1.1 Mechanisms of exposure to chemicals AND how chemicals may harm you
Chemicals can have a range of unwanted effects on the body. Some of these are short term and relatively insignificant. Others may be of a more serious nature or even fatal and not all of the harmful effects of chemicals are immediately apparent.
Chemicals pose a risk of harm to individuals in two ways. They may harm the outer surface of the body such as the skin and mucous membranes. Alternatively, they may cause damage by their actions from within the body if they are permitted to enter the blood stream, or any of the tissues or organs. Below are brief descriptions of some ways in which chemicals may enter your body.
Skin or other surface contact
Simple contact with skin is all that is necessary for some chemicals to cause harm. Chemicals such as strong acids or alkalis have the ability to cause serious damage to skin after only a short exposure.
Some chemicals, e.g. organic solvents, can extract natural oils and fats from the skin. Over a period of time this may lead to dermatitis or other skin conditions.
Effects on the skin and body surfaces
Effects on the skin can include blistering or allergic reaction and the severity of harm is often significantly greater if these chemicals contact the less well protected tissues of the eyes, mouth or inside the nose. The respiratory tract and the gut are also easily contaminated by chemicals. Although not outside the body, the linings of the gut could certainly be regarded as a body surface and these are particularly sensitive to some chemicals.
The effects of chemicals on these parts of the body are often quickly apparent and in cases where the damage has not been severe recovery will occur. Severe damage may result in scarring or other damage that is permanent.
Sometimes, however, exposure to a chemical can result in an allergic sensitisation that is not immediately apparent until a future exposure to the same or a similar chemical, at which time a reaction will occur. The reaction generally subsides when exposure to the chemical stops. Any reaction may be mild or severe but it is likely to remain a permanent feature in which case future exposures would trigger a similar reaction. In some cases the severity of the reaction increases with each exposure. Occasionally reactions may be fatal.
Some chemicals have properties that allow them to be absorbed directly through the skin. This is sometimes called cutaneous absorption. They enter the bloodstream and may cause harm to various tissues or organs in the body. These chemicals may also cause direct damage to the skin itself.
Dimethyl sulphoxide can pass easily through the skin and enter the blood stream where it can cause damage to the central nervous system. It can also carry with it any chemicals that are dissolved in it presenting the risk of further toxicity.
Breaks or cuts in the skin
A break or cut in the skin is a breach of the body’s natural defences and has the potential to allow chemicals direct access to our blood stream or to the tissues beneath the skin.
Eyes and mucous membranes
The eyes and mouth/nose area are parts of our body where there is less defence against chemicals. They are particularly susceptible to direct damage and also pose a lesser barrier to chemicals that may be absorbed directly. Gases, fumes and vapours are particularly hazardous and the absorption of this form of chemicals via the eye is a route that is often overlooked in risk assessments.
Formaldehyde and ammonia vapours are chemicals that can damage the eyes directly. Even a small exposure will irritate and cause the eyes to water. Larger exposures can lead to blindness.
If the chemical enters the body by being breathed in through the nose or the mouth harm can be caused directly to the linings of the respiratory tract, alternatively the chemical may be absorbed into the blood stream. The physical form of chemicals that cause this type of harm is generally one of the following:
Vapours and Gases
Many vapours, and gases are not capable of being detected by the human senses as they have no odour or colour. They may be absorbed into the blood stream where they may circulate to internal organs and result in harm.
Carbon monoxide gas is undetectable by smell, taste or visibility, but when inhaled it rapidly enters the bloodstream and reduces the ability of blood to transport oxygen.
Mercury vapour when inhaled is absorbed into the blood stream and may accumulate in various organs, including the brain. Depending on the exposure level the effects may range from relatively minor to fatal. Effects on the central nervous system include impaired speech and vision, loss of memory, hallucinations or delirium.
Fumes and Mists and Aerosols
Fumes are solid particles produced by condensation from the gaseous phase. They usually arise from heating a solid to its melting point and the subsequent cooling of the gas produced.
Mists are formed by aerosols from the surface of liquids and are often visible as white clouds although coloured mists are possible. They are therefore more easily detected than vapours and gases.
Melting solder forms fumes which when inhaled can lead to occupational asthma.
Hydrochloric acid readily forms mists which when inhaled are corrosive to the lining of the respiratory tract.
Dusts are particulate materials that are, or have been. airborne. ‘Inhalable dusts’ are normally comprised of a range of particle sizes the larger of which can only enter the upper respiratory tract. The smaller the particle the deeper into the lungs it can penetrate, where it may settle out onto the lining.
The damage caused by the dust may be chemical in nature, causing allergic reactions, toxic effects etc. It may, however, have no chemical effect and cause only physical irritation
Many chemicals pose a hazard if they are eaten or swallowed. There is, of course, a risk of direct physical damage to the lining of the gut, but equally serious is the risk of absorption of some chemicals after they have been ingested. The gut is designed to maximise absorption of materials and any chemical that crosses into the blood stream may then easily reach other tissues or organs where it may have an adverse effect.
Ingestion of sodium cyanide can cause damage to the central nervous and cardiovascular systems resulting in coma or death.
This is a less common route for accidental exposure to a chemical. None the less it is an extremely efficient method of having a chemical enter the tissues or blood stream and for this reason is chosen as the method of administration for many chemicals that are used as medication. Accidental injection can occur by means of a hypodermic needle/syringe when an individual is delivering an injection to an animal or human patient. It may also occur when any sharp object that is contaminated with a chemical pierces the skin. Any natural defences present in the skin are bypassed and transport of the chemical to critical tissues or organs can occur very rapidly.
The veterinary anaesthetic Large Animal Immobilon contains etorphine and acetylpromazine. Accidental injection of even a small quantity to humans causes dizziness, nausea, respiratory depression, cyanosis, hypotension, loss
of consciousness and death.
Effects on the body’s function (systemic effects)
Once a chemical has penetrated the body’s outer protective barriers there is little to stop it reaching the internal tissues and organs. The type of disruption to body functions varies widely as do the symptoms, their severity and their duration.
With topical exposure it may be some time before any harmful effects become apparent. One reason for this may be that harm has resulted from repeated exposures to small quantities of a chemical for which the harmful effects are cumulative. Particularly serious long-term effects can arise from exposure to chemicals that are carcinogenic or mutagenic. Carcinogenic chemicals have the potential to cause cancer whereas mutagens cause changes to the hereditary genetic material that is a part of body cells. Mutation may also result in the subsequent development of cancer. In each of these cases the time between exposure occurring and the development of symptoms can be extremely lengthy, typically many years.
It is also possible to develop allergic reactions to chemicals that have entered the body. As with the topical allergic reactions described above, these reactions can vary in severity and the period of time to onset and may also be fatal.
Individuals who are particularly at risk
Young people and pregnant women are examples of groups of the population who may be at a higher risk of suffering harm from exposure to some chemicals. Section 3.2 deals with how to carry out a risk assessment but you should note that during the risk assessment process you should pay particular attention to any especially vulnerable group of individuals and ensure that any appropriate additional controls are used.
Updating our view of chemical hazards
Research continues to improve our understanding of the functions of the body and how they are controlled. As our knowledge increases so too does our understanding of the effects of chemicals on these critical processes. This may modify our perception of the safety of chemicals and trigger changes to the legal exposure limits and the classification. For this reason it is essential to:
- completely eliminate exposure where it is practicable to do so
- keep all chemical exposure to an absolute minimum
- keep abreast of current exposure limits (e.g. MEL and ODS definitions of these terms)
- routinely review risk assessments