People are warm, and we control our body temperature by emitting different amounts of heat. This happens 24 hours a day and is completely automatic, requiring no additional ventilation. The fact that we give off heat like this creates a natural airflow (natural convection) of 10–15 litres of air a second that tries to rise to the ceiling like a balloon.
When we stand up, the airflow starts around our feet and follows the body upwards. Just like a chimney. The airflow changes in size and speed depending on how warm we are. This means that the airflow has a greater volume and speed when we have been training and are sweating, for instance, compared to times when we feel chilly.