The COVID-19 pandemic has heightened awareness among the general population about the importance of indoor air quality in unprecedented ways. While this acute, disaster-borne focus is clearly something that most would have preferred to avoid, there is growing momentum to position our industry to better respond to future pandemics in the wake of this global tragedy.

As described in an AIHA report published in 2020 (PDF), multiple studies have shown that better IAQ can result in higher worker productivity, better cognitive performance, increased job satisfaction, reduced absenteeism, and less reported job stress. Specific interventions associated with these types of improvements include higher ventilation rates, lower indoor pollutant emissions, more efficient air filtration, improved temperature and humidity control, and reduced dampness and mold in buildings. For the most part, however, building owners, operators, and managers have not recognized or appreciated the potential financial benefits of good IAQ, instead focusing on energy costs. This predilection persists even though the value of increased productivity among employees is more than 150 times greater than the associated energy costs for enhanced ventilation, according to a 2015 paper published in the International Journal of Environmental Research and Public Health.

As described in the journal Atmosphere, for the most part, the efficiency of ventilation systems has been prioritized over effectiveness at controlling and minimizing contaminants, so it should come as no surprise that many ventilation systems commonly found in buildings are not designed to—and are quite ineffective at—controlling airborne transmission of such agents. Concerns over ventilation and COVID-19 reached a boiling point in 2021, when hundreds of healthcare workers, scientists, engineers, and industrial and occupational hygienists signed open letters calling on government officials in the United States, the United Kingdom, Canada, Australia, and elsewhere to address substandard IAQ and take steps to reduce airborne transmission of SARS-CoV-2.

Traditionally, mechanical ventilation has been used primarily to provide thermal comfort by ensuring that temperatures throughout a building do not vary widely. Controlling relative humidity levels in buildings has typically been achieved as an ancillary consideration. A secondary aim of mechanical ventilation is to reduce the buildup of indoor air contaminants, and thereby mitigate potential adverse health effects, by mixing “fresh” outdoor air with the indoor air. While such dilution ventilation is generally acceptable for controlling low-toxicity gaseous contaminants such as carbon dioxide, it is much less effective at preventing the transmission of highly infectious viruses or other microorganisms.

In response to the COVID-19 pandemic, various agencies and professional associations have recommended upgrading, to the extent possible, filtration on heating, ventilation, and air-conditioning (HVAC) systems and increasing the amount of outdoor air brought into buildings. However, the impact of these types of interventions on energy usage is often a major consideration for companies and institutions that are looking to reduce their carbon footprint. The Journal of Building Engineering recently published a study that simulated ventilation rates and filtration methods across 13 U.S. cities and reported that, on average, the annual energy consumed by an improved filtration system was 31 percent lower than the energy consumed by 100 percent outdoor air ventilation. A similar study published in Energy and Buildings found that increasing outdoor air ventilation from 7 liters per second per person (l/s/person) to 21 l/s/person tripled HVAC energy consumption. The same increase in outdoor air, combined with sensible heat recovery, resulted in a more modest 40 percent increase in energy usage.

The practice of recycling a majority of air (for example, 90 percent or more, in some constant-volume systems) in mechanically ventilated buildings, in order to conserve energy, has long been recognized as a likely contributor to complaints about “sick building syndrome” and reported adverse health effects. Energy conservation in the 1970s and 1980s was largely driven by the oil embargo in 1973–1974 and the ensuing spike in oil prices. Some energy conservation measures, undertaken to reduce the operational costs of HVAC systems, resulted in insufficient amounts of outdoor air or inadequately conditioned air being supplied to the occupied space. Today, however, energy conservation efforts are increasingly motivated by concerns about climate change, a primary driver of which are greenhouse gases released when fossil fuels are burned to produce energy. A corresponding push for decarbonization of buildings and the U.S. energy system has established a goal of net-zero carbon emissions by 2050.

AIR CLEANERS AND FILTRATION HVAC filters are designed to remove particles from the air before supplying it to the occupied spaces in a building. Particles are either physically trapped by the fibers of the filter media or become stuck to the media due to an electrostatic charge. Filter performance depends on many variables, including airflow rate and velocity, filter media thickness and composition, particle size and mass, amount of dust on the filter, and amount of air leakage bypassing the filter.

NEWER ENERGY-EFFICIENT TECHNOLOGIES Heat recovery ventilators (HRV) and energy recovery ventilation (ERV) systems provide controlled ventilation while minimizing energy loss from directly exhausting conditioned air outdoors. Unlike HRV systems, ERVs transfer both heat and humidity, and thereby affect both the temperature and the moisture of the supply air. In the winter, ERVs capture heat from the indoor air being exhausted and transfer it to the outdoor supply air being brought into the building. In the summer, the opposite occurs: the indoor air being exhausted helps to pre-cool the incoming outdoor supply air. ERCs therefore reduce the costs of both heating and cooling air and can be anywhere from 50 to 98 percent efficient in recovering the energy that would otherwise be exhausted outdoors.

Air source heat pumps work by transferring heat absorbed from the outside air to an indoor space. Certain heat pumps can also work as cooling systems in the summer months by transferring heat from the indoors to the outside air. Air-to-air heat pumps can deliver up to three times more heat energy than electrical energy consumed. Enhanced, high-performance air-to-water heat pumps can provide 350 percent greater energy efficiency than typical gas boilers in heating mode. The International Energy Agency estimates that heat pumps could satisfy 90 percent of global space and water heating needs; yet in 2020, heat pumps accounted for only 7 percent of residential heat demand, while fossil fuel-based technologies made up nearly half of heating equipment sales globally. As focus on decarbonization and electrification of buildings grows, heat pumps are poised to become much more prevalent.

According to the National Renewable Energy Laboratory (NREL), existing technology to cool and dehumidify buildings using refrigerants and cooling coils is reaching practical and theoretical limits (PDF). Typically, to control indoor humidity levels, supply air is over-cooled to below the dew-point temperature and then reheated to an appropriate temperature, wasting energy in the process. Newer technologies that divide cooling and humidity control into two separate processes show potential to improve efficiency by 40 percent or more. One such technology is liquid desiccant-based cooling. Desiccant liquids are often high-concentration salt-water solutions that can absorb water directly from air and reduce the need for mechanical dehumidification. Thermal energy is then used to dry the desiccant solution after water has been absorbed. NREL researchers note that liquid desiccants fundamentally change the way humidity is controlled and have a theoretical efficiency limit 10 times higher than technology based on an electrically driven vapor compression cycle (for example, heat pumps, chillers, and direct expansion units). In addition, liquid desiccants can be implemented to shift latent loads to times when energy is cheaper, either because energy demand is low or when renewable energy is abundant.

IAQ AND ENERGY CHALLENGES GOING FORWARD The COVID-19 pandemic has clearly shown that current ventilation standards and practices for acceptable IAQ in non-healthcare settings are inadequate when dealing with infectious agents. History teaches us that pandemics eventually run their course or transition to endemic issues, similar to influenza or seasonal colds. While this outcome will be welcomed, concerns regarding ventilation and IAQ, climate change, energy savings, and decarbonization will persist. Ventilation and IAQ need to become resilient and adaptable to the future we face—not only with respect to airborne infectious diseases, but also to disasters such as wildfires, extreme heat, and pollution-driven air quality concerns.

New technologies for heating, cooling, and ventilation do more than deliver large gains in efficiency. They can also improve the way building HVAC systems meet occupant needs—improving comfort, productivity, and health. IAQ sensors that measure specific contaminants will become ubiquitous, enabling demand-control ventilation and IAQ efforts that are targeted, efficient, and continuously optimized. Improved ventilation, filtration, and air cleaning, plus sensing and controls, will make buildings more resilient because they can be adapted to whatever situation is at hand. In the buildings of the future, combining ventilation, filtration, and air cleaning will be the key to lowering energy usage and improving IAQ. The use of combined natural and mechanical (hybrid) ventilation, air-to-air and air-to-water heat recovery (where possible), more and better demand-control sensors, economizers, dedicated outdoor air supplies, and better air delivery can make IAQ improvements achievable, while also decarbonizing buildings.

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