Depends if you care about CO2 and presumably the CO2 from breathing humans. The VOCs from cars in a busy city will mask/overwhelm/confound any measure of human

Depends if you care about CO2 and presumably the CO2 from breathing humans. The VOCs from cars in a busy city will mask/overwhelm/confound any measure of human body produced CO2 in that location. But a more suburban or rural location will be fine.