30+ years of knowledge creation: Indoor Air 1991–2021

Abstract

The Indoor Air journal was established in 1991. As stated on the inside cover of the first issue, the goal “is to provide a single, identifiable forum for reporting original research in the broad area defined by the indoor environment of non-industrial buildings.” Among the key features of the journal are these three: it is international, multidisciplinary, and a research journal.1 The importance of being an international journal is that what is learned in one geographic area can be relevant for other regions. Being multidisciplinary means that the journal is open to the full set of topics relevant to indoor environment and health. Also, significantly, this journal aims to serve a broad and diverse readership. As a research journal, the articles describe systematic investigations that create new knowledge. The journal's aims and scope remain largely intact from its founding. As stated on the inside of the current (virtual) cover, “The research results will provide the basic information to allow designers, building owners and operators to provide a healthy and comfortable environment for building occupants.” As to being a journal about health, the 1946 definition in the Constitution of the World Health Organization2 is useful: “Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.” The research published in this journal is motivated by the goal of making buildings more supportive of the health and well-being of occupants. Since its founding, the journal has experienced sustained and substantial growth in scale and influence. Measured by the number of articles published, it has tripled in size during the quarter century between 1991–1995 and 2017–2021. Over that time, the journal's impact factor rose from less than 1.0 when it was first ranked, to consistently above 4.0 beginning about a decade ago,3 to 6.55 in the most recent (year 2021) evaluation. It ranks among the better journals in the three areas in which it is grouped: Construction & Building Technology; Environmental Engineering; and Public, Environmental & Occupational Health. Cumulatively, ~1700 research papers were published in Indoor Air during the years 1991–2021. To identify major topics addressed in the journal, I selected a subset of 64 key papers, spanning the full 31-year publication history, based on either high citation rates or having been previously chosen as a “best paper” award winner.4 From the titles and abstracts of these 64 papers, I identified seven major clusters or themes. The remainder of this editorial highlights these themes, emphasizing what we have learned as reported in this journal. The full list of 64 key papers is provided in the Supporting Information. Before proceeding, I want to emphasize two points. First, these seven themes do not fully represent the scope of the 64 key papers. The breadth and diversity of topics covered in the journal extends well beyond the scope of this editorial. Second, the seven topics are much more substantially addressed in the totality of the journal than briefly summarized here. To substantiate this latter point, a set of Web of Science searches was conducted to identify articles published in Indoor Air during 1994–2021, with the search topic being a key word from one of the seven themes (SBS, dampness, school*, biomass, infect*, thermal, residen*, where * is a wildcard). The seven searches yielded a median of 197 articles (range = 85 to 299). The key point is that each of these seven topics has been addressed by a substantial proportion of the totality of research published in Indoor Air. The term, “sick building syndrome” (SBS) was coined in the early 1980s to describe the occurrence of nonspecific symptoms associated with building occupancy, such as headache and eye irritation.5 Research efforts have focused substantially on office buildings. During the heating seasons of 1993–1994, Bluyssen et al. surveyed more than 6500 occupants of 56 office buildings spanning 9 countries.6 In all, 27% of the occupants surveyed found the indoor air quality at their workplace was “not acceptable.” The self-reported prevalence of lethargy, headache, and dry eyes was particularly high. Several studies have reported an association between ventilation system characteristics or ventilation rates and SBS symptoms in offices. An extensive survey in the Netherlands found that the “prevalence of symptoms was higher in air-conditioned buildings than in naturally or mechanically ventilated buildings.” 7 Review articles synthesized evidence in terms of the influence of outdoor-air ventilation rates. Seppänen et al.8 wrote, “Almost all studies found that ventilation rates below 10 L/s per person in all building types were associated with statistically significant worsening in one or more health or perceived air quality outcomes.” Fisk et al.9 elaborated, “As the ventilation rate drops from 10 to 5 L/s per person, relative SBS symptom prevalence increases approximately 23% … and as ventilation rate increases from 10 to 25 L/s per person, relative prevalence decreases approximately 29%.” Other investigations have highlighted the role of volatile organic compound (VOC) exposures. In an early study that describes the potential importance of oxidative chemistry indoors, Wolkoff et al.10 concluded that, “Chemical reactions between oxidizable VOCs and oxidants, such as ozone … can form irritants which may be responsible for the reported symptoms.” Ten Brinke et al.11 developed exposure metrics that utilized clustered VOC data. The authors reported, “Using data from 22 office areas in 12 California buildings, seven VOC exposure metrics were developed and their ability to predict self-reported SBS irritant symptoms of office workers was tested.” Their results “show a link between low-level VOC exposures from specific types of indoor sources to SBS symptoms.” In an experimental study conducted in Denmark, Wargocki and collaborators12 had subjects carry out simulated office work in a room that either contained, or did not, a VOC-emitting used carpet