Robert T. Gregory


To assess airborne pathogen infection risk, concentrations of interior carbon dioxide and changes in relative humidity serve as proxies for human respiration products. Yanes and Yapp, 2010, demonstrated that interior air carbon dioxide concentrations and 13C/12C ratios display a two-component mixing relationship between respiration carbon dioxide which is highly concentrated (>100x atmospheric) in respiration air CO2 at >40,000 ppm compared with ambient air (~410 ppm). In the absence of combustion carbon dioxide, the concentration of carbon dioxide, in an interior space, depends on the rate of respiration CO2 addition relative to the amount of fresh atmosphere mixed back into the air circulation system. The number of occupants and the proportion of outside air introduced to determine a series of steady-state room concentrations punctuated by transient CO2 concentrations that follow first-order rate laws with linear trends in plots of ln(f) vs. time where f tracks the progression of concentrations between unoccupied and occupied states of a building. Previous studies have shown several school classrooms achieve CO2 concentrations 2 to 5 times atmospheric, with buildup times on minute to hour timescales depending on the size of the room and number of occupants. Preliminary data show that removal time scales exceed rise timescales ~1/2 of the time and are at least ten to thirty minutes longer than the steady-state rise times, i.e., the transmission of pathogens can occur before their removal by the filtration system. Using American Society of Heating Refrigeration and Air-Conditioning Engineering (ASHRAE) standards and isotopic ratio measurements measured ppm concentrations of CO2 transform into percentages of classroom respiration CO2 whichtypically exceeds 40% and can be >60% inhaled respiration carbon dioxide. CO2 can be translated into infection risk with increasing CO2 values correlating to higher risk of infection and a decrease in overall exposure time prior to an infection.

Degree Date

Spring 5-2022

Document Type


Degree Name



Earth Sciences


Robert T. Gregory

Second Advisor

Crayton J. Yapp

Third Advisor

Matthew Hornbach

Subject Area

Health Sciences, Immunology, Public Policy

Number of Pages




Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

CO2_Data (2).zip (949 kB)