Data centers are essential in today's digital world, supporting everything from cloud computing to artificial intelligence. As digital demand grows, so does the need for data center infrastructure. Despite challenges like limited power supply, North America's data center inventory grew by 24.4% year-over-year in Q1 2024.1 Globally, data storage is expected to double from 10.1 zettabytes in 2023 to 21.0 zettabytes by 2027.2,3 Rapid growth increases HVAC demand for higher heat loads while maintaining efficiency and reliability.

Indoor air quality (IAQ) and energy efficiency are often perceived as competing priorities in building operation. However, artificial intelligence (AI) offers tools that may synergistically optimize both, but its promise must be weighed against challenges in deployment and management. Drawing on insights from the Stanford IAQ Forum, ASHRAE Guideline 36, and emerging AI deployments in HVAC optimization, this paper explores how AI-enabled control systems can enhance IAQ while reducing energy waste. By leveraging high-frequency sensor data and standardized control sequences, AI can unlock real-time optimization, fault detection, and adaptive performance. This approach supports the implementation of IAQ performance standards without sacrificing sustainability or cost-effectiveness. Interim, scalable approaches are needed, as broad adoption faces technical, economic, and organizational barriers.

Clean indoor air is vital for health in all settings, especially in locations where extreme climates restrict outdoor activity, such as in the United Arab Emirates (UAE). UAE summer temperatures routinely exceed 42°C (108°F), discouraging outdoor activity and limiting natural ventilation of structures. Yet, little research is available on indoor air quality in the UAE. To inform the design of a new indoor air quality program, the Abu Dhabi Public Health Centre commissioned a study to characterize and prioritize knowledge gaps on indoor air quality and its relationship to health in the UAE and to identify potential partners for the program. Research gaps and priorities were identified by a panel of 16 international and local indoor air quality experts through a two-day structured, in-person workshop and follow-up survey. Key partners were identified through a stakeholder mapping exercise and e-mail survey of 79 government agencies and nongovernment organizations. The expert panel concluded that the most important short-term research need is to characterize the major sources of indoor air pollution and the most frequently occurring pollutants. The panel recommended establishing a national indoor air quality observatory encompassing a wide range of settings, including residences, schools, mosques, healthcare facilities, shopping malls, and other public spaces. Indoor air quality monitors would be permanently placed to establish baseline indoor air quality, provide data to estimate source contributions, and enable tracking of changes over time. The stakeholder mapping exercise identified ten agencies that should be involved in planning, including the Abu Dhabi Public Health Centre, Department of Health–Abu Dhabi, Environment Agency–Abu Dhabi, Abu Dhabi Department of Energy, and Emirates Public Health Association. While focused on the UAE, the methods and research priorities in this study may be useful for planning indoor air quality improvement campaigns in other high-income nations.

BACKGROUND:Engineering infection controls include a wide range of interventions used indoors to reduce occupants' exposure to respiratory pathogens. PURPOSE:To identify and describe primary studies evaluating the effects of engineering infection control interventions designed to reduce the spread of respiratory infections transmitted through indoor air. DATA SOURCES:MEDLINE, Embase, Global Health, Cochrane Central Register of Controlled Trials, CINAHL, Scopus, and Environmental Science Collection from database inception to 12 December 2023. STUDY SELECTION:English-language primary research articles evaluating engineering infection control interventions. DATA EXTRACTION:Publication information, population characteristics, intervention details, and all relevant outcomes were abstracted by a reviewer and verified by a second, senior reviewer. DATA SYNTHESIS:A total of 672 studies published between 1929 and 2024 were identified. Most (n = 606) evaluated environmental samples only, 57 included human participants, and 9 included sentinel animal subjects. About half of the studies included at least 1 intervention classified as pathogen inactivation (n = 405), with fewer involving pathogen removal (n = 200) or air exchange or dilution (n = 143). Across all studies, about half (n = 332) measured the quantity of viable nonpathogenic organisms from air samples, followed by the quantity of nonbiological particulates (n = 197) or viable pathogenic organisms (n = 149). Harms, such as toxic byproducts, were rarely measured. LIMITATION:Exclusion of non-English-language publications and gray literature. CONCLUSION:There is substantial heterogeneity in the available evidence. Gaps in evidence include studies measuring efficacy outcomes that are highly relevant for human infection transmission or harms. Refinements in classification of interventions and outcomes could strengthen reporting of these evaluations. PRIMARY FUNDING SOURCE:National Institute for Occupational Safety and Health at the Centers for Disease Control and Prevention. (Registered on Open Science Framework [https://osf.io/5zmhd]).

