Elsevier

Journal of Building Engineering

Volume 22, March 2019, Pages 295-304
Journal of Building Engineering

Personal CO2 bubble: Context-dependent variations and wearable sensors usability

https://doi.org/10.1016/j.jobe.2018.11.015Get rights and content

Highlights

  • High CO2 concentration in the inhalation zone was introduced as a personal CO2 bubble.

  • Regular office variability such as desk settings, activity, and personal difference was explored.

  • Using air movement devices such as a desk fan significantly reduces inhaled CO2.

  • A data driven method for predicting inhaled CO2 from wearable CO2 sensors was developed.

Abstract

High CO2 concentration in inhaled air has been shown to negatively impact work performance and increase acute health symptoms. As respiratory CO2 is constantly exhaled, it may not dissipate in surrounding air in absence of adequate air movement and is instead re-inhaled into the airways (breathing in a CO2-rich bubble). In this study, we explored the impacts of context-dependent factors such as office activities, desk settings, and personal differences on the inhalation zone CO2 concentration and on concentrations at a below-neck wearable sensor. We found that all factors significantly impact measurements at both measuring points of our test subjects. Presence of a small portable desk fan was also found to significantly reduce the CO2 concentration. On average, we observed a 177 ppm reduction in CO2 concentration when using a fan, which is 25 ppm higher than the background CO2 measurement (650 ppm). Among 41 test subjects, we found distinct relationships between the inhalation zone CO2 concentration and the wearable sensor measurements and, by applying a hierarchical clustering algorithm, we found 4 clusters of relationships. While below-neck wearable sensors could be used as an exact measure of inhalation of CO2 concentration for 29% of the subjects, we identified a boundary point (917 ppm) separating high and low inhalation zone CO2 concentration measurements.

Introduction

According to the Occupational Safety and Health Administration (OSHA 2012) and the American Conference of Government Industrial Hygienists (ACGIH 2011), the maximum recommended occupational exposure to CO2 concentrations for an 8-hr workday is 5000 ppm. Exposure to an indoor concentration above 5000 ppm carries potential health risks. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.1, “Ventilation for Acceptable Indoor Air Quality” also addresses CO2 levels that target occupant satisfaction from the odor perspective. The standard states that an indoor CO2 concentration of 700 ppm above outdoor air levels is the accepted threshold given the activities found in typical office buildings so, considering that outdoor CO2 concentrations range from 300 to 500 ppm, the indoor CO2 concentrations should not exceed a range of 1000–1200 ppm [1]. CO2 levels can be used to adjust outdoor air supply flow rate as a proxy for ventilation [2]. Higher levels of CO2 within the range found in normal indoor settings are associated with perceptions of poor air quality, increased acute health symptoms (e.g., headache, mucosal irritation), slower work performance [3], [4], [5], [6], and increased absenteeism [7], [8]. Recent studies have shown that increase in indoor CO2 concentration in indoor environments could negatively impact productivity and especially decision making [9]. Their results show that a 400 ppm increase in CO2 is associated with a 21% decrease in a typical participant's cognitive scores across a variety of tests [10]. Results also suggest that at approximately 950 ppm (levels that are common in indoor spaces and considered acceptable by ASHRAE 62.1), statistically significant declines in cognitive function scores are observed [10]. With the increased interest in occupant productivity and well-being and evolution of system-centric to human-centric standards, these results suggest the importance of understanding the environmental and personal factors impacting the inhaled CO2 and monitoring the exact inhalation zone CO2 concentrations between upper indoor levels and outdoor levels.

The primary source of CO2 in office buildings is respiration of the building occupants [11]. In a sedentary condition, the total exhaled volume (~6 L/min) of air contains 4–5% CO2, a 100-fold increase compared to the inhaled ambient air [12]. It is commonly assumed that the generated CO2 quickly dissipates into the surrounding air, making the CO2 concentration uniform throughout the indoor environment. However, the exhaled respiratory (i.e., metabolic) CO2 distribution around the human face and body is not uniform [13], [14]. Higher particle concentrations in a person's breathing zone was first reported as a “personal cloud” by Ozkaynak et al. in the late 90 s [15]. Subsequent research examined various pollutants such as ozone and trace CO2 from prior exhalations in the breathing zone and found significant differences from the background levels [13], [16]. The pollutant concentration in the inhalation zone is a function of both the physical characteristics of the human body and its environment, and the occupants’ dynamic traits (i.e., occupants’ dynamic breathing patterns) [13], [16], [17]. In addition, the thermal plume flow generated by the temperature gradient adjacent to the body can lead to a high concentration of suspended particles in the breathing zone [18], [19], up to 1.6–13 μg/m3 [19], [20]. In summary, a polluted breathing zone exists for various pollutants when there is insufficient air movement or human movement. A high concentration of CO2 can happen in any type of environment because it is respiratory-induced. Fig. 1 depicts results from Computation Fluid Dynamics simulation {{581 Sørensen, Dan Nørtoft 2003}} showing qualitative visualization of a high CO2 concentration in the inhalation zone.

Body posture and nose- and mouth geometry are static factors that influence shape and concentration of the CO2 bubble. Dynamic factors are occupant activity (spatial motions), indoor environment/furniture geometry, and air movements in the microclimate. Static and dynamic factors make a collective impact on the inhalation zone concentration, making it different from the background level.

