The term respiration has two major definitions. On a cellular level, respiration is the chemical process by which food molecules are oxidized to release energy. In the process, carbon dioxide and water are also produced. Cellular respiration is, therefore, the reverse of photosynthesis, the process by which carbon dioxide and water are combined to form complex carbohydrate molecules. On an organismic level, respiration refers to the act of inhaling and exhaling. Respiratory diseases are among the most common of all environmental illnesses. Inhaling particulate matter in polluted air may result, for example, in various forms of pneumoconiosis.