Abstract: Systems, devices, and methods are provided for monitoring air in an indoor space. An example system includes at least one sensor located in the indoor space to generate monitoring data, a communication interface, and a processor connected to the communication interface and in communication with the at least one sensor. The at least one sensor includes at least a carbon dioxide sensor or a particle sensor. The processor is operable to receive monitoring data for an extended time period, which includes at least one period when the indoor space is occupied and at least one period when the indoor space is unoccupied. The processor is operable to identify at least one region of interest in the monitoring data; and determine an air change rate for the indoor space based on the region of interest. The processor can be further operable to determine an occupancy or an airborne infection risk based on real-time monitoring data.