situated behind a screen. They reported that, “Reducing the pollution load on indoor air proved to be an effective means of improving the comfort, health and productivity of building occupants.” Synthesizing the field and laboratory evidence, several studies have concluded that building factors are important contributors to SBS prevalence and that the economics are favorable for intervening to improve conditions. Mendell13 wrote, “Overall evidence suggested that work-related symptoms among office workers were relatively common, and that some of these symptoms represented preventable physiologic effects of environmental exposures or conditions.” Fisk and Rosenfeld14 assessed the cost–benefit relationship, finding that, “Characteristics of buildings and indoor environments significantly influence rates of respiratory disease, allergy and asthma symptoms, sick building symptoms and worker performance.” They concluded that the “potential financial benefits of improving indoor environments exceed costs by a factor of 18 to 47.” Milton et al.15 determined that, “currently recommended levels of outdoor air supply may be associated with significant morbidity, and lost productivity on a [US] national scale could be as much as $22.8 billion per year.” Another broad concern is the occurrence of dampness in buildings along with associated adverse health risks. As moisture and dampness also affect the prevalence of fungi, I have bundled this topic with the broader interest in microbiomes of the built environment. Over the past few decades, several book-length reviews have been prepared addressing this theme.16-19 Members of ISIAQ, who have published in Indoor Air, contributed as coauthors to each of these volumes. Bornehag et al.20 undertook a systematic multidisciplinary review of the relationship between building dampness and health effects in the Nordic context. Their report concluded that “’dampness' in buildings appears to increase the risk for human health effects in the airways, such as cough, wheeze and asthma.” They went on to say that “Even if the mechanisms are unknown, there is sufficient evidence to take preventive measures against dampness in buildings.” Considerable effort has focused on understanding causal processes, with several candidates identified, but no firm conclusions. To illustrate, consider the results of an experimental study in Finland,21 in which it was found that the bacterial strains associated with moldy building materials “are potent inducers of inflammatory responses and thus possibly related to adverse health effects of the inhabitants.” In a major review by Nevalainen and colleagues,22 the authors stated that “assessment of fungal exposures is notoriously challenging due to the numerous factors that contribute to the variation of fungal concentrations in indoor environments.” Regarding the possibility of exposure to mycotoxins produced by fungi, they noted that “just as microbes are everywhere in our indoor environments, so too are their metabolic products.” In a Swedish study, Lorentzen et al.23 showed that the wood preservative pentachlorophenol could be transformed by microbes into chloroanisoles, which have a low odor threshold. They speculated that these species could “contribute to adverse health effects by evoking odor which, enhanced by the belief of the exposure being hazardous, induces stress-related and inflammatory symptoms.” During the past few decades, we have come to a profound new understanding of the importance of the human microbiome for health. We are also learning that there are two-way interactions between the human microbiome and the microbiology of indoor spaces. With the advent of quantitative PCR and related DNA-based measurement tools, researchers have been able to probe more deeply into the microbiology of indoor environments. Qian et al.24 sampled airborne size-resolved bacteria and fungi in a university classroom, finding much higher levels when the room was occupied than when it was vacant. They quantified the average bioaerosol emissions rate per occupant to be 37 million genome copies per hour for bacteria and 7 million genome copies per hour for fungi, the majority of which they attributed to shedding and resuspension. Based on their microbiological investigations in a university building, Meadow et al.25 stated that “Both occupancy patterns and ventilation strategies are important for understanding airborne microbial community dynamics in the built environment.” Degraded indoor environmental quality in schools might adversely affect student learning. Such was the conclusion of review articles published almost two decades ago. Daisey et al.26 wrote that “ventilation is inadequate in many classrooms, possibly leading to health symptoms.” Mendell and Heath27 concluded that “immediate actions are warranted in schools to prevent dampness problems, inadequate ventilation, and excess indoor exposures to substances such as NO2 and formaldehyde.” Two large experimental studies conducted in the United States reinforce and expand the findings of these early reviews. Haverinen-Shaughnessy et al.28 measured 100 elementary-school classrooms in the southwestern United States and found that 87 had ventilation rates below the guideline value of 7.1 L/s per person. They determined that, within the range 0.9–7.1 L/s per person, each 1 L/s per person increase in the ventilation rate increased the proportion of students passing a standardized test by 2.9% for math and 2.7% for reading. Analogously, Mendell et al.29 studied ventilation and student performance in 150 classrooms in California and their findings “suggest potential small positive associations between classroom [ventilation rates] and learning.” Globally, billions of people eat food that is cooked over an open flame using solid fuels. The associated air pollution exposures are extraordinarily high with severe adverse health effects expected as a result. Kirk Smith, a member of the ISIAQ community, was a pioneer in studying this subject. In 2002, he wrote30 that “indoor air pollution … from household