In 1919, the year of its creation, the International Labour Organization (ILO) declared that anthrax was an occupational disease. In 1925, the first ILO List of Occupational Diseases was established by the Workmen’s Compensation (Occupational Diseases) Convention (No. 18). There were three occupational diseases listed. Convention No. 42 (1934) revised Convention No. 18 with a list of ten occupational diseases. In 1964, the International Labour Conference adopted the Employment Injury Benefits Convention (No. 121), this time with a separate schedule (List of Occupational Diseases) appended to the Convention, which allows for amending the schedule without having to adopt a new Convention (ILO 1964).

Definition of Work-Related Diseases and Occupational Diseases

In the third edition of the ILO’s Encyclopaedia of Occupational Health and Safety, a distinction was made among the pathological conditions that could affect workers in which diseases due to occupation (occupational diseases) and diseases aggravated by work or having a higher incidence owing to conditions of work (work-related diseases) were separated from conditions having no connection with work. However, in some countries work-related diseases are treated the same as work-caused diseases, which are in fact occupational diseases. The concepts of work-related diseases and occupational diseases have always been a matter of discussion.

In 1987, a joint ILO/WHO expert committee on occupational health offered the suggestion that the term work-related diseases may be appropriate to describe not only recognized occupational diseases, but other disorders to which the work environment and performance of work contribute significantly as one of the several causative factors (Joint ILO/WHO Committee on Occupational Health 1989). When it is clear that a causal relationship exists between an occupational exposure and a specific disease, that disease is usually considered both medically and legally as occupational and may be defined as such. However, not all work-related diseases can be defined so specifically. The ILO Employment Injury Benefits Recommendation, 1964 (No. 121), paragraph 6(1), defines occupational disease as follows: “Each Member should, under prescribed conditions, regard diseases known to arise out of the exposure to substances and dangerous conditions in processes, trades or occupations as occupational diseases.”

Nevertheless, it is not always that easy to designate a disease as being work-related. In fact, there is a wide range of diseases that could be related in one way or another to occupation or working conditions. On the one hand, there are the classical diseases that are occupational in nature, generally related to one causal agent and relatively easy to identify. On the other hand, there are all sorts of disorders without strong or specific connections to occupation and with numerous possible causal agents.

Many of these diseases with a multifactorial aetiology may be work-related only under certain conditions. The subject was discussed at an international symposium on work-related diseases organized by the ILO in Linz, Austria, in October 1992 (ILO 1993). The relationship between work and disease could be identified in the following categories:

• occupational diseases, having a specific or a strong relation to occupation, generally with only one causal agent, and recognized as such
• work-related diseases, with multiple causal agents, where factors in the work environment may play a role, together with other risk factors, in the development of such diseases, which have a complex aetiology
• diseases affecting working populations, without causal relationship with work but which may be aggravated by occupational hazards to health.

Criteria for Identification of Occupational Diseases in General

Two main elements are present in the definition of occupational diseases:

• the exposure-effect relationship between a specific working environment and/or activity and a specific disease effect
• the fact that these diseases occur among the group of persons concerned with a frequency above the average morbidity of the rest of the population.

It is apparent that the exposure-effect relationship must be clearly established: (a) clinical and pathological data and (b) occupational background and job analysis are indispensable, while (c) epidemiological data are useful, for determining the exposure-effect relationship of a specific occupational disease and its corresponding activity in specific occupations.

As a general rule, the symptoms of such disorders are not sufficiently characteristic to enable occupational diseases to be diagnosed other than on the basis of the knowledge of the pathological changes engendered by the physical, chemical, biological or other factors encountered in the exercise of an occupation. It is therefore normal that, as a result of the improvement of knowledge regarding the action processes of the factors in question, the steady increase in the number of substances employed, and the quality used or the variety of agents suspected, it should be more and more possible to make an accurate diagnosis while at the same time broadening the range of these diseases. Parallel with the boom in the research in this field, the development and refinement of epidemiological surveys have made a substantial contribution towards furthering the knowledge of exposure/effect relationships, making it easier, inter alia, to define and identify the various occupational diseases. The identification of a disease as being of occupational origin is, in reality, a specific example of clinical decision-making or applied clinical epidemiology. Deciding on the cause of a disease is not an exact science but rather a question of judgement based on a critical review of all the available evidence, which should include a consideration of:

