Cigarette smoking can adversely affect health both directly and indirectly. One key concern is that cigarettes contain various levels of the chemical nicotine, a highly addictive substance. Therefore, smoking may easily become a controlling habit, driven by both psychological and physical dependence. The resulting frequency of use generally aggravates the already undesirable health effects of other damaging substances in the cigarettes. For example, a number of serious and generally irreversible heart and lung conditions may result from inhaled smoke; the other body systems often face many secondary consequences as well. Effective functioning of the body's heart, lungs, and metabolism normally requires proper interaction of internal chemical 'messengers.' However, cigarette smoke contains over 4,000 harmful chemicals that can interfere with these messengers. Over time, this disturbance in proper function can lead to the development of chronic heart and lung diseases. In the case of the heart, the most common problems generally include atherosclerosis (ath-er-oh-skluh-ROW-sis) or hardening and narrowing of the coronary arteries, coronary blood clot formation, and coronary artery spasms. Any of these conditions may create a high risk for sudden heart attack or chronic heart disease. With regard to the lungs, typical disease conditions include several respiratory system cancers, permanently narrowed airways or emphysema (em-fih-ZEE-muh), chronic swelling and irritation of airways, and eventually, permanently obstructed airways. These conditions can occur as smoke damages the airway lining's ability to carry toxin-filled mucus away from the lungs for expulsion. In addition, the smoke may introduce irritants into the lungs' tiny air sacs, causing inflammation and disintegration of the air sac structure itself. The disintegration can decrease the area in the lungs available for oxygen exchange, leading to shortness of breath, fatigue, and chronic cough with increased secretions. Further, these structural alterations may lead to secondary respiratory problems such as chronic nose and throat inflammation, and frequent, long-lasting respiratory infections. Not only does cigarette smoking generally harm the heart and lungs, but it may also adversely affect female hormonal balance, leading to osteoporosis and early-onset menopause. In men, it may cause a decrease in sperm mobility, creating fertility problems. In both men and women, smoking may reduce the effectiveness of medications, accelerate the aging process, cause chronic hoarseness, snoring, and sleep apnea. Cigarette smoking can also create a higher risk of a wide variety of cancers, including cancer of the lip, tongue, salivary gland, mouth, larynx, esophagus, middle and lower pharynx, stomach, bladder, kidney, cervix, and pancreas. There also seem to be significant links between long-term smokers and the incidence of leukemia.