Transient and continuous effects of indoor human movement on nanoparticle concentrations in a sitting person's breathing zone
Graphical abstract
Introduction
People spend more than 80% of their time in indoor environments and indoor air quality has a great impact on human health (Klepeis et al., 2001). In recent decades, the development of nanotechnology has led to advances in science, and there has also been an increase in public exposure to nanoparticles. An experimental study showed that, in a college canteen, the inhaled particles were mainly contributed by nanoparticles (Zhang et al., 2017). In offices, printer-emitted particles and particles emitted from ultrasonic humidifiers were dominated by particles at the nanoscale (Koivisto et al., 2010; Serfozo et al., 2018; Yao et al., 2020). During 3D printing and some engineering processes, the indoor nanoparticle number concentration may reach the order of 106#/cm3 (Byrley et al., 2020; Walser et al., 2012). For construction activities, 14–17% of the airborne particles were found to be below 100 nm in diameter during bag-emptying and cutting processes (Batsungnoen et al., 2020). Nanoparticles are likely to enter the lower respiratory tract and lungs, posing potential health risks to humans. Accumulation of nanoparticles in lung tissue can lead to lung damage, followed by inflammation (Kim et al., 2011; Nho, 2020). Nanoparticles may even be absorbed into the brain via the olfactory pathway (Shang et al., 2021). In addition, many viruses are also at the nanoscale, such as SARS-CoV-2 (~100 nm) (Zhu et al., 2020b) and influenza viruses (80–500 nm) (Fabian et al., 2008). It is therefore important to study nanoparticle concentration and personal inhalation in typical indoor settings.
Personal inhalation can be influenced by air distribution and ventilation (Li, 2007; Liu et al., 2018), the occupants of the space (Zukowska et al., 2012), the location of pollution source (Mazumdar et al., 2010), and the ambient temperature and humidity (Feng et al., 2020). Human activities can change the concentration distribution of aerosols, and many activities can produce aerosols or particles (Géhin et al., 2008). Researchers have drawn different conclusions about the effect of human movement on indoor particles. Some have found that human movement can increase indoor aerosol concentration levels and span the decay time (Cao et al., 2017), while others have found that an increase in walking speed may reduce the overall number of suspended droplets (Wang and Chow, 2011). The concentration distribution of particulate matter (PM) in different areas of a room was found to be different (Licina et al., 2015a), especially in a room with poor ventilation. (Licina et al., 2015a) showed that it is the particle concentration in a person's breathing zone that determines the inhalation or the risk of infection (for viral particles).
In addition, there are often multiple people in many indoor spaces. Some studies, however, focused only on the influence between the individual and the environment, ignoring the interaction between people. When no one is moving, the airflow speed in a room is usually less than 0.2 m/s (Wu and Gao, 2014). Once someone starts moving at 1 m/s, however, the air velocity in some areas has been found to increase significantly, reaching a peak of around 2 m/s (Han et al., 2014b). With the disturbance of airflow, its influence on the diffusion of pollutants cannot be ignored. The thermal plumes around the human body can also influence the inhalation ratio (Voelker et al., 2014; Zhu et al., 2020a). Such plumes are generated by the temperature difference between the human body and the surrounding environment, also known as the human convective boundary layer (CBL). Licina et al. conducted a series of experiments to study the human CBL (Licina et al., 2015b; Licina et al., 2015c; Licina et al., 2014). In a windless room, the peak velocity of the CBL in front of a standing manikin decreased from 0.23 m/s to 0.16 m/s when the temperature increased from 20 °C to 26 °C, but the shape of the CBL did not change. The indoor airflow had a clear influence on the CBL, and even transverse flow at 0.175 m/s interfered with CBL in the breathing zone. Another study of the inhalation ratio showed that, if thermal plumes were ignored, a 5% to 20% error may be observed for the inhalation ratio of particles in an indoor environment of 20 °C (Naseri et al., 2017). Thus, as well as breathing, the thermal effect of the human body should be taken into account when studying particle concentration in the breathing zone.
