Chemical hazards

Hazardous substances present one of the major hazards that Occupational Safety and Health (OSH) professionals need to consider regarding health at work.

Hazardous substances are used for a variety of tasks, from simple cleaning operations to complex chemical processes. Hazardous substances have the potential to harm human health. They may be solids, liquids or gases; they may be naturally occurring, manufactured as a single substance or mixture, or can be a by-product of an industrial process. When  used in the workplace, these substances often generate vapours, fumes, dusts and mists. A wide range of industrial, laboratory and agricultural chemicals are classified as hazardous, as are many biological agents and nanoparticles.

Hazardous substances can cause a wide range of health effects, from localised (skin irritation or burns from irritating or corrosive substances) to systemic (such as occupational cancers and lung disease).

Although hazardous substances can also lead to fire, explosion and environmental risks, this guidance will only focus on  the health risks arising from occupational exposure to hazardous chemicals.

The physical state of chemicals

Chemicals can exist in many physical states. They can be:

• solids
• liquids
• gases
• dusts (inhalable and respirable), fumes, vapours, and mists.

The physical state of chemicals may change, depending on the environmental factors and / or processes. They are important to consider when assessing the routes of exposure.

The classification of chemical hazards

Substances hazardous to health are  classified as:

• corrosive – for example, oven cleaners and caustic soda.
• hazardous to the environment - for example, pesticides and mercury
• explosive – for example, fireworks.
• acute toxicity - such as lead or sodium cyanide
• health hazard - such as chemical irritants like adhesives
• sensitiser – substances (for example, isocyanate paints) that can cause an allergic reaction or effect, usually affecting the skin or respiratory system.
• oxidising – for example, hair dyes and bleaches.
• flammable – for example, liquefied petroleum gas (LPG) or methylated spirits.
• serious health hazard (which includes carcinogens, mutagens and reproductive toxins) - for example, asbestos, silica dust and diesel fumes.

Routes of exposure

There are many ways that a chemical can either enter or come into contact with the human body. To understand how chemicals can affect the human body, an OSH professional must understand the mechanics    of normal operation of the various organs and processes in a healthy individual.

Through the lungs

The respiratory system is made up of tissues and organs that allow a person to  breathe. It consists of the airways (which are made up of nasal cavities, mouth, larynx or voice box and trachea or windpipe), the lungs (bronchioles and alveoli) and the linked blood vessels. Through breathing (inhalation and exhalation), the respiratory   system facilitates the exchange of gases between the air and the blood and between the blood and the body’s cells.

Diagram of the lungs

The physical state and size of the particles will determine how far chemical substances can penetrate the respiratory system and beyond.

• An inhalable substance can only penetrate as far as the bronchioles and can be deposited along the respiratory system to this It can therefore affect the mouth, nose, larynx, trachea and bronchioles.
• A respirable substance can penetrate through to the alveoli and can interfere with the gas exchange Some respirable substances can also be respired themselves and can enter the bloodstream.
• Gases that are inhaled can cause acute (sudden or brief) effects on the airways and lungs. Examples include chemicals such as chlorine and ammonia. These gases react with the water content in tissues (especially in the mouth, nose and throat) to produce hydrochloric acid and ammonium hydroxide respectively. The concentration of these reacted substances will determine how severe the irritation or corrosion will be to these organs.
• Gases can also create chronic (long-term or prolonged) effects when inhaled or respired. These effects can be toxic, sensitisation, carcinogenic or mutagenic effects.

Through the skin

Skin is comprised three primary regions – the outer layer (epidermis), the middle layer (dermis) and the inner layer (hypodermis or subcutaneous tissue).

Diagram of the skin

Chemicals can enter the skin by:

• penetration
• permeation
• absorption.

As well as being absorbed through the skin, chemicals can be transported into the body via mucous members – the ears, eyes, nose and mouth.

Chemicals such as corrosives can almost immediately affect living tissue by damage or destruction of the proteins (called amide hydrolysis). There are three levels of burn category.

• Injury to the top layer of skin, or the epidermis, is called a superficial burn. This was formerly called a first-degree burn.
• Injury to the second layer of skin, or the dermis, is called a partial thickness injury or dermal injury. This was formerly called a second-degree burn.
• Injury to the third layer of skin, or subcutaneous tissue, is referred to as a full thickness injury. This was formerly called a third-degree burn.

Hydrochloric acid and sodium hydroxide are, respectively, common examples of acid and alkali corrosives.

Irritants affect the skin and mucous membranes by causing inflammation. Irritants work in the same way as corrosives but produce less acute effects. Common irritants to the skin include liquids such as oils, lubricants and solvents. These cause reddening, dryness and cracking of the skin on contact.

Skin sensitisers usually affect the hands and forearms, as these are the parts of the body that are most likely to be exposed to these substances. The sensitisation can also spread to other parts of the body. Common chemical skin sensitisers include chromium in cements, some textile dyes and pigments and isocyanates and epoxy resins in paints and varnishes.

Irritant contact dermatitis The most common form of occupational dermatitis is irritant contact dermatitis. This is caused when a specific irritant in a high enough concentration comes into contact with the skin for a sufficiently long time to damage the skin cells. If a mild irritant is in contact with the skin it will wash away the stratum corneum lipids and prevent the skin’s natural barrier from protecting and regenerating. If the exposure takes place over time the lipids will fail to regenerate and, as a consequence, dermatitis will often occur in the area of exposure on the skin. This type of dermatitis, which occurs over time, is sometimes referred to as chronic irritant contact dermatitis.

Through the digestive system

The digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body. It is primarily made up of the mouth, oesophagus, stomach, small intestines and large intestines.   

The oesophagus is a muscular tube connecting the mouth to the stomach. It is around 25 cm long in adults. When food is swallowed, the walls of the oesophagus contract to move the food down to the stomach.  The stomach breaks down much of the food chemically and mechanically, using powerful stomach acids and stomach muscle movement. The food is then transferred into the small intestine.

Effects of absorption of chemicals through the digestive system or by direct contact

Chemicals that are ingested often affect the throat or stomach lining first and, if absorbed, then affect the liver. The chemicals can either be broken down or transported to other parts of the body where they can accumulate. As with inhalation, chemicals can also react more directly with linings of the mouth, throat, stomach or bowel. Corrosives and irritants have an acute effect on the digestive system. They react chemically with the linings of the oesophagus and stomach and either inflame the tissues (in the case of irritants) or destroy them (corrosives). Corrosives can also cause ulcerations to occur in the stomach.

Other chemicals that are ingested have more chronic effects. Toxins, carcinogens and mutagens may be absorbed through the intestines and are transported to other organs via the circulatory system. Accumulation of these chemicals can either poison the organs or potentially cause the uncontrolled growth of abnormal cells or change of normal cells.