Secondary organic aerosol (SOA) forms indoors when ozone reacts with terpenes, generating a range of low- and semi-volatile compounds, over 50% of which partition into the particle phase. This study investigated the formation of SOA in indoor spaces under heterogeneous thermal conditions resulting from the combined effects of HVAC systems and heat emitted by human occupants. The core of this study involved integrating the volatility basis set (VBS) model with computational fluid dynamics (CFD) simulations. The resulting VBS-CFD framework was used to simulate SOA formation from ozone-terpene reactions, with terpenes originating from human emissions. Model accuracy was assessed using experimental data from previous measurement studies and a material balance model. Results indicate that semi-volatile compound concentrations are substantially higher near occupants compared to ambient levels, while SOA concentrations are lower near humans due to temperature gradients. The study results further revealed notable spatial variability in SOA concentrations under both cooling and heating scenarios, despite maintaining a consistent average indoor temperature. These findings highlight the important role of semi-volatile compounds in influencing particle concentrations near occupants, with over 50% of these compounds potentially contributing to aerosol formation-and thereby increasing human exposure to indoor aerosols.

Poor indoor air quality (IAQ) is one of the world's largest unmet public health threats. Although we spend 90% of our time indoors, building codes base IAQ requirements on comfort and harm prevention rather than health and wellness. Post-construction, there is little regulation of IAQ. The cost of unhealthy indoor air is massive, with US healthcare spending of >$45B for respiratory infections and >$39B for allergies and asthma. Yet, while health risks from unhealthy indoor air are increasing due to increasing wildfire frequency, longer allergy seasons, and new respiratory viruses, the momentum for developing and implementing mandatory health-based IAQ standards in the aftermath of the COVID-19 pandemic has waned, with little evidence to date of concrete policy and regulatory advances. In response, this paper outlines a practical, feasible, and actionable policy and regulatory roadmap to achieve health-based IAQ standards, anchored in a bottom-up approach linked to a strategic plan that can scale nationally. First, we review the public health, economic, and national security case for health-based IAQ standards. Second, we establish guiding principles, primary goals, and current state assessment of health-based IAQ standards relative to our guiding principles and primary goals. Third, we present two minicase studies focused on national energy and life safety codes, which are possible models of the road ahead for health-based IAQ standards. Fourth, we define an action-focused critical path, with clear phases and key stakeholder roles and responsibilities, to achieve health-based IAQ standards. Finally, we conclude with a call to action to provide clear direction for future efforts.

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

If some countries lead by example, standards may increasingly become normalized

Due to increasing concerns related to airborne virus spread indoors, more reactive species air cleaners are being utilized in classrooms. Reactive species generated by air cleaners decompose airborne pathogens chemically, decreasing the risk of infection. Due to the high reactivity of these oxidants, reactive species may be distributed nonuniformly in indoor environments, as are viral aerosols emitted by infectors. Heterogeneous distributions of reactive species may cause spatially non-uniform removal rates of viral aerosols. However, there is little information regarding spatial distributions of either reactive species or viral aerosols in ventilated classrooms. Thus, the objective of this study was to investigate spatial distributions of reactive species and infectious aerosols and to examine how operating conditions of air cleaners affect viral aerosol removal rates. A CFD model simulated the operation of a reactive species air cleaner generating hydrogen peroxide (H2O2) in a mechanically ventilated 237 m3 classroom with nine occupants. The reactive species air cleaner showed a 3-20 times higher equivalent air change rate to a HEPA filter air cleaner with the same inlet and outlet flows. During the operation of reactive species air cleaners, elevated viral aerosol concentration was confined to regions near infectors. This was due to the high reactivity of reactive species, decreasing the infection probability of receptors from 3.2% to 0.1% with a 1-hour exposure time. As the room average concentration of reactive species increased from 15.6 to 50.4 ppb, both below the US Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL) of 1000 ppb, the room average infection probability decreased from 0.3% to 0.1%. Due to the residence times of reactive species, the location of reactive species air cleaners affected the inactivation rate of viral aerosol, resulting in a 24% variation of concentration difference of infectious aerosol with air cleaner locations.

