The Texas Elementary School Indoor Air Study (TESIAS) involved several phases, including single-day continuous monitoring of carbon dioxide (CO2) in 120 randomly selected classrooms in two school districts. The median time-averaged and peak CO CO2 concentrations were 1,286 ppm and 2,062 ppm, respectively. The time-averaged CO2 concentration exceeded 1,000 ppm in 66% of the classrooms. The peak CO2 concentration exceeded 1,000 ppm in 88% of the classrooms and 3,000 ppm in 21% of the classrooms. Mean and peak occupied CO2 concentrations were statistically different (a = 0.05) between the two districts, and peak CO2 concentrations were statistically greater in classrooms that employed packaged terminal air conditioning (PTAC) systems. Statistically significant differences in both mean time-averaged and peak CO2 concentrations were not observed for portable vs. traditional classrooms, classrooms with outside vs. inside entries, or when data were separated amongst teacher responses to questions related to classroom odors.