Carbon dioxide (CO2) at levels that are unusually high indoors may cause occupants to grow drowsy, get headaches, or function at lower activity levels. Humans are the main indoor source of carbon dioxide. To eliminate most Indoor Air Quality complaints, total indoor carbon dioxide should be reduced a difference of less than 600 ppm above outdoor levels. Indoor air concentrations of carbon dioxide that exceed 1,000 ppm are considered a marker suggesting inadequate ventilation. It is recommended that carbon dioxide levels not exceed 700 ppm above outdoor ambient levels. OSHA limits carbon dioxide concentration in the workplace to 5,000 ppm for prolonged periods, and 35,000 ppm for 15 minutes. A common cause of excess CO2 are leaks in metal exhaust pipes.