A hybrid network integrating absorption and compression chillers is a suitable solution to respond to the variable cooling demand in a large-scale chiller plant, in regions with multiple energy sources. However, the design and control challenges of such networks remain uncharted, lacking a comprehensive approach. This study presents a general procedure for designing a hybrid chiller network for a building with an arbitrary annual cooling demand distribution. The procedure determines the optimal configuration considering the different capacity ratios of absorption to compression chillers, chiller numbers, and arrangements. Utilizing the Particle Swarm Optimization algorithm, the optimal chiller loading distribution is found for each configuration. Life cycle cost analysis aids in selecting the optimal configuration. Simulations conducted in TRNSYS, reveal that optimal energy and economic choices depend on the natural gas and electricity price ratio. The best energy performance occurs at a low capacity ratio, while the economic trend varies with capacity ratio for different price ratios. The potential reduction in life cycle cost associated with the configuration, when compared to the full absorption baseline and the full compression baseline, can reach up to 7,210,000 $ (72.6%) and 724,000 $ (24.5%), respectively. From an environmental perspective, compared to the full absorption baseline and the full compression baseline, hybrid chiller configurations reduce the CO2 emissions for up to 899 tons (68.8%) and 52 tons (11.3%), respectively.

Transmission of respiratory pathogens occurs primarily in indoor settings, interventions to reduce the risk of their transmission include increases in outdoor air introduction, filtration, and ultraviolet germicidal irradiation (UVGI). However, validating these interventions is challenging, particularly in actual applications. This study introduces an aerosol tracer system utilizing DNA as tracer molecule, aimed at quantitative characterization of the performance of indoor air cleaning systems. Two DNA tracers, designed one to be relatively UV-resistant and another relatively UV-sensitive, were employed to assess air quality changes related to filtration and ventilation and the contributions of UVGI fixtures in various built environments. We conducted controlled UV exposure experiments of DNA-tagged tracers on foil coupons, aerosolization studies in a test chamber, and in a commercial building conference room. The DNA tracer results provided insights at the point of sampling into the effects of complex airflow dynamics. Additionally, a way to scale the DNA tracer results to MS2 bacteriophage is proposed. Four distinct UV devices challenged with MS2 in a chamber test produced equivalent clean airflow rates of 13-147 CFM. Scaled equivalent clean airflow rates in a commercial building setting using the UV-sensitive tracer varied from 47 percent less to 101 percent more than the chamber results, possibly due to differences in airflow patterns, equipment configuration, and other factors. Our findings provide quantitative understanding of the interaction between UV-sensitive aerosols and the built environment, with implications for environmental monitoring, measuring UVGI fixture impact in field settings, and addressing current technologies limitations for assessing UVGI disinfection efficacy.

Abstract This is an account that should be heard of an important struggle: the struggle of a large group of experts who came together at the beginning of the COVID-19 pandemic to warn the world about the risk of airborne transmission and the consequences of ignoring it. We alerted the World Health Organization about the potential significance of the airborne transmission of SARS-CoV-2 and the urgent need to control it, but our concerns were dismissed. Here we describe how this happened and the consequences. We hope that by reporting this story we can raise awareness of the importance of interdisciplinary collaboration and the need to be open to new evidence, and to prevent it from happening again. Acknowledgement of an issue, and the emergence of new evidence related to it, is the first necessary step towards finding effective mitigation solutions.