The current study was designed to investigate the impacts of office activities, desk settings, and personal differences on the inhalation zone CO2 concentration and on concentrations at a below-neck wearable sensor, and the ability of neck-worn wearable CO2 sensors to measure inhalation zone CO2 concentration. For ground truth we measured the inhaled CO2 concentration at the nostril for several dynamic and static factors: activity (e.g., looking straight, looking down, steady speaking by counting numbers, and freestyle), desk settings (e.g., empty desk, desk with a rotating fan, desk with a desktop monitor, and desk with a laptop), and personal differences (i.e., inherent and experiential factors unique to each individual). We used an n-way ANOVA to assess the statistical relationships between the factors in the inhalation zone CO2 concentration. The measurements from the wearable CO2 sensor were then compared to the inhalation zone concentration measurements.

The paper is structured as follows. In Section 2, we describe our data collection procedure and statistical analysis tool used for studying impacts of different factors affecting inhalation zone CO2 concentrations. 3 Inhalation zone CO, 4 Wearable CO cover the results and statistical analysis of measurements for the inhalation zone concentration and wearable sensor, including the use of the hierarchical clustering algorithm. A comparison between measurements taken for inhalation zone and wearable sensor CO2 concentrations are presented in Section 5, and Section 6 discusses the generality of the results.

Section snippets

Methods and experimental procedure

There were 41 participants recruited to perform different office activities in a controlled climate chamber while wearing a sensor to measure the CO2 concentration in the inhalation zone. Participants were primarily college students (age between 18 and 28) and uniformly random male and female groups. The climate chamber, located at the Center for the Built Environment (CBE) in UC Berkeley, USA, has dimensions of 5.4 m × 5.4 m × 2.65 m (length × width × height). A mixing ventilation was supplied

Inhalation zone CO2 measurements results

Fig. 3 shows the mean personal inhalation zone CO2 distributions for different desk settings, activity, and subjects. As it can be seen in Fig. 3a, having an empty desk, desk with monitor, and laptop are not considerably different. However, the condition including fan is considerably different (~400 ppm lower median value compared to other cases). In addition, the distribution of CO2 concentration during the desk with fan scenario is considerably smaller. In Fig. 3b, the looking down and free

Wearable CO2 sensor measurements results

Fig. 4 shows the mean wearable sensor CO2 measurements for different desk settings, activity, and subjects. As it can be seen in Fig. 4a, similar to the inhalation zone CO2, using a fan reduces the CO2 measurement by ~100 ppm. However, the empty desk, desk with monitor, and laptop scenarios are all lower (~600 ppm) than inhalation zone CO2 and desk with fan is ~300 ppm lower. In Fig. 4b, the looking down and freestyle activity are similar, and higher than looking straight and counting numbers.

Comparing inhalation zone with wearable sensor CO2 concentration measurements

Based on clustering, we examined the temporal distribution of inhalation zone and wearable sensor CO2 concentration measurements. Fig. 7 shows two different cases: results from subject 16 demonstrates high correlation between the subjects’ inhalation zone CO2 concentration measurements and wearable sensor measurements whereas the results for subject 29 demonstrates a case where wearable sensor measurements are different than their inhalation zone CO2 concentration measurements. Results for

Discussion

In this study, we investigated the potential of wearable sensors to evaluate the inhalation zone CO2 concentration as a function of activity, desk settings, and personal differences. We used scenarios that realistically mimic regular office work conditions. However, the number of activities and desk settings might be expanded to generate more generalizable results. The ranking of the factors is only based on the data collection procedure in this study. The main takeaway is that all of the

Conclusion

This study was pursued in response to the premise that measuring CO2 concentrations is important for air quality, occupant health and building occupant productivity. One avenue to measuring indoor CO2 concentrations is via wearable sensors. Our experiment specifically explored the feasibility of using a neck-worn wearable sensor to measure the CO2 concentrations in the inhalation zone. Our methodology involved designing environments with different activity and desk configurations to mimic that

Acknowledgement

This study was funded by the U.S. General Services Administration (GSA) under interagency agreement # GX0012829 with the U.S. Department of Energy and Lawrence Berkeley National Laboratory. GSA's Wellbuilt for Wellbeing Group is a multidisciplinary research project team (GSA Contract # GS-00-H-14-AA-C-0094) consisting of the following members: Kevin Kampschroer, Judith Heerwagen and Brian Gilligan of GSA. Esther Sternberg, Perry Skeath, Casey Lindberg,and Matthias Mehlof the University of

References (39)

  • A. Ghahramani et al.

    An online learning approach for quantifying personalized thermal comfort via adaptive stochastic modeling

    Build. Environ.

    (2015)
  • S. Ahmadi-Karvigh et al.

    Real-time activity recognition for energy efficiency in buildings

    Appl. Energy

    (2018)
  • S. Ahmadi-Karvigh et al.

    One size does not fit all: understanding user preferences for building automation systems

    Energy Build.

    (2017)
  • A. Ghahramani et al.

    Towards unsupervised learning of thermal comfort using infrared thermography

    Appl. Energy

    (2018)
  • A. Ghahramani et al.

    Infrared thermography of human face for monitoring thermoregulation performance and estimating personal thermal comfort

    Build. Environ.

    (2016)
  • A. Ghahramani et al.

    Learning occupants' workplace interactions from wearable and stationary ambient sensing systems

    Appl. Energy

    (2018)
  • ASHRAE Standard, Standard 62. 1-2010. Ventilation for acceptable indoor air quality, Atlanta, GA, in: Proceedings of...
  • W.J. Fisk, CO2 Monitoring for Demand Controlled Ventilation in Commercial Buildings,...
  • C.A. Erdmann, M.G. Apte, Mucous Membrane and Lower Respiratory Building Related Symptoms in Relation to Indoor Carbon...
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