Of the carcinogenic substances in tobacco smoke, which is the most damaging to the lungs?

The most damaging components of tobacco smoke are:

Tar – this is the collective term for the various particles suspended in tobacco smoke. The particles contain chemicals, including several cancer-causing substances (carcinogens). Tar is sticky and brown, and stains teeth, fingernails and lung tissue. Tar contains the carcinogen benzo(a)pyrene.

Carbon monoxide – this odourless gas is fatal in large doses because it takes the place of oxygen in the blood. Each red blood cell contains a protein called haemoglobin that transports oxygen molecules around the body. However, carbon monoxide binds to haemoglobin better than oxygen. In response, the body makes more red blood cells to carry the oxygen it needs, but it makes the blood thicker. This means that when the body demands more oxygen during exercise, less oxygen reaches the brain, heart, muscles and other organs.

Hydrogen cyanide – the lungs contain tiny hairs (cilia) that help to clean the lungs by moving foreign substances out. Hydrogen cyanide stops this lung clearance system from working properly, which means the poisonous chemicals in tobacco smoke can build up inside the lungs. Other chemicals in smoke that damage the lungs include hydrocarbons, nitrous oxides, organic acids, phenols and oxidising agents.
Oxidizing chemicals – these highly reactive chemicals (which include free radicals) can damage the heart muscles and blood vessels. They react with cholesterol, leading to the build-up of fatty material on artery walls. Their actions lead to heart disease, stroke and blood vessel disease.
Metals – tobacco smoke contains dangerous metals including arsenic, cadmium and lead. Several of these metals are carcinogenic.
Radioactive compounds – tobacco smoke contains radioactive compounds that are known to be carcinogenic.