Are occupants net sources or sinks of indoor air pollutants? 

In the past, building materials and furnishing were considered the dominant sources of indoor air pollution. As emissions from building products were gradually reduced, the role of human emissions in indoor air quality began to be increasingly recognized. The presence of people indoors leads to chemical changes occurring both in the gas phase and on indoor surfaces. It has been reported that the compounds associated with the presence of humans contribute up to 40% of the measured daytime volatile organic compounds (VOC) concentration in indoor spaces (Liu et al. 2016). In another study, human-emitted VOCs were the dominant source during occupied periods in a well-ventilated classroom (57%) together with ventilation supply air, which was the second most important source of pollution (35%) (Tang et al. 2016). 

Human emission can undergo chemical transformations, generating a suite of new chemicals. Human emissions impact perceived indoor air quality and some chemical compounds emitted by humans, and the products of their chemical transformations, may have adverse health effects. Recent studies have highlighted the potential for human emissions to affect cognitive performance and cause neurobehavioral symptoms in people. Some ozonolysis products are suspected respiratory irritants or have previously been shown to cause skin irritation (Bekö et al. (2021) and references therein). 

Human breath has been widely investigated as a source of VOCs. However, the role of the human respiratory system as a sink for VOCs is much less studied. At the same time, exposure to volatile organic compounds might lead to adverse health effects. It is therefore important to study the removal of indoor air pollutants by the human body. 

Hundreds of VOCs are released from the human body via the breath, skin, and intestinal gases. These compounds are derived from endogenous metabolic processes in and on the human body and from exogenous sources such as airborne VOCs from environmental or occupational exposures, foods, and personal care products. Primary emissions can result from microbial activity and can lead to odor nuisance and decreased perceived air quality. The emitted compounds can undergo chemical transformations, such as ozone-initiated reactions, which produce a range of new compounds. The reactions of ozone with squalene and other constituents of skin oils have been shown to significantly alter the composition of indoor air and alter its OH reactivity. 

Direct measurements of chemicals in both indoor air and breath are rare. He et al. (2019) measured the concentrations of numerous VOCs in inhaled and exhaled air in different environmental conditions. They found that human breath could be a source for some VOCs and a sink for others. VOCs are absorbed by the human body during respiration, and they are transferred via gas exchange through the alveolar membrane to the capillaries. They then diffuse into the pulmonary veins and reach internal organs via blood circulation. At the same time, VOCs and carbon dioxide generated from internal metabolism are also released as exhaled breath. Multiple factors may influence the interactions between ambient VOCs and human respiratory systems. Gender, age, smoking habit, the time of sampling within the day, and previous exposure may influence whether a person is a source or sink, and the conversion from one to the other. 

Volatiles and semivolatiles with certain physicochemical properties and dermal permeabilities can penetrate the skin directly from air (Weschler and Nazaroff, 2012; 2013; Weschler EHP 2015). Modeling studies suggest that compared to unoccupied conditions, including a human occupant in an indoor setting reduces the concentrations of chemicals with low vapor pressures by as much as two orders of magnitude if one assumes that the chemical is only degraded in the gas-phase. Even if degradation of a chemical occurs in other indoor compartments, occupancy may reduce indoor concentrations of certain chemicals by up to 60% (Weschler, 2016; Zhang et al., 2014). Whether dermal uptake from air or uptake to clothing alone can influence indoor concentrations and resulting human emissions, needs to be investigated. Similarly, measurements of the effect of whole-body exposure/emissions on indoor concentrations of compounds present before occupancy warrants attention. 

Hundreds of primary species have been routinely measured in studies looking at exhaled breath and dermal emissions from humans. Primary emissions from humans, especially via exhaled breath, have been studied for a long time with the purpose to identify disease biomarkers and advance disease diagnostics. Breath VOCs are typically present at ppb (or lower) concentrations and include compounds that are generated in the body and taken up from the environment. Endogenous VOCs are primarily blood-borne compounds released via the lungs. Exogenous VOCs include compounds inhaled from the external environment or absorbed through the skin, in addition to compounds produced following oral ingestion of food, and those derived from smoking cigarettes. Additionally, largely different are the compounds emitted via breath and skin. 

Human emissions can also undergo chemical transformation and result in a suite of new compounds in indoor air. These reactions occur in the presence of humans and in previously occupied spaces, as skin oils are transferred to surfaces through touching, and via skin flakes and their constituents shed by people in a process called desquamation contribute to the organic films on indoor surfaces. The literature with discussion of specific compounds can be found in Bekö et al. (2021) and references therein. 

