As cigarette smoking is a major cause of lung cancer nowadays, it is important to appreciate how smoking affects and injures the lungs. This is because smoke inhalation damages the normal cleaning process by which the lung protects itself from injury.
The bronchi which conduct inhaled air to the lung tissues are lined with a single coating of cells on which lies a defensive coating of mucus. The hair-like cilia on these cells beat in a regular rhythm to advance mucus upwards continually from the lung removing any inhaled particles which may have become trapped in the process.
The competence of this cleaning mechanism is damaged very quickly by smoke inhalation. The cilia disappear and the coating they lie in thickens in an attempt to protect the fine underlying tissues from injury. Once this damage has occurred, the lung can no longer keep itself uncontaminated.
As a result, the cancer-producing agents in cigarette smoke remain ensnared in the mucus on the surface lining of the airway. They then pass into the cells before being removed by coughing which is the only cleansing mechanism remaining.
Once they are within the body, these chemicals, and their by-products, alter the very nature of the cells in the lungs slowly and increasingly until finally cancer develops.
Most lung cancers start in the lining of the bronchi, though some cancers also begin in the trachea, bronchioles, or alveoli.
Lung cancer is thought to develop over a period of years. To start with there is probably areas of pre-cancerous changes in the lung, but these changes don’t form a mass or tumour.
Unfortunately they can’t be seen on an x-ray and they don’t cause any symptoms. As the cancer develops, it continues to grow and forms a tumour which is then large enough to be seen on x-rays.
These pre-cancerous changes can be exposed by analysing cells in the lining of the airways of smoke-damaged lungs. Molecular abnormalities considered to be pre-cancerous have also been recognized in cells from people who are at high risk of developing lung cancer. These pre-cancerous changes usually progress to true cancer.