Smoking is the most common method of consuming tobacco, and tobacco is the most common substance smoked. The agricultural product is often mixed with additives and then combusted. The resulting smoke is then inhaled and the active substances absorbed through the alveoli in the lungs or the oral mucosa. Combustion was traditionally enhanced by addition of potassium or nitrates. Many substances in cigarette smoke trigger chemical reactions in nerve endings, which heighten heart rate, alertness and reaction time, among other things. Dopamine and endorphins are released, which are often associated with pleasure. As of 2008 to 2010, tobacco is used by about 49% of men and 11% of women aged 15 or older in fourteen low-income and middle-income countries (Bangladesh, Brazil, China, Egypt, India, Mexico, Philippines, Poland, Russia, Thailand, Turkey, Ukraine, Uruguay and Vietnam), with about 80% of this usage in the form of smoking. The gender gap tends to be less pronounced in lower age groups.