Ultraviolet germicidal irradiation (UVGI) systems inactivate microorganisms indoors. Upper-room UVGI systems use wall- or ceiling-mounted fixtures to create an air disinfection zone above the occupied zone. The performance of upper-room UVGI systems varies with indoor airflow patterns induced by mechanical ventilation and thermal plumes from indoor heat sources. Little information is available on the effects of ventilation strategies on upper-room UVGI system performance for the control of viral aerosols in occupied spaces. This study simulated the effects of ventilation system characteristics in an office space on the ability of an upper-room UVGI system to inactivate viral aerosols with UV-C susceptibility representative of coronaviruses. UVGI reduced viral aerosol concentration by two orders of magnitude relative to the concentration without UVGI. Air change rates and air distribution strategy (mixing vs. displacement) had notable effects on the effectiveness of the UVGI system. For mixing ventilation, as the recirculation airflow rate increased from 0 to 5.3 h−1 for a room volume of 108 m3 with a fixed outdoor air change rate of 0.7 h−1, UVGI inactivation increased by 96.7%. Mixing ventilation with 100% outdoor air of 0.7 h−1 yielded airborne virus inactivation that was double that of displacement ventilation, due to enhanced air mixing.

On June 24, 2023, ASHRAE approved the publication of Standard 241-2023 Control of Infectious Aerosols. The purpose of Standard 241 is "to establish minimum requirements for control of infectious aerosols to reduce risk of disease transmission in the occupiable space of buildings" by defining "the amount of equivalent clean airflow necessary to substantially reduce the risk of disease transmission during infection risk management mode." This column provides a high-level overview of key aspects of the new standard and a discussion of its historical context and its potential impact on design and operation of buildings to achieve improved indoor air quality (IAQ). Future columns will provide more detailed information on the background and use of the standard.

In sparsely occupied large industrial and commercial buildings, large-diameter ceiling fans1 (LDCFs) are commonly utilized for comfort cooling and destratification; however, a limited number of studies were conducted to guide the operation of these devices during the COVID-19 pandemic. This study conducted 223 parametrical computational-fluid-dynamics (CFD) simulations of LDCFs in the U.S. Department of Energy warehouse reference building to compare the impacts of fan operations, index-person, and worker-packing-line locations on airborne exposures to infectious aerosols under both summer and winter conditions. The steadystate airflow fields were modeled while transient exposures to particles of varying sizes (0.5-10 mu m) were evaluated over an 8-h period. Both the airflow and aerosol models were validated by measurement data from the literature. It was found that it is preferable to create a breeze from LDCFs for increased airborne dilution into a sparsely occupied large warehouse, which is more similar to an outdoor scenario than a typical indoor scenario. Operation of fans at the highest feasible speed while maintaining thermal-comfort requirements consistently outperformed the other options in terms of airborne exposures. There is no substantial evidence that fan reversal is beneficial in the current large space of interest. Reversal flow direction to create upward flows at higher fan speeds generally reduced performance compared with downward flows, as there was less airflow through the fan blades at the same rotational speed. Reversing flow at lower fan speeds decreased airflow speeds and dilution in the space and, thus, increased whole-warehouse concentrations.

To prevent Natural Gas (NG) frozen during pressure reduction process, a heater is contrived for preheating NG. In this study, a novel system is suggested to supply part of heat demand of a NG City Gate Station (CGS). The system consists of a heat pump in which the compressor input work is supplied by the turbo-expanders. In the proposed system, pressure reduction is carried out by turbo-expanders instead of Regulator Valve (RV) to recover the energy of high pressure NG. The recovered energy by turbo-expanders meets the heat pump cycle work demand. A CGS is selected as a case study, and the suggested system is assessed in terms of energy, exergy, and economics. The amount of the annual fuel savings is calculated 2.42 x 106 kg (70.08%). Exergy analysis shows that the most exergy destruction occurs in the RV and heater. The exergy factor of the proposed system in the days that the system works, is about 2.2-5.1%. Based on the economic analysis, the value of simple payback period and discounted payback period are calculated 2.63 and 3.08 years, respectively. The annual revenue that comes from saving fuel is 310,000 $/year based on the current price of NG.

In a large-scale chiller plant, a network of chillers is a suitable solution to respond to the variable cooling demand. The energy consumption of a chiller network depends on the configuration and the control strategy of the chiller network in different conditions. This study presents a general procedure for designing a chiller network for a building with an arbitrary annual cooling demand distribution. The procedure determines the optimal configuration considering the quantity, the size ratio, and the energy performance of chillers. The particle swarm optimization (PSO) algorithm is used for each configuration to find the optimal chiller loading distribution. Then, the optimal configuration is selected through a life cycle cost analysis. In order to predict a general chiller performance curve with an arbitrary nominal capacity, an artificial neural network model is developed based on 20 available commercial chillers in the market. The chiller performance prediction includes determination of COP and actual capacity of a chiller in terms of nominal capacity, chilled water temperature, cooling water temperature, and partial load ratio. The simulation is carried out in TRNSYS, which linked to MATLAB to implement the PSO optimization strategy. The results show that for networks with two, three, and four chillers, the optimal selection of chiller network configuration under the PSO strategy reduces the energy consumption by 26.30, 26.06, and 26.18%, respectively, compared to the conventional configuration under the baseline strategy. The life cycle cost for these configurations is also reduced by 17.93, 17.69, and 18.56%, respectively.