In multi-person offices, factories, trains, buses, and aircraft cabins, it is common for one person to walk past a sitting person. This movement has an obvious impact on the nearby flow field (Han et al., 2014a; Luo et al., 2017). Vortex wakes follow the moving people and may affect the trajectory of PM in the nearby air (Luo et al., 2018). The closer the moving object is to the source of the pollution, the greater the change in the surrounding pollutant concentration (Mazumdar et al., 2010). On the other hand, the airflow speed of the whole room quickly returned to below 0.2 m/s within 4 s after the man stopped. Thus, the effects of human movement on indoor airflow do not last for long, and the transient variation of PM concentration should be considered. Moreover, when a pollutant source was sitting, the disturbance from object movement was greater due to the lower height of the pollutant emission. The PM concentration stratified in the direction of height, and the change in the lower height was more obvious than the upper one (Wu and Gao, 2014). Many previous studies have focused on the inhalation of standing people (Li et al., 2013) or moving people (Tao et al., 2020), but a sitting person may have a higher risk of exposure and may be more affected by movement than a moving person.
In general, previous experimental studies have tended to focus on micron particles. Due to the small size effect, nanoparticles are greatly affected by Brownian force and the turbulence effect, and so their motion characteristics are different from those of micron particles. Moreover, previous studies have mainly focused on the average exposure and have not discussed transient fluctuations, even though transient and continuous effects may differ. To date, there are few experimental studies on the effect of human movement on indoor nanoparticle concentration. In this study, we used two manikins with real human body shape to conduct full-scale experiments and the transient nanoparticle concentration fluctuation in the sitting manikin's breathing zone was recorded and analyzed. Then the transient and continuous effects of indoor human movement on nanoparticle concentrations in a sitting person's breathing zone were compared.
Section snippets
Experimental setup
The experimental site was located in Hefei Institute for Public Safety Research, Tsinghua University, Hefei, Anhui Province, China. Experiments were carried out in an experimental chamber with dimensions of 6.4 m × 5 m × 3 m (L × W × H) in which the temperature and humidity was controlled, and there was a rail 16.5 m long (Fig. 1). The volume of the chamber was 96 m3. During the experiments, the ambient temperature was between 20.1 °C and 22.1 °C, and the relative humidity was between 49.5% and
Environmental background concentration of nanoparticles
Fig. 3 shows the environmental background concentration of nanoparticles. The concentration was mainly between 103 #/cm3 and 104 #/cm3, consistent with typical indoor environmental sampling (Afshari et al., 2005). The results also showed that the breathing behavior of the STBM (breathing flow rate Q = 7.5 L/min, respiratory cycle T = 4 s) had a significant effect on the nanoparticle concentration in the breathing zone. In the absence of respiration, the nanoparticle concentration was relatively
Conclusions
In this study, a set of full-scale experiments was carried out to sample the nanoparticle concentration in a STBM's breathing zone. The transient concentration fluctuations were analyzed. The transient and continuous effects of human movement on a sitting person's inhalation were compared. The main conclusions are as follows.
When the standing manikin moved (at 1 m/s) past the STBM, nanoparticle concentration decreased by 37.6(±5.7) % compared with the peak value in a cycle. The average
CRediT authorship contribution statement
Jialin Wu: Methodology, Software, Validation, Formal analysis, Data curation, Writing – original draft, Visualization, Supervision. Wenguo Weng: Conceptualization, Resources, Investigation, Writing – review & editing, Project administration, Funding acquisition. Liangchang Shen: Validation, Formal analysis, Data curation, Writing – original draft. Ming Fu: Formal analysis, Investigation, Project administration, Funding acquisition.
Declaration of competing interest
We declare that we have no conflicts of interest.
Acknowledgements
This study was supported by National Natural Science Foundation of China (Grant No. 72034004), National Science Fund for Distinguished Young Scholars of China (Grant No 71725006), and National Natural Science Foundation of China (Grant No. 52074163), Anhui Provincial Natural Science Foundation for Distinguished Young Scholars (Grant No. 1908085J22). The authors are deeply grateful to these supports.
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