The document summarizes the harmful substances found in cigarettes and their effects on the lungs. It lists the main substances in cigarettes as nicotine, tar, and carbon monoxide. It then describes several other toxic and carcinogenic elements and chemicals found in tobacco and cigarette smoke, including benzopyrene, methanol, acetic acid, cadmium, formaldehyde, arsenic, ammonia, butane, nitrogen dioxide, endrin, and hydrogen cyanide. All of these substances can damage and destroy lung tissue over time.