Ventilation is an organised way of exchanging air in the house. Efficient ventilation system allows eliminating carbon dioxide, water vapour, saprophytes and other allergens from the air. There are two kinds of ventilation systems: gravitational, being the most common, and mechanical, employing fans. The principle of a good ventilation system is that the air flows to the less polluted rooms first, and only then to those with higher pollution level, so in the case of houses - through dayrooms to the kitchen, bathrooms, toilets and windowless rooms. The process is possible only when sufficient amount of air gets into the house, and the chimney duct is in working order.