The list of commonly measured compounds emitted via breath or skin and their known reactions products can be found in this table. For further information on relevant compounds see Weschler 2016, Wang et al., 2022. See also references in these sources. If comprehensive monitoring of all compounds using high-end instrumentation is not feasible, the target compounds should be carefully selected with the consideration of the research question. 

The uptake potential of the human body could be measured for a long range of chemicals that are routinely measured in breath and/or indoor air. He et al. (2019) list the 40 VOCs most frequently detected in inhaled and exhaled air of 98 subjects. Regarding the effect of dermal uptake on indoor concentrations, compounds with the right set of properties to be transported from bulk air to the skin and across the skin need to be measured (Weschler and Nazaroff, 2012; 2013). To investigate the effect of occupancy in general on indoor concentrations of common indoor air pollutants, those common compounds originating mostly from building materials, furnishing, consumer products and occupant activities need to be measured. Zhang et al. (2014) modelled the influence of human intake on indoor chemical fate for 40 compounds. 

Investigation of human emissions rarely focuses on a specific compound. Such measurements more often encompass the measurements of a long range of compounds, which requires sophisticated instruments with high resolution, low detection levels and high accuracy. Preferably on-line instrumentation is recommended (WG4b, 6.3; Vera et al., 2022; Spinazze 2021). Measurements of radicals and OH reactivity are also recommended in connection with studies of human emission. For relevant instrumentation, see WG4e (Gomez Alvarez, 2022). For measuring selected target compounds (e.g. acetone, acetic acid, isoprene, toluene, aldehydes,…) at lower costs both passive and active sampling methods can be used. While passive sampling is applicable in real environments monitored over a longer period of time, active sampling is more suitable for controlled experiments studying human emissions and the underlying personal and environmental factors, including relevant additional target compounds described in section “What species should we measure?” (WG4b, 6.2, Vera et al., 2022). High-end precision instruments, such as cavity ring-down spectrometers, as well as lower price range instruments are available for certain specific compounds (e.g. CO2, CH4, ammonia, …) (Spinazze 2021, Vera et al., 2022). Low cost sensors are suitable for measurements of supporting parameters, such as temperature, relative humidity, ozone, CO2, but are not recommended in targeted human emission studies (Garcia 2022). In connection with human emissions, measurements of biological particles and nanoparticles have received some attention. For relevant instrumentation, see WG4d report, section 8.2.2 and Bergmans et al. (2022), sections 3.1.3, 3.1.4. For characterization of particles in relation to occupant emissions, see WG4d report, section 8.3 and Duarte et al., 2022, section 2.1. For an example of instrumentation used in a comprehensive human emission study, see Bekö et al. (2020). 

In order to study the isolated effects of a range of parameters on human emissions, controlled studies are best. These can be performed in climate chambers of various size (e.g. testing the effect of age, sex, health condition, personal care products, clothing level, environmental factors, interpersonal variations), specific rooms or even in real indoor environments (e.g. testing the effect of dynamic environmental conditions – changing activity level, temperature, ventilation, ozone concentrations, etc.). Studies probing the impact of occupancy on indoor air chemistry and exposure would benefit from being performed in parallel under occupied and unoccupied, but otherwise identical conditions in realistic environments. This would allow for the direct consideration of the interaction between human emissions and other indoor pollutants and surfaces. Measurements of group emissions under different circumstances can be performed in school classrooms or other premises. For example, repeated measurements in a cinema showed that airborne chemicals in air varied distinctively and reproducibly with time for a particular film, even in different screenings to different audiences. In such studies, additional measurements may include environmental parameters (e.g. temperature, relative humidity, ozone concentration, etc.). A good understanding of ventilation, occupancy, occupant activities, appliance use, and other factors that may have an influence on the results should be considered. For a list of potentially important parameters, see WG5 List of parameters and WG5 Final Report. 

For measurements of exhaled breath or skin emissions in an isolated manner, controlled laboratory conditions with appropriate facilities are most appropriate (e.g. Bekö et al., 2020;  Sun et al., 2017a, 2017b; Zou and Yang, 2022a, 2022b). As well as instrumentation for the measurements of target compounds, surveys and sensory assessments can be applied (see e.g. Tsushima et al., 2018). If the research question includes the relationship between physiology and human emissions or their effects, physiological measurements (e.g. heart rate, heart rate variability, core body temperature, skin temperature, respiratory indicators, sleep quality, biomonitoring, etc. ), should be considered. Studies of the link between dermal emissions and indoor air can be supplemented with analyses of the target species in skin wipes. Additionally, studies probing the effects of occupants on indoor air chemistry would benefit from sampling of indoor surfaces and considering the role of surface chemistry.