Colorectal Cancer

Background

The Canadian Cancer Society estimates that colorectal cancer will be the 3rd most commonly diagnosed cancer in Canada in 2020 [1] . Increased colorectal cancer screening is improving early diagnosis and treatment, but colorectal cancer remains the 2nd leading cause of cancer deaths [1,2]. Colorectal cancer risk is primarily attributed to lifestyle factors including obesity, physical inactivity, poor diet, alcohol consumption and smoking [3]. One study estimated that as many as 11-15% of cases may be attributable to occupational exposures [4]. However, there are no well established occupational risk factors for colorectal cancer.


Possible occupational risk factors
    • X-radiation, gamma-radiation [5]
    • Asbestos [5]
    • Night shift work [5]
    • Sedentary work [6]
    • Diesel engine exhaust [7]
Key Findings

The greatest occupational risks for colorectal cancer in the ODSS appeared to be related to mineral and metal processing, and mining and quarrying. Excess risk was also observed for police officers, firefighters, truck drivers, and bus drivers. Occupational differences in lifestyle factors may play a significant role in the observed occupational differences in colorectal cancer risk. Sedentary work has been posited as a risk factor for colorectal cancer, but no excess risk was observed among most office workers in the ODSS. Non-occupational exposure to ionizing radiation is a risk factor for colorectal cancer. A 13% increased risk was observed among radiological technologists and technicians in the ODSS, but the number of workers employed in occupations and industries with radiation exposure is relatively small.

 

Mineral and metal processing

Workers in mineral and metal processing may be exposed to a variety of dusts, fumes and other substances, including asbestos, which may contribute to the excess colorectal cancer risk observed.

    • Metal smelting, converting and refining furnacemen: 1.79 times the risk
    • Mineral ore treating occupations: 1.34 times the risk
    • Iron and steel mills industry: 1.17 times the risk

 

Mining and quarrying

Miners may be exposed to ionizing radiation, which could contribute to risk of colorectal cancer. No excess was observed among uranium miners. Protective measures in uranium mines may more effectively protect miners than in other mine types [8]. Additionally, asbestos exposure may