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Work-related Asthma and Occupational Asthma - An introduction

What is asthma?

Asthma is a common condition which affects the small airways or tubes in the lungs. It often develops in childhood and may be caused by allergy to house dust mite, pollen, and /or pet dander. Often there is a family history of asthma. It is easy to recognise allergic asthma caused by pollen because symptoms appear and disappear with the seasons. The body produces antibodies which react with the allergic agent to trigger cells in the body to produce chemicals such as histamine. These chemicals cause intermittent spasm of the muscles around the small airways causes the airways to narrow leading to episodes or bouts of symptoms of chest tightness, difficulty breathing, wheezing and coughing. Inflammation of the airways may also be accompanied by the production of sputum or phlegm. Most people who suffer from asthma have periodic attacks of symptoms separated by symptom-free periods. Symptoms can be aggravated by cold air and cigarette smoke and are often worse at night or early in the morning. Symptoms often improve or disappear as children approach adulthood. However, in some children asthma can persist into adulthood; and sometimes asthma can present for the first time in adulthood – usually without any associated allergies.

What is work-related asthma?

Work-related asthma is present when the timing of bouts of symptoms can be related to being at work.  There may be a time lag of a few hours between breathing in something at work and the onset of symptoms; sometimes symptoms may develop when you get home. The way to identify a relationship with work is to observe if symptoms are better on days away from work such as weekends and holidays. There are two types of work-related asthma:

  • Work-aggravated asthma, which is asthma that a patient has already and which is aggravated by non-specific agents at work, such as dust, cold air and dry air.
  • Occupational asthma, that is asthma which is caused by inhaling a substance hazardous to heath in the workplace.

What is occupational asthma?

As noted occupational asthma is asthma that is caused by inhaling a substance hazardous to health in the workplace. There are two main types of occupational asthma:

  • Irritant-induced occupational asthma – where a single or a few very high exposures to an irritant substance at work induces the onset of asthma. This can happen with gases or vapours such as chlorine or ammonia.
  • Sensitizer-induced occupational asthma – where a person develops hypersensitivity (an allergy) to a substance at work after many repeated exposures, typically over months or a few years.

How common is occupational asthma?

The most reliable studies suggest that occupational asthma accounts for as many as 15% of cases of adult-onset asthma. Sensitizer-induced occupational asthma is by far the most common type accounting for about 90% of all cases of occupational asthma. Hence the remainder of the guidance provided here relates to this type of occupational asthma.

What are the main symptoms of occupational asthma?

The symptoms of asthma are the same whatever the cause, although in the early stages of the disease a distinguishing feature occupational asthma is that symptoms deteriorate when at work and improve regularly when away from work i.e. weekends and holidays.

What are the related symptoms?

The respiratory system which we use to breath extends from the nose to the lungs. It is not surprising that people who are allergic to something in the air and who suffer from asthma may also have allergic rhinitis (hay fever type symptoms) with a runny nose and sneezing.  Allergic rhinitis can also be accompanied by allergic conjunctivitis which leads to symptoms of watering, itchy and red eyes. About two-thirds of people with occupational asthma also suffer from rhinitis (sneezing, runny nose) and/or conjunctivitis (itchy red eyes). It is important to recognise that symptoms of rhinitis and conjunctivitis may develop at the same as the onset of asthma; or they may precede the development of asthma by as long as one year. The new onset of rhinitis and / or conjunctivitis is someone exposed to occupational causes of asthma ought to be considered to be an early warning sign for the development of occupational asthma and should be investigated.

Who is at risk of developing occupational asthma?

Broadly speaking there are two risk factors to consider:

  • Personal susceptibility – it is not possible to predict who will and who will not develop occupational asthma since anyone can develop the condition. However, people who have a history of asthma are more susceptible to developing occupational asthma caused by high molecular weight allergens. Smokers are at increased risk for developing occupational asthma to some chemicals such as isocyanates and platinum salts.
  • Workplace exposure – the level of exposure to agents that cause occupational asthma is the major risk factor for the development of occupational asthma; therefore, reducing exposure is the lynchpin of prevention.