cooking and space heating apparently causes substantial ill-health in developing countries where the majority of households rely on solid fuels.” Armendáriz Arnez et al.31 measured particulate matter concentrations and exposures in rural Mexican households contrasting a traditional open stove with an improved Patsari stove, which has an enclosed combustion chamber and flued exhaust. Installation of the improved stove “resulted in 74% reduction in median 48-h PM2.5 concentrations in kitchens and 35% reduction in median 24-h PM2.5 personal exposures.” However, even with the improved stoves, the median indoor 48-h PM2.5 level was about 200 μg/m3, well above health-based target values for outdoor air. Field and laboratory experiments have probed the health consequences of exposure to cookstove smoke from wood and other biomass fuels. Dutta et al.32 reported an association of hypertension with biomass use in a study in West Bengal, contrasting exposures with the use of a cleaner combustion fuel, liquified petroleum gas. Hawley and Volkens33 conducted an in vitro test using a line of bronchial epithelial cells and reporting that “cells exposed to emissions from the cleaner burning stoves generated significantly fewer amounts of pro-inflammatory markers than cells exposed to emissions from a traditional three-stone fire.” Excessive indoor pollution can create community-scale air quality problems. In a study of a very densely populated neighborhood in Dhaka, Bangladesh, Salje et al.34 reported that “indoor pollution in clean fuel households may be determined by biomass used by neighbors. … Community interventions to reduce biomass use may reduce exposure to high concentrations of PM2.5 in both biomass and non-biomass using households.” Airborne infectious disease transmission indoors has been an important subject of research reported in Indoor Air for at least the past two decades. Rudnick and Milton35 introduced, as a quantitative indicator of infection risk, the “rebreathed fraction,” which is estimated through measured levels of carbon dioxide, a marker of exhaled breath.36 They reported that “there is likely to be an achievable critical rebreathed fraction of indoor air below which airborne propagation of common respiratory infections and influenza will not occur.” Following the SARS epidemic of 2002–2004, infectious disease research efforts were substantially boosted in the indoor air community. In 2005, Li and colleagues37 reported on a retrospective investigation of a SARS outbreak in a hospital ward, which “demonstrated the cross-infection risks of respiratory infectious diseases in hospitals if a potential highly infectious patient was not identified and isolated.” In a systematic multidisciplinary review, Li and collaborators38 determined that “there is strong and sufficient evidence to demonstrate the association between ventilation, air movements in buildings and the transmission/spread of infectious diseases such as measles, tuberculosis, chickenpox, influenza, smallpox and SARS.” Airborne infectious disease transmission received another large boost of research attention because of the COVID-19 pandemic, which is caused by the SARS-CoV-2 virus. In 2021, Lidia Morawska, a long-time leader in the ISIAQ community, was recognized in Time Magazine as among the 100 most influential people that year.39 The citation reads, in part, “Lidia Morawska stands out among peers for her work in recognizing the importance of aerosol transmission and marshalling the data that would convince the World Health Organization and other authoritative bodies to do the same. … Her advocacy helped change practices everywhere from schools to workplaces, making these environments safer for more people around the world.” In 2006, Morawska40 reviewed the aerosol aspects of virus transmission indoors. “Every day tens of millions of people worldwide suffer from viral infections …. There is, however, only minimal understanding of the dynamics of virus-laden aerosols, and so the ability to control and prevent virus spread is severely reduced, as was clearly demonstrated during the recent severe acute respiratory syndrome epidemic.” Recent papers in Indoor Air have reported on the spread of COVID-19 in indoor environments. Miller et al.41 undertook a retrospective assessment of an early superspreading event in the United States. “An outbreak occurred following attendance of a symptomatic index case at a weekly rehearsal … of the Skagit Valley Chorale … After that rehearsal, 53 members … among 61 in attendance were confirmed or strongly suspected to have contracted COVID-19 and two died. Transmission by the aerosol route is likely.” Qian et al.42 synthesized evidence about the importance of indoor environments as sites of transmission. “Case reports were extracted from the local Municipal Health Commissions of 320 prefectural municipalities in China. … All identified outbreaks of three or more cases occurred in indoor environments, which confirm that sharing indoor spaces with one or more infected persons is a major SARS-CoV-2 infection risk.” These works are especially important because indoor aerosol transmission of SARS-CoV-2 was not widely understood to play such an important role early in the COVID-19 pandemic. The design and operation of isolation rooms in healthcare systems are important for controlling infectious disease transmission. Qian and Li43 combined full-scale chamber experiments with computational modeling to investigate how ventilation system design would influence the removal process for respiratory emissions. They reported that “gaseous [species] and fine particles were … removed more efficiently by ceiling-level exhausts than by floor-level exhausts. Large particles were mainly removed by deposition rather than by ventilation. … An improved ventilation design is hence recommended.” An outbreak of the Middle East respiratory syndrome occurred in a Korean hospital in May 2015. Xiao et al.44 investigated, using a multiagent modeling framework. 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Symptom Clusters