Course:SPPH381B/Essay 2/ Occupational Cancers - Soham

Introduction

Cancer refers to a group of diseases, rather than a single illness, that results from uncontrolled cell division that begins in one tissue and spreads or metastasize to various regions of the body . The mechanisms that govern these uncontrolled cellular divisions are complex processes which may be attributed partially to exposure to cancer-causing substances known as carcinogens, along with a wide range of factors including genetics, diet, smoking, and physical activity . Occupational cancers are more specifically defined as cancers originating partially or completely from carcinogenic exposure in the workplace (CCOHS, 2017). There may be a workplace component in many cancer cases, although long latency periods (period between exposure and disease) make it difficult to attribute causation . The annual percentage of work-related deaths attributable to cancer worldwide has been estimated to be as high as 32%, representing over half a million deaths (Takala, 2014). Among industrialized countries, including the EU, cancer accounts for 57% of occupationally related deaths (Takala, 2014). For a prominent example of an occupational cancer, in the United States, 85%-90% of mesotheliomas (cancers of inner lining of lungs) are attributed to workplace exposure (CCOHS, 2017). Thus, occupational cancers represent a high priority area of research in epidemiology.

Figure 1.1 Work-related annual deaths

Carcinogens and Categorization

Carcinogens or cancer-causing agent are identified using epidemiological studies (observational research), in vitro studies (cellular level), in vivo studies (involving whole organisms such as mice or other laboratory animals), as well as mechanism studies (involving procedures such as chemical analysis of an agent) (CCOHS, 2017). Additionally, scientists can test the ability of a particular agent to cause cellular mutations and interactions (CCOHS, 2017). Organizations including the International Agency for Research on Cancer (IARC), American Conference of Governmental Industrial Hygienists (ACGIH), US National Toxicology Program (NTP) categorize lists of substances or agents that are known to cause cancer.

The IARC produces one of the most widely used and commonly accepted list of carcinogenic categorization (CCOHS, 2017). The IARC list of carcinogens is categorized on research-based evidence of carcinogenicity (CCOHS, 2017). An agent may be placed into any of the following five categories based on evidence of carcinogenicity: Group 1 – carcinogenic to humans, Group 2A – Probably carcinogenic to humans, Group 2B – Possibly carcinogenic to humans, Group 3 – Not classifiable as to its carcinogenicity to humans, Group 4 – Probably not carcinogenic to humans (CCOHS, 2017). Out of the agents examined through research so far, 119 belong to group 1, 81 belong to group 2A, 292 belong to group 2B, 505 belong to Group 3, and only 1 belongs to group 4 (WHO, 2017). It is worth mentioning that these figures only represent the agents rigorously researched through evidence-based procedures, and many more carcinogens may be identified in the future (Davies, 2017).

Group	Carcinogenicity	Number of identified agents	Examples
1	Carcinogenic to humans	119	Formaldehyde
2A	Probably carcinogenic to humans	81	DDT
2B	Possibly carcinogenic to humans	292	Phenobarbital
3	Not classifiable as to its carcinogenicity to humans	505	Phenylbutazone
4	Probably not carcinogenic to humans	1	Caprolactam

Cancer Clusters

Cancer clusters represent an important phenomenon within epidemiology, as they may indicate an occupational cause of disease. Cancer clusters refer to a greater than expected number of cancers in a group (of workers, families or people residing in a region) (Davies, 2017). The investigation to identify a potential cancer cluster takes into consideration the specific type of cancer, the number of cases, statistical significance and causes (Davies, 2017). In the United States, out of an average of 1000 inquiries about potential cancer clusters received by US government agencies, 75% of them are identified as non-clusters (Thun & Sinks, 2004). Even out of the 5% to 15% of inquiries identified as cases of cancer clusters, further rigorous epidemiological testing does not result in confident findings (Thun & Sinks, 2004). Thus, cancer clusters have generally involved very rare cancers only (Thun & Sinks, 2004). A historic example of a cancer cluster involved the carcinogen polyvinyl chloride (PVC) in the United States (Davies, 2017). Four men working at a tire plant in Kentucky were diagnosed with angiocarcinoma of the liver (CDC MMWR, 1997). No men had long-term history of alcohol abuse or exposure to other liver toxins (CDC MMWR, 1997). Angiocarcinoma is a very rare cancer, and it was identified as anomalous in the context of four plant workers in the same department. After examining more cases, animal studies in other countries, epidemiological studies, and mutagenicity studies, PVC was identified as responsible for the cancer cluster (Davies, 2017).

Pleural Mesothelioma - An occupational cancer example

Pleural mesothelioma represents a prominent example of an occupational cancer, largely attributable to occupational asbestos exposure (Hemminki & Li, 2003). Although controls have been implemented to prevent asbestos usage or exposure, mesothelioma has a long latency period, or time between exposure and disease onset (Hemminki & Li, 2003). Consequently, it is estimated that new diagnoses of mesothelioma (and subsequent deaths resulting from the disease) will continue to peak until 2020, with the current decade demonstrating the biggest rise in incidence (Hemminki & Li, 2003). In Sweden, an epidemiological study indicated that the cities with large shipbuilding industries had a higher incidence of mesothelioma, while agricultural employees had the slowest (although still quite significant) increase in incidence (Hemminki & Li, 2003), see Figure 1. Consequently, seamen in Sweden had the highest increase in risk from 1981 to 1998, despite no cases being recorded prior to 1971 (Hemminki & Li, 2003). In particular, male manual workers had the highest and greatest increase in incidence across all socioeconomic groups (Hemminki & Li, 2003). Notably, college-educated groups had an up to six times higher incidence ratio than farmers, indicating the damaging effects of indirect exposure as well due to asbestos use in buildings (Hemminki & Li, 2003). Overall, this data demonstrates how occupational status can be intrinsically tied with the disease-process of a cancer such as mesothelioma, disproportionately affecting some populations more than others.

Figure 1.2 Data collected in Swedish cities for standardized incidence ratios (SIRs) for different occupations from 1961-1998 ((Hemminki & Li, 2003)

Table of Common Occupational Cancers

Cancer	Exposure
Mesothelioma (pleural and peritoneal)	Inhalation of Asbestos fibres
Lung	Inhalation of Asbestos fibres
Leukaemia	Inhalation and some dermal exposure to Benzene
Breast	Inhalation, ingestion, wound contamination and dermal absorption of Ionizing radiation