Transient and continuous effects of indoor human movement on nanoparticle concentrations in a sitting person's breathing zone

Highlights

• The nanoparticle concentration fluctuation affected by a moving manikin was recorded.

• Nanoparticle concentration decreased 37.6 (±5.7) % compared with the peak value.

• Short-term exposure of a sitting person may decrease 5.18 (±0.99) %.

• Continuous exposure of a sitting person may increase 2.88 (±1.24) %.

Abstract

Particle concentration in a sitting person's breathing zone can be influenced by human movement around the person, and the transient and continuous effects may differ. In this study, a set of full-scale experiments was conducted to sample the nanoparticle concentration in the breathing zone of a sitting thermal breathing manikin (STBM). The transient fluctuation of the nanoparticle concentration was recorded continuously and analyzed. The results showed that when a manikin moved (at 1 m/s) past the STBM, the nanoparticle concentration in the STBM's breathing zone decreased and reached its lowest after the standing manikin had passed, decreasing 37.6 (±5.7) % compared with the peak value. The average concentration in the STBM's breathing zone during influence periods was 5.18 (±0.99) % less than that during non-influence Periods (NP). This finding reflected the fact that the transient inhalation (over several seconds) of the STBM may be reduced by manikin movement. On the other hand, the exposure of the STBM increased 2.88 (±1.24) % when there was a continuously moving manikin compared with the stable state in a 10-min observation. This finding may be explained by the fuller mix of indoor air and nanoparticles caused by manikin movement, as well as the increase of nanoparticle suspension time. The difference in the transient and continuous effects of the manikin movement on the STBM's exposure shows the importance of considering these effects separately in different scenarios.

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.