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Breathing is the most common avenue for the introduction of poisons or harmful agents into the human system. Whether it involves a dust, gas, fume, vapour, biological agent, fibre, spores or enzymes, the effects can vary from irritation to devastating, permanent, disabilities or death. The mechanisms for these consequences vary with the offending agent and the effects vary with the agent and the circumstances of exposure.

The flow of air into the lungs begins at the nose and mouth and passes into the windpipe and bronchii. There are many bronchii or tubes, of decreasing diameter and these fine tubes extend into every part of the lungs and terminate in the alveoli, small leaf-shaped chambers where the oxygen in the air is passed into the blood stream.

The air breathed in is cleaned by a combination of hairs in the nose and mucus on the walls of the bronchii. The mucus is constantly being ferried up to the nose and mouth with its burden of dirt and particles. With one exception, only a gas, vapour or fume can pass into the system to reach the alveoli.

That exception is respirable dust or fibre, which means the dust or fibre particle must be of a size equal to or smaller than 5 µm (one µm is called a micron). For dust to be visible in the air, it needs to be at least 50 µm in size. Fibres of asbestos can range down in size as small as 0.015 µm in diameter. These can only be seen with an electron microscope. There is no filter available to catch particles of this size.

1 metre = 39.7 inches
1 metre = 100 centimetres (cm)
1 cm = 100 millimetres (mm)
1 mm = 1,000 microns (µ)

A vapour is a suspension of liquid droplets in air or gas. A fume is a suspension of metal droplets in air or gas.

The following are some categories of lung disease, based on the mechanisms of damage:

1. Asthmatic responses
2. Pneumoconioses
3. Irritations
4. Hypersensitive reactions.
5. Non-irritants