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Entry or Exposure Routes

Major routes of exposure are through the skin (topical), through the lung (inhalation) or through the gastrointestinal tract (ingestion).  In general, inhalation is likely to cause more damage than ingestion, which, in turn, is more harmful than skin exposure.

Skin Absorption
This is the least likely route of penetration since the natural thickness of the skin plus its natural coating of grease and sweat provide some protection against chemicals. However, some materials are capable of penetrating intact, healthy skin e.g. aniline, hydrogen cyanide, some steroid hormones, organic mercury compounds, nitrobenzene, organophosphate compounds and phenol.  Phenol itself can be lethal if absorbed for a sufficient time through a few square centimetres of skin and inappropriate protective clothing e.g. incorrect gloves may cause absorption rate to increase.

The natural protection of the skin may also be bypassed through cuts, abrasion or puncture wounds e.g. needle-stick injury.

Inhalation
Gases and vapours, aerosols and fumes are readily inhaled and may cause harm (including asphyxiation) anywhere in the respiratory system and may also be absorbed into the bloodstream but inhalation of particles depends upon their size and shape -the smaller the particle, the further into the respiratory tract it can go.

• Large particles are filtered off in the nose.
• Smaller particles, or those breathed in by mouth, settle on the walls of the upper respiratory tract or throat and are coughed up and either ejected or swallowed.  If swallowed, they may enter the gut and cause damage as if they had been ingested.
• The smallest particles of dust and fibres can be inhaled down into the lungs where they can cause local damage, sometimes by interaction with the cells in the lungs which normally remove bacteria.  These particles may also be absorbed into the bloodstream.

Ingestion
Airborne particles that are eventually swallowed are the most likely source of ingested chemical. Otherwise, ingestion of potentially toxic substances is likely to be accidental on contaminated food, drink or make-up. Once absorbed through the stomach or intestine, the route to excretion may be complex and damaging.