Ultraviolet germicidal irradiation (UVGI) is a highly effective means of inactivating many bacteria, viruses, and fungi. UVGI is an attractive viral mitigation strategy against coronaviruses, including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the coronavirus disease-2019 (COVID-19) pandemic. This investigation measures the susceptibility of two human coronaviruses to inactivation by 254 nm UV-C radiation. Human coronavirus NL63 and SARS-CoV-2 were irradiated in a collimated, dual-beam, aqueous UV reactor. By measuring fluence and integrating it in real-time, this reactor accounts for the lamp output transients during UVGI exposures. The inactivation rate constants of a one-stage exponential decay model were determined to be 2.050 cm2/mJ and 2.098 cm2/mJ for the NL63 and SARS-CoV-2 viruses, respectively. The inactivation rate constant for SARS-CoV-2 is within 2% of that of NL63, indicating that in identical inactivation environments, very similar UV 254 nm deactivation susceptibilities for these two coronaviruses would be achieved. Given the inactivation rate constant obtained in this study, doses of 1.1 mJ/cm2, 2.2 mJ/cm2, and 3.3 mJ/cm2 would result in a 90%, 99%, and 99.9% inactivation of the SARS-CoV-2 virus, respectively. The inactivation rate constant obtained in this study is significantly higher than values reported from many 254 nm studies, which suggests greater UV susceptibility to the UV-C than what was believed. Overall, results from this study indicate that 254 nm UV-C is effective for inactivation of human coronaviruses, including SARS-CoV-2.

SARS-CoV-2 is transmitted mainly through short and long range airborne transmission.1 2 The omicron variant shows faster transmissionandgreater vaccine escape thanprevious variants.3 Furthermeasures are needed to contain transmission. In 2020 we argued in The BMJ that “rigid safe distancing rules are an oversimplification based on outdated science.”4 We produced risk charts for SARS-CoV-2 transmission incorporating multiple variables: indoors versus outdoors (and level of ventilation if indoors), roomoccupancy (loworhigh), time spent together (short or long), vocalisation (silent, speaking, shouting, or singing), and masking (yes or no). We have now developed a mathematical model to quantify further these relative risks with updated data.5 Our model, which assumes a single enclosed space in which virus containing aerosols exhaled by a single infected human mix rapidly, is based on models developed for infectious disease spread through the air (such as measles6). It takes account of the disease specific emission rate of virus carrying particles, the increase in emission of viral particles with vocalisation and exercise, room volume, room occupancy (assumed to be stable and continuous), rate of particle removal either naturally (through, for example, opening windows) or mechanically (through, for example, replacement with outdoor air or filtration), and the efficiency with which virus carrying particles penetrate masks. Details of these calculations are published in the technical paper.5 One of the attack rate charts from the resultant model is reproduced in a linked table (https://docs.google.com/document/d/1BWWCpUWiPIlntWRFfPwqBSa1Z777KFIl/edit), with an interactive risk calculatormade available online (http://tinyurl.com/COVID-Tables).7 Findings strongly affirm the validity of the low, medium, and high risk social situations set out in our original paper in The BMJ,4 with the addition of exercise whereby heavy breathing greatly increases both viral emission and viral intake. The model, however, does not account for all variables—notably, overlapping breathing zones between individuals and known airflow heterogeneity indoors.8 As transmissionescalatesdespite vaccination, fuelled by shedding from asymptomatic carriers,9 we should note the perils of mixing unmasked in crowded and under-ventilated indoor spaces, especially when singing or exercising. When prevalence decreases after the current wave, more activities will become low risk. Competing interests: None declared.