When carbon monoxide is inhaled, it takes the place of oxygen in haemoglobin to form carboxyhaemoglobin in the body. Because carbon monoxide binds to haemoglobin several hundred times more strongly than oxygen, its effects are cumulative and long-lasting, causing oxygen starvation throughout the body. Concentrations as low as 20 or 30 parts per million (PPM) can be harmful if exposed for several hours. Exposure at 2,000 PPM for one hour will cause unconsciousness.