Then what causes sick – building syndrome in an office building or school, where people don’t smoke or burn oil, wood, or gas? Experts have discovered several sources of sick – building syndrome; among these are mold and bacteria, synthetic products, and lack of ventilation – or the movement of fresh air into and out of the building. In many buildings, rain has leaked in and caused water damage to walls and carpets. This allows mold and bacteria to grow. Air conditioning systems are another place where mold and bacteria can grow. Synthetic (that is, man – made) products such as paint, carpeting, and furniture can be found in all offices and schools. These products release toxic (poisonous) chemicals into the air. Perhaps the most common cause of sick – building syndrome, however, is lack of ventilation. Most modern office buildings are tightly sealed; in other words, the windows don’t open, so fresh air doesn’t enter the building. In a building with mold, bacteria, or toxic chemicals, lack of ventilation makes the situation more serious.