Why there must be a 1/H air exchange?

This is a standard construction
requirement for any habitable space. As the length of the fireplace
increases, so does the amount of re-circulated air, which means a larger air
volume is needed. This ensures that the recirculated air does not lower the overall
air quality in the room within an hour. This is the minimum air exchange
requirement. Increasing the air
exchange rate will further enhance air quality by reducing the
concentration of recirculated air.