more than one student showed that the risk of the self-reported common cold is reduced if ventilation is increased until 5 ℓ/s per person and that any further increase in the ventilation rate will provide only a very small benefit. Generalizations of this result to other buildings and other occupant densities are difficult but a recent study of Li et al. [26] provides some support in which ventilation rates as low as 0.9 ℓ/s per person were suspected to cause the outbreak of COVID-19 in the overcrowded restaurant. Assuming that mild infections cause short-term absence rates, a study by Milton et al. [20] showed that ventilation rates of 24 ℓ/s per person would significantly reduce the risk of absence compared with 12 ℓ/s per person. However, these results were obtained in a cross-sectional study and were not confirmed in intervention studies where the levels of ventilation were about 40–45 ℓ/s per person [58]. Still, another study showed that virus might survive on a filter at ventilation rates as high as about 40 ℓ/s per person, the rate being back-calculated from the measured CO₂ above the outdoor level of 100–200 ppm [59]. With respect to the current COVID-19 pandemic, REHVA recommends ventilation rates that ensure that CO₂ is at or below 800 ppm (i.e., around 10 ℓ/s per person), while ASHRAE recommends that the rates should meet at least the minimum code requirements with improved filtration efficiency [60]. If we consider that during a pandemic, the risk of infection due to airborne pathogens in buildings will be similar to what occurs in hospital wards (patient rooms), it may be inferred that a total ventilation rate corresponding to 6 h⁻¹ would be necessary with high-efficient filtration, of which 2 h⁻¹ would be outdoor air; these are the rates prescribed by ASHRAE Standard 170 for health care facilities [61]. The rate of 6 h⁻¹ would correspond to 17–20 ℓ/s per person in a 100 m² classroom with 25 students or about 5 ℓ/s per m² floor, of which the outdoor component should be at least 30% as in ASHRAE Standard 170 [61]. Hence, this outdoor component would be around 20 ℓ/s per person, which is close to what was found by Milton et al. [20] to reduce absenteeism in practice.

With the evidence described above we can assume that the ventilation rate in buildings can be anywhere between 4 and 25 ℓ/s per person, as also shown in Figure 1.

The future

There are many beliefs regarding ventilation, and many are only partly true. Among the few, it is assumed that more ventilation will always improve indoor air quality, that low ventilation rate always means poor air quality, that it is simple to measure ventilation, that ventilation can be used as a metric predicting human responses, that outdoor air and the air supplied indoors is clean, that ventilation systems are clean, and that the air indoors is fully mixed within the air volume. Irrespective of the different beliefs and opinions about ventilation, and whether or not they are true, we need ventilation in buildings and it must satisfy a number of different criteria. It must be reliable, flexible and well-functioning, adaptable, and responsive to different needs and unusual events. The urgent task of defining ventilation requirements that meet these different criteria must be solved as a high priority and may require out-of-the-box thinking and new advanced solutions for ventilation [62]. In other words, a new paradigm and a framework for defining ventilation in buildings are urgently required.

More research is also required. Current knowledge should be used and supplemented by population- representative measurement campaigns of indoor exposures in all major building types to support improved design. They will fill the gaps in knowledge on the effects of ventilation (and indoor exposures to poor air quality) on health and other relevant human responses. These measuring campaigns should include much better characterization of ventilation and exposure than was achieved earlier and should examine in detail the influence of indoor and outdoor sources of pollution on chronic health problems, and determine the environmental conditions responsible for the most severe exposures. Particular consideration should be given to people with special needs, such as patients with chronic respiratory diseases, the elderly, and children.

Epilogue

Let us conclude by quoting the 1964 version of “Basic principles of ventilation and heating” by Bedford. He wrote back then that “Great care is devoted to ensure that we have a pure water supply, and no one would suggest that in the interest of economy we should be doomed to drink polluted water. On aesthetic grounds alone, it should be one’s right to be allowed to live and work in a clean atmosphere which is free of objectionable odours”. Independently of Bedford, WHO [63] endorsed this opinion by publishing the document “The Right to Healthy Indoor Air”. This document states that “Under the principle of the human right to health, everyone has the right to breathe healthy indoor air.” Ventilation is and will continue to be one of the means to safeguard this principle. Therefore, we must agree on consistent principles and criteria for defining ventilation requirements in buildings occupied by people and follow them strictly. These criteria should consider different effects on building occupants and ensure that the systems are designed to fulfil their needs and are used efficiently. The paradigm change required should not only concern new design requirements but also new ventilation solutions and new ways to design, operate and maintain them that take account of all of the costs, not only the cost of energy but also the cost of the negative consequences of poor ventilation for health, productivity and socio-economic wellbeing. The new paradigm for ventilation in buildings should have every single building occupant at the centre of all recommendations.

Acknowledgments

This article is mainly based on a presentation made during the special workshop organized by REHVA at the CLIMA 2019 conference in Bucharest, Romania; parts of it were also included in a plenary talk at the Healthy Buildings Europe conference organized in 2017 by ISIAQ in Lublin, Poland. It includes the HealthVent project results supported by a grant from the European Commission, Executive Agency for Health and Consumers (Contract no 2009 12 18) and carried out between 2011 and 2013. Many thanks go to the entire HealthVent Team for their outstanding collaboration. Special thanks go to Prof. Olli Seppänen for his encouragement to prepare this article.

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