We have all experienced at least once the bad effects of overcrowding in a cinema, bar or in a theater. This happens due to physical factors as there is no evaporation of heat from the surface of skin. The prevention of heat loss by radiation causes heat stagnation and people suffer from drowsiness, headache, lassitude, nausea, vomiting which may lead to unconsciousness or death. In the past, it was believed that these symptoms were caused entirely by the changes in the air by respiration. However, it is proved that the increase of carbon dioxide and organic poisons from the expired air as well as the decrease of oxygen are not sufficient by themselves or even in combination to produce these symptoms unless other conditions, such as the stagnation of air, increased humidity and rise of temperature which are present in the overcrowded room.
Natural sources of ventilation are not possible, where a large number of people are congregated for a considerable long period and where climatic conditions do not permit free use of open doors and windows. Therefore, artificial ventilation is largely resorted to in these places and the mechanical means are used to facilitate the renewal of air.