Human personal air pollution clouds in a naturally ventilated office during the COVID-19 pandemic

Abstract

Personal cloud, termed as the difference in air pollutant concentrations between breathing zone and room sites, represents the bias in approximating personal inhalation exposure that is linked to accuracy of health risk assessment. This study performed a two-week field experiment in a naturally ventilated office during the COVID-19 pandemic to assess occupants' exposure to common air pollutants and to determine factors contributing to the personal cloud effect. During occupied periods, indoor average concentrations of endotoxin (0.09 EU/m³), TVOC (231 μg/m³), CO₂ (630 ppm), and PM₁₀ (14 μg/m³) were below the recommended limits, except for formaldehyde (58 μg/m³). Personal exposure concentrations, however, were significantly different from, and mostly higher than, concentrations measured at room stationary sampling sites. Although three participants shared the same office, their personal air pollution clouds were mutually distinct. The mean personal cloud magnitude ranged within 0-0.05 EU/m³, 35-192 μg/m³, 32-120 ppm, and 4-9 μg/m³ for endotoxin, TVOC, CO₂, and PM₁₀, respectively, and was independent from room concentrations. The use of hand sanitizer was strongly associated with an elevated personal cloud of endotoxin and alcohol-based VOCs. Reduced occupancy density in the office resulted in more pronounced personal CO₂ clouds. The representativeness of room stationary sampling for capturing dynamic personal exposures was as low as 28% and 5% for CO₂ and PM₁₀, respectively. The findings of our study highlight the necessity of considering the personal cloud effect when assessing personal exposure in offices.

Keywords: Endotoxin; Gaseous pollutants; Indoor air quality; Particles; Personal exposure.

Fig. 1

The layout of the office. The office was regularly occupied by three occupants (P1, P2, and P3) during the COVID-19 pandemic. There were four stationary sites for sampling indoor air pollutants: S1, S2, S3, and S4, of which S4 was the location of the room thermostat. Personal exposure was sampled at breathing zones of P1, P2, and P3. The atrium measurement station was located 10 cm outside the window. P1 was involved in the semi-controlled experiment in the second week. Geometrical data were annotated in the layout. In addition, the sizes of main furniture were as follows (L × W × H, m): desk (1.6 × 0.8 × 0.8), cabinet (1.2 × 0.4 × 1.1), coffee table (1.0 × 0.6 × 0.5), and sofa (2.0 × 0.8 × 0.8).

Fig. 2

Endotoxin concentrations detected at the atrium, indoor (unoccupied and occupied), and personal sites (P1, P2, and P3). Numerical labels on the top of P1, P2, and P3 bars indicate magnitudes of individual personal endotoxin clouds (difference between personal and room average levels). Diamonds represent the number of times using sanitizer for each participant during the sampling period in the first week (right axis). The error bar of the occupied room represents the standard deviation of room stationary samples.

Fig. 3

Example of time-series of room average and P1 personal CO₂ concentrations, and the magnitude of CO₂ personal cloud (difference between personal and room average levels) in a day. The red band represents the standard deviation of CO₂ values at room stationary sampling sites. The yellow background indicates the period when P1 was seated in the office. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

CO₂ personal cloud of three participants (A) and the correlation with room average CO₂ concentration of 1-min resolution (B). The triangle in the box plot represents the mean value. The difference in CO₂ personal cloud magnitude was significant among the three participants (p < 0.001). R² represents the coefficient of determination.

Fig. 5

Example of time-series of room average and P1 personal PM₁₀ concentrations, and the magnitude of PM₁₀ personal cloud (difference between personal and room average levels) in a day. The red band represents the standard deviation of PM₁₀ values at room stationary sampling sites. The yellow background indicates the period when P1 was seated in the office. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6

PM₁₀ personal cloud of three participants (A) and the correlation with room average PM₁₀ concentration of 1-min resolution (B). The triangle in the box plot represents the mean value. The difference in PM₁₀ personal cloud magnitude was significant among three participants (p < 0.001). R² represents the coefficient of determination.

Fig. 7

Influence of four factors on PM₁₀ personal cloud magnitude of one participant (P1). The factors include wearing clothing that was previously worn during office working, applying body cream before experiments, using hand sanitizer during the experiments, and wearing clothing that was worn during cooking. The triangle in the box plot represents the mean value. The difference was not significant (p = 0.94).

Fig. 8

Scatter plot of (A) CO₂ concentrations and (B) PM₁₀ concentrations measured at personal sites (P1, P2, and P3) and room stationary sites (S1, S2, S3, and S4). Refer to Fig. 1 for specific locations of the personal and room stationary sites in the office. R² represents the coefficient of determination.