The cartilage rings are C-shaped because the back of the trachea presses against the esophagus. The cartilage opens at the esophagus and is replaced by connective tissue and muscle. This gives the esophagus space to allow food to be swallowed easily. The rings of the trachea are stacked one on top of the other, with a small space between them.
In 2014, a survey of the Australian working population revealed that the most common carcinogenic exposures in the workplace were solar ultraviolet radiation, diesel engine exhaust, environmental tobacco (second-hand) smoke, benzene, lead and silica.
Carcinogens do not cause cancer in everyone who is exposed; the ability of a carcinogen to cause cancer depends on many factors, including the amount of exposure, the length of exposure, the health of the individual, and other factors in the person's life that either raise or lower the risk of cancer.