The occupational groups at greatest risk for developing occupational asthma include: animal handlers, bakers and pastry makers, cleaners, nurses, chemical workers, food processing workers, hairdressers, health care workers, paint sprayers, woodworkers and welders.

Which agents cause occupational asthma?

Several hundred agents have been identified to cause occupational asthma. Chemicals which are known to be able to cause occupational asthma are usually identified on manufacturer’s safety data sheets using the hazard statement H334 - may cause allergy or asthma symptoms or breathing difficulties if inhaled or H335 – may cause respiratory irritation. The causative agents are sometimes divided into two categories:

  • High molecular weight agents are highly allergenic that is to say they can cause hypersensitivity (allergy). They include animal and vegetable proteins; those most frequently reported to cause occupational asthma being animal dander, flour, grain dust and latex. Others include castor bean, enzymes, green coffee beans, salmon, shellfish and many more besides.
  • Low molecular weight agents include reactive chemicals that readily take part in some chemical reaction; those most frequently reported to cause occupational asthma including aldehydes, isocyanates (in twin pack spray paint) and persulfates (hair bleaches). Others include platinum salts, natural acids in wood dust and solder fumes.

How is it prevented?

In order of effectiveness the best ways of preventing exposure to the causative agent are:

  1. elimination of the substance or process;
  2. replacement with a less hazardous substance;
  3. enclosure of the process;
  4. ventilation of the local area or of the entire workplace;
  5. respiratory protective equipment (RPE). However, the use of RPE does not completely prevent the onset of occupational asthma and should not be relied upon as the main or only means of prevention.

Health surveillance

Employers should arrange regular health surveillance for workers who are at risk of developing occupational asthma.  This is because the early identification of health effects offers the best chance of cure or of preventing someone’s asthma getting worse.  If the risk assessment shows that the risk is low, then the health check might simply involve employees filling in a questionnaire which asks questions about relevant symptoms.  Where there is higher risk, employees should be seen at least annually by an occupational health nurse or technician who will assess employee’s lung function by asking them to blow into a machine called a spirometer.

How is work-related asthma managed?

A diagnosis of work-related asthma requires specialist investigation to determine whether or not work is either causing or aggravating someone’s asthma. The occupational management differs according to the type of asthma. Any restrictions at work should be recommended by an occupational physician (a doctor who has knowledge, experience and training in work-related health).

If an employee has work-aggravated asthma then they should avoid the triggers that aggravate the asthma at work or use respiratory protective equipment.

If an employee has occupational asthma it is important that they avoid further exposure to the cause completely and early in the course of the disease.  This is best managed by being relocated to another area of the workplace and/or to another job, ideally within a year of symptoms developing.  This offers the best chance of recovery.

The legal position - relevant laws and regulations

The main health and safety laws and regulations relating to occupational asthma include:

  • The Health and Safety at Work Act 1974 requires employers to “ensure, so far as is reasonably practicable, the health, safety and welfare of their employees at work”.
  • The Management of Health and Safety at Work Regulations 1999 require employers to assess the and control risks to health in the workplace; to appoint competent persons to assist them with their responsibilities for health and safety and to provide their employees with appropriate information and training.
  • The Control of Substances Hazardous to Health Regulations 2002 (COSHH) require employers to control substances that are hazardous to health and defines when health surveillance must be performed.
  • The Personal Protective Equipment Regulations 1992 require employers to provide and maintain appropriate personal protective equipment to employees.
  • The Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013 (RIDDOR) requires employers and self-employed people to report cases of occupational asthma “where the person’s work involves significant or regular exposure to a known respiratory sensitizer”.

For more information see the HSE webpage on regulations.


Compiled by Dr Paul J Nicholson OBE, a specialist in occupational medicine.