• Strength of association. An occupational disease is one where there is an obvious and real increase in disease in association with exposure to the risk.
• Consistency. The various research reports have generally similar results and conclusions.
• Specificity. The risk exposure results in a clearly defined pattern of disease or of diseases and not simply in an increasing number of causes of morbidity or mortality.
• Appropriate time relationship. The disease follows after the exposure and with an appropriate time interval.
• Biological gradient. The greater the level of exposure, the greater the prevalence of severity of diseases.
• Biological plausibility. From what is known of toxicology, chemistry, physical properties or other attributes of the studied risk, it does really make biological sense to suggest that the exposure leads to a certain disorder.
• Coherence. A general synthesis of all the evidence (human epidemiology, animal studies and so on) leads to the conclusion that there is a causative effect in its broad sense and in terms of general common sense.

The magnitude of the risk is another basic element generally used for determining whether a disease is to be considered occupational in origin. Quantitative and qualitative criteria play an important role in evaluating the risk of contracting an occupational disease. Such a risk may be expressed either in terms of its magnitude—for instance, the quantities in which the substance is employed, the number of workers exposed, the prevalence rates for the disease in different countries—or in terms of the seriousness of the risk, which may be assessed on the basis of its effects upon workers’ health (e.g., the likelihood of its causing cancer or mutations or having highly toxic effects or leading in due course to disablement). It should be noted that the figures available as to prevalence rates and the degree of seriousness of occupational diseases should be viewed with some circumspection due to the differences in procedures for reporting cases and compiling and evaluating data. The same is true for the number of workers exposed, as figures can only be approximate.

Finally, at the international level, another very important factor must be taken into account: the fact that the disease is recognized as being occupational by the law of a certain number of countries constitutes an important criterion on which to base a decision to include it in the international list. It may indeed be considered that its incorporation in the list of diseases carrying entitlement to benefit in a large number of countries shows that it is of considerable social and economic importance and that the risk factors involved are recognized and widely encountered.

To summarize, criteria for determining a new occupational disease to be added on an international list are: the strength of the exposure-effect relationship, the occurrence of the disease with specific activity or specific work environment (which includes the occurrence of the event and a specific nature of this relationship), the magnitude of the risk on the basis of the number of workers exposed or the seriousness of the risk, and the fact that a disease is recognized on many national lists.

Criteria for Identification of an Individual Disease

The exposure-effect relationship (relation between exposure and the severity of the impairment in the subject) and the exposure-response relationship (connection between exposure and the relative number of subjects affected) are important elements for the determination of occupational diseases, which research and epidemiological studies have greatly contributed to developing in the last decade. This information pertaining to the causal relationship between diseases and exposure in the workplace has allowed us to achieve a better medical definition of occupational diseases. Therefore it follows that the legal definition of occupational diseases, which was a rather complex problem before, is becoming more and more linked to the medical definitions. The legal system entitling the victim to compensation varies from country to country. Article 8 of the Employment Injury Benefits Convention (No. 121), which indicates the various possibilities regarding the form of the schedule of occupational diseases entitling workers to a compensation benefit, states:

Each Member shall:

1. prescribe a list of diseases, comprising at least the diseases enumerated in Schedule I to this Convention, which shall be regarded as occupational diseases under prescribed conditions; or
2. include in its legislation a general definition of occupational diseases broad enough to cover at least the diseases enumerated in Schedule I to this Convention; or
3. prescribe a list of diseases in conformity with clause (a), complemented by a general definition of occupational diseases or by other provisions for establishing the occupational origin of diseases not so listed or manifesting themselves under conditions different from those prescribed.

Point (a) is called the list system, point (b) is the general definition system or overall coverage system while point (c) is generally referred to as the mixed system.

While the list system has the disadvantage of covering only a certain number of occupational diseases, it has the advantage of listing diseases for which there is a presumption that they are of occupational origin. Frequently it is very difficult if not impossible to prove that a disease is directly attributable to the victim’s occupation. Paragraph 6(2) of Recommendation No. 121 indicates that “Unless proof to the contrary is brought, there should be a presumption of the occupational origin of such diseases” (under prescribed conditions). It also has the important advantage of indicating clearly where prevention should take place.

The general definition system covers theoretically all occupational diseases; it affords the widest and most flexible protection, but leaves it to the victim to prove the occupational origin of the disease, and no emphasis is placed on specific prevention.

Because of this marked difference between a general definition and a list of specific diseases, the mixed system has been favoured by many ILO Member States because it combines the advantages of the two others without their disadvantages.

