during the past 30 days. This represented approximately three in 10 high school students (31.2%) and approximately one in eight middle school students (12.5%). Rates of smoking continue to rise in developing countries, but have leveled off or declined in developed countries. Smoking rates in the United States have dropped by half from 1965 to 2006, falling from 42% to 20.8% in adults. In the developing world, tobacco consumption is rising by 3.4% per year. Health effects Chemicals Tobacco smoking harms health because of the toxic chemicals in tobacco smoke, including carbon monoxide, cyanide, and carcinogens, which have been proven to cause heart and lung diseases and cancer. Thousands of different substances in cigarette smoke, including polycyclic aromatic hydrocarbons (such as benzopyrene), formaldehyde, cadmium, nickel, arsenic, tobacco-specific nitrosamines, and phenols contribute to the harmful effects of smoking. According to the World Health Organization, tobacco is the single greatest cause of preventable death globally. WHO estimates that tobacco caused 5.4 million deaths in 2004 and 100 million deaths over the course of the 20th century. Similarly, the United States Centers for Disease Control and Prevention describe tobacco use as "the single most important preventable risk to human health in developed countries and an important cause of premature death worldwide." Due to these health consequences, it is estimated that a 10 hectare (approximately 24.7 acre) field of tobacco used for cigarettes causes 30 deaths per year – 10 from lung cancer and 20 from cigarette-induced diseases like cardiac arrest, gangrene, bladder cancer, mouth cancer, etc. The harms caused by inhaling tobacco smoke include diseases of the heart and lungs, with smoking being a major risk factor for heart attacks, strokes, chronic obstructive pulmonary disease (emphysema), and cancer (particularly cancers of the lungs, larynx, mouth, and pancreas). Cancer is caused by inhaling carcinogenic substances in tobacco smoke. Inhaling secondhand tobacco smoke (which has been exhaled by a smoker) can cause lung cancer in nonsmoking adults. In the United States, about 3,000 adults die each year due to lung cancer from secondhand smoke exposure. Heart disease caused by secondhand smoke kills around 46,000 nonsmokers every year. In children, exposure to secondhand tobacco smoke is associated with a higher incidence and severity of respiratory illnesses, middle ear disease, and asthma attacks. Each year in the United States, secondhand smoke exposure causes 24,500 infants to be born with low birthweight, 71,900 preterm births, 202,300 episodes of