A ventilation system and a method for discharging hazardous air during cooking are disclosed. The ventilation system allows hazardous air generated by cooking to flow in a backward direction of a cooking device so as to quickly transfer the hazardous air to a hood, such that the hazardous air is prevented from being inhaled by a user. As a result, the hazardous air may be concentrated into a predetermined region without being dissipated, and then subsequently discharged outside. Information about a quality of air generated in a current state of a cooking space is transferred to a user so that the user may more actively use the ventilation system, thereby better protecting the user's health.