List of Occupational Diseases

Convention No. 121 and Recommendation No. 121

The ILO list plays a key role in harmonizing the development of policy on occupational diseases and in promoting their prevention. It has in fact achieved considerable status in the field of occupational health and safety. It presents a clear statement of diseases or disorders that can and should be prevented. As it is, it does not include all occupational diseases. It should represent those that are most common in the industries of many countries and where prevention can have the greatest impact on the health of workers.

Because the patterns of employment and risks are changing greatly and continuously in many countries, and because of the evolution of knowledge on occupational diseases through epidemiological studies and research, the list must be modified and added to, reflecting an updated state of knowledge, to be fair to the victims of these diseases.

In developed countries, heavy industries such as steel fabrication and underground mining have greatly diminished, and environmental conditions have improved. Service industries and automated offices have risen in relative importance. A far greater proportion of the workforce is made up of women who still, for the most part, manage the home and care for children in addition to working on the outside. The need for day care for children is increasing while these developments place added stress on women. Night work and rotating shift work have become a normal pattern. Stress, in all aspects, is now an important problem.

In developing countries, heavy industries are rising rapidly to supply local and export needs, and providing employment to these burgeoning populations. Rural populations are moving to cities in search of employment and to escape poverty.

The human health risks of some new chemicals are known, and special emphasis is given to short-term biological tests or to long-term animal exposures for the purpose of toxicological and carcinogenic incidence. Exposures of working populations in most developed countries are probably controlled at low levels, but no such assurance can be assumed for the use of chemicals in many other nations. A particularly important example is provided by the use of pesticides and herbicides in agriculture. Although there can be no serious doubt that they increase crop yields in the short term as well as increasing the control of vector-borne diseases such as malaria, we do not know clearly in which controlled conditions they can be used without major impact on the health of agricultural workers or those who eat the foods so produced. It seems that in certain countries, very large numbers of agricultural workers have been poisoned by their use. Even in well industrialized countries the health of farm workers is a serious problem. The isolation and lack of supervision place them at real risk. A prominent issue is provided by the continued manufacture of some chemicals in countries where their use is banned, in order to export these chemicals to countries where no such ban exists.

The design and function of enclosed modern buildings in industrialized countries and of the electronic office equipment within them have received close attention. Continuous repetitive movements are widely considered to be the cause of debilitating symptoms.

Tobacco smoke in the workplace, although not seen as a cause of occupational disease by itself, seems likely to be an issue in the future. Non-smokers are increasingly intolerant of the perceived health hazard from the smoke emitted by smokers in the vicinity. The pressure to sell tobacco products in developing countries is likely to produce an unprecedented epidemic of diseases in the near future. Exposure of non-smokers to tobacco smoke pollution will have to be taken as a matter of increasing consideration. Relevant legislation is already in place in some countries. A most important hazard is associated with health care workers who are exposed to a wide variety of chemicals, sensitizers and infections. Hepatitis and AIDS provide special examples.

The entry of women into the workforce in all countries underlies the problem of reproductive disorders associated with workplace factors. These include infertility, sexual dysfunction and effects on foetus and pregnancy when the women are exposed to chemical agents and workplace factors, including ergonomic strain. There is increasing evidence that the same problems may affect male workers.

Within this framework of changing populations and changing patterns of risk, it is necessary to review the list and add those diseases identified as being occupational. The list appended to Convention No. 121 should accordingly be brought up to date so as to include the disorders most widely recognized as being of occupational origin and those involved in most dangers to health. In this regard, an informal consultation on the revision of the list of occupational diseases appended to Convention No. 121 was held by the ILO in Geneva in December 1991. In their report, the experts proposed a new list, which is shown in table 1.

Table 1. Proposed ILO list of occupational diseases

In their report, the experts indicated that the list should be brought up to date regularly to contribute to harmonizing social security benefits at the international level. It was clearly indicated that there is no moral or ethical reason to recommend standards in one country that are lower than those in another. Additional reasons to revise this list frequently include (1) stimulating the prevention of occupational diseases by facilitating a greater awareness of the risks involved in work, (2) encouraging combating the use of harmful substances, and (3) keeping workers under medical surveillance. The prevention of occupational diseases remains an essential objective of any system of social security concerned with the protection of workers’ health.

A new format has been proposed, breaking down the list into the three following categories:

1. diseases caused by agents (chemical, physical, biological)
2. diseases of target organ systems (respiratory, skin, musculoskeletal)
3. occupational cancer.

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