|Injury can be caused by chemicals only if they reach sensitive parts of the body at a sufficiently high concentration and for a sufficiently long time (high enough exposure).|
|Exposure: is a function of the amount (or concentration) of the chemical involved and the time of it interaction. Effects may vary depending upon the nature of exposure - continuous or repeated short intervals- and may depend on the substance's ability to accumulate in the body and on the "victim's" age and/or state of health.|
|The potential to cause damage or toxicity can vary enormously from substance to substance e.g. LD50 values (the single dose that can be expected to kill 50% of a population) can vary from 7000mg/kg of body weight for Ethanol to 0.02mg/kg for Dioxins.|
|The injury depends on the properties of the potentially toxic substance, the exact nature of the exposure circumstances and the health of the person at risk.|
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.
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.
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.
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.
- Adverse Effects may be local or systemic.
- Local Effects occur at the site of exposure e.g. corrosives and often irritants.
- Systemic Effects occur at a target organ or at site remote from the point of contact following absorption and distribution around the body.
Some substances produce both effects e.g. lead tetraethyl damages the skin on contact and is the absorbed and transported to the central nervous system where it may cause further damage.
Adverse Effects may also be acute or chronic.
- Acute Effects are immediate such as the effect from inhaling chlorine.
- Chronic Effects are much slower, often cumulative following repeated exposures. Chronic effects can be the most difficult to avoid because damage may not become evident for many years.
Adverse Effects may also be reversible or irreversible.
- Reversible Effects: damage can be repaired by the body's natural processes.
- Irreversible Effects: cannot be repaired e.g. dead nerve cells cannot be replaced.
Adverse Effects may be psychological as well as purely physical.
Response of the Body
- Respiratory: Chemicals can irritate the nose and upper respiratory tract causing sneezing, coughing and, in some cases, bronchitis. They may also damage lung tissue.
- Skin and Eyes: Chemicals on skin often cause dermatitis (a rash). Solvents can de-grease the skin causing it to become red and sore. Corrosive material such as strong acids and bases can be irritants in dilute form but cause severe burns when concentrated. Eyes are particularly vulnerable and damage can be permanent.
- Respiratory: Some chemicals can cause sensitisation leading to asthma. Once sensitised, even minimal exposure can cause a severe allergic response.
- Skin: Sensitisation can occur through contact with a chemical and the skin. As before, once sensitised, minimal contact can cause an allergic response causing severe itching and other discomfort.
The most serious of long-term effects is cancer, a disorder of cell growth. Here the effect of exposure may not be evident for many years. Carcinogenic materials are considered in more detail here.
Possible effects are loss of fertility in both men and women, heritable genetic damage or harm to the unborn child.