You could add a deadband to the program, , which could be something like:

IF (Num_People > 0) && (Sensor_CO2 >=800 + deadband/2),
  SET Window = 1,
ELSIF (Sensor_CO2 <= 800 - deadband/2),
  SET Window = 0,
ENDIF

Or you could use trend variables and add hysteresis.

You could a deadband, which could be something like:

IF (Num_People > 0) && (Sensor_CO2 >=800 + deadband/2),
  SET Window = 1,
ELSIF (Sensor_CO2 <= 800 - deadband/2),
  SET Window = 0,
ENDIF

You could a deadband, deadband, which could be something like:

IF (Num_People > 0) && (Sensor_CO2 >=800 + deadband/2),
  SET Window = 1,
ELSIF (Sensor_CO2 <= 800 - deadband/2),
  SET Window = 0,
ENDIF

You could a deadband, which could be something like:

IF 
If (Num_People > 0) && (Sensor_CO2 >=800 + deadband/2),
 SET Window = 1,
ELSIF (Sensor_CO2 <= 800 - deadband/2),
 SET Window = 0,
ENDIF