I often carry my Aranet4 CO₂ monitor with me when I go places. CO₂ concentration is a common way to measure the amount of fresh air being circulated into a space—a factor in covid transmission risk. In general, the higher the CO₂ levels, the more air you are breathing in that other people have breathed out. One limitation of this measurement is that, while it is a good proxy for measuring ventilation, it doesn't measure filtration (or virus inactivation factors like su">