Energy efficiency vs indoor air quality conundrum and possible solutions

Chiara Giorio 
University of Cambridge
cg525@cam.ac.uk
ECG Bulletin January 2022

Recipient of the 2021 Environment, Sustainability and Energy Division Early Career Award (https://www.rsc.org/prizes-funding/prizes/2021-winners/dr-chiara-giorio)

With increased energy demand and the need to improve energy efficiency to limit human-caused climate change, new buildings and renovation of old buildings will need to comply with increasingly strict national standards and guidelines. Given that most of the population spend about 90% of their time indoors, actions to improve energy efficiency should not come at the expense of indoor air quality and comfort.
 

Figure 1. This infographic depicts the importance of indoor air quality in energy efficient buildings.

​A large part of the population, especially in urban areas, is exposed to air that does not meet European standards nor World Health Organisation Air Quality Guidelines, with consequences in increased morbidity and mortality mainly due to cardiovascular and pulmonary diseases  (1). Air pollution kills more than malaria and HIV combined (1) or the novel coronavirus disease (COVID-19) on a global scale, and represents the largest environmental risk factor behind premature deaths (2). Mounting evidence indicates that COVID-19 might leave a significant proportion of the population with permanent lung damage, making them even more susceptible to the detrimental effects of air pollution (3).
 

Air pollution epidemiology has so far primarily relied on fixed outdoor air quality monitoring stations and static populations which cannot capture the high heterogeneity of personal exposure, as individuals move between different microenvironments, spending as much as 90% of their time indoors (4) at home and school/work. Indoor air (gases and particulate matter) contains a complex mixture of abiotic and biotic components that may be generated by indoor sources or may come from outdoor pollution. Indoor generated pollutants include volatile organic compounds (VOCs) as well as aerosols (primarily- and secondarily-sourced) emitted by consumer products (e.g. cleaning products, personal care products, furniture, electronics), building materials and wall painting, people-related emissions (e.g. exhaled volatile compounds, cigarette smoke including thirdhand exposure), activity-related emissions (e.g. cooking) ,and a variety of natural organic particles including living (microbes such as bacteria and fungi) and non-living matter (e.g. hairs and fragments of biological tissues).
 

According to a recent Eurobarometer survey, most European citizens are concerned about air quality with more than two-thirds of respondents (71%) saying they think the EU should propose additional measures to address air quality-related problems in Europe. The European Green Deal has recently introduced the ambitious commitment to a ‘zero-pollution action plan for air, water, and soil’ to protect humans and the environment. Within the same framework, the European Commission adopted a package of proposals for reducing net greenhouse gas emissions by at least 55% by 2030, compared to 1990 levels, to turn Europe into the first climate-neutral continent by 2050. The UK government has recently announced the target to reduce UK's emissions by at least 68% by 2030 compared to 1990 levels.
 

Owing to climate change concerns, improving the energy efficiency of homes, public and private buildings is of paramount importance. With new “nearly zero energy” buildings, and retrofits to improve the energy efficiency of old buildings, it may be tempting to increase airtightness. The EU directive 2010/31/EU leaves flexibility in the implementation plan and the choice of how to balance the different factors such as energy consumption, indoor air quality, and comfort levels so each country has its own standards and regulations (5). This leads to the so-called “energy efficiency/indoor air quality dilemma” (6).

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The ventilation rate of a building, usually defined as the rate at which external air flows inside (5), is very important in controlling air quality in the indoor environment. The ventilation rate determines the rate at which outdoor pollutants can enter a building but also the rate at which indoor generated pollutants can be dispersed out of a building. Ventilation is also important to reduce the transmission of airborne viruses and bacteria (7). With an increase in airtightness, occupants of a building may experience the “sick building syndrome” (8), which is characterised by nonspecific symptoms such as headache, eye or nasal irritation, skin rash or itch, malaise, or difficulty concentrating (9). As an example, in schools, a satisfactory indoor environment in terms of airflows, temperatures, and air quality is important for children's well-being and their performance in the classroom (10, 11). Ventilation guidelines recommend that CO₂ in indoor spaces should be maintained below 1000 ppm. At these levels, CO₂ is not viewed as a pollutant of concern (even if it is, especially above 2500 ppm), but rather as an indicator of how well both people-related and activity-related indoor pollutants are controlled (9, 10).

To make sure that we do not pursue energy efficiency at the expense of indoor air quality, actions need to be taken to improve both. The use of consumer products labelled as “low emissions”, coupled with behaviour changes to reduce emissions related to indoor activities such as cooking, may not be enough to mitigate people-related indoor air emissions (e.g. exhaled CO₂ and VOCs). A recent review by Frisk et al. (12) reported that green retrofits in old residential buildings in Europe and the US that added ventilation and improved temperature comfort led to improved asthma symptoms amongst both adults and children. Introducing mechanical ventilation (e.g. a network of ducts powered by fans), with either outdoor air or purified outdoor air for buildings in areas characterised by elevated outdoor pollution, may be the way to go  (13). While this may increase energy consumption, the trade-off may be in the order of 1-2% decreased energy efficiency (14) compared to the situation where there is no mechanical ventilation. A passive and lower-energy alternative would be to manage/control natural ventilation, for example, through air vents and/or dynamic insulation. In the latter case, building envelopes could be made of porous material in which airflow is achieved by pressure differential and heat is transmitted by conductance through the material to the air. With sufficiently small airspeed, the temperature gradient at the external surface may be reduced virtually to zero, and the heat loss would be only that of the ventilation (5). There are also instances in which ventilation may promote energy efficiency. As an example, ventilation, either natural or mechanical, may be exploited in the warm seasons for thermal storage/night cooling purposes in which high ventilation rates at night would remove heat from the building envelope that has been stored during the day, so that in the following day the building envelope would absorb heat from the internal air to provide passive cooling (5).

Finding a trade-off between energy efficiency and indoor air quality does not seem impossible given the aforementioned solutions and new solutions that may arise with further interdisciplinary research involving — among others — architects, engineers, material scientists, and atmospheric scientists.
 

References
For other technologies that address problems raised by this author, see https://www.cse.org.uk/advice/advice-and-support/mechanical-ventilation-with-heat-recovery
 

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