The Control of Metabolic CO2 in Public Transport as a Strategy to Reduce the Transmission of Respiratory Infectious Diseases

Abstract

The global acceptance of the SARS-CoV-2 airborne transmission led to prevention measures based on quality control and air renewal. Among them, carbon dioxide (CO₂) measurement has positioned itself as a cost-efficiency, reliable, and straightforward method to assess indoor air renewal indirectly. Through the control of CO₂, it is possible to implement and validate the effectiveness of prevention measures to reduce the risk of contagion of respiratory diseases by aerosols. Thanks to the method scalability, CO₂ measurement has become the gold standard for diagnosing air quality in shared spaces. Even though collective transport is considered one of the environments with the highest rate of COVID-19 propagation, little research has been done where the air inside vehicles is analyzed. This work explores the generation and accumulation of metabolic CO₂ in a tramway (Zaragoza, Spain) operation. Importantly, we propose to use the indicator ppm/person as a basis for comparing environments under different conditions. Our study concludes with an experimental evaluation of the benefit of modifying some parameters of the Heating-Ventilation-Air conditioning (HVAC) system. The study of the particle retention efficiency of the implemented filters shows a poor air cleaning performance that, at present, can be counteracted by opening windows. Seeking a post-pandemic scenario, it will be crucial to seek strategies to improve air quality in public transport to prevent the transmission of infectious diseases.

Keywords: CO2; COVID-19; SARS-CoV-2; airborne; collective transport; epidemiology; filtration; infectious diseases; public health; tramway.

Figure 1

Schematic representation (a) of the meters distribution in the Tram and (b) installed sensors in the Tram. Where, # refers to the meter ID.

Figure 2

Route of the Zaragoza Tram. Where, # refers to the station ID.

Figure 3

Performance test. (a) Diagram of the equipment used to characterize the filters and (b) particle concentration distribution for efficiency determination measurements in the range 0.1–1.0 μm.

Figure 4

CO₂ increment average levels (a) in all weekday and (b) in all weekend routes, and (c) ppm/person ratio average and maximum gap along routes. The error bars in (a,b) correspond to the difference between the maximum/minimum data and the average data of all studied routes.

Figure 5

∆CO₂ increment and tram occupancy as (a) a function of time, and as (b) a function of tram occupancy. The error bars correspond to the difference between the maximum/minimum data and the average data of all the studied routes.

Figure 6

Distribution of ∆CO₂ in the Tram (z-axis) as a function of time (x-axis) and ∆CO₂ measures (y-axis) on routes #2, #4–#25, and #32–#33, where there is homogeneity in the CO₂ measurement along the tram and a strong relationship with occupancy.

Figure 7

Registered ppm/person values depending on the air return.

Figure 8

∆CO₂ per person ratio (ppm/person) in (a) Set A, and in (b) Set B depending on time; (c) difference between Set B and Set A depending on time.

Figure 9

CO₂ measurements depending on Tram speed. (a) ∆CO₂ per person ratio, and (b) average reduction rate of ∆CO₂ depending on the Tram speed.

Figure 10

Coarse 75% filter retention efficiency depending on the particle diameter at different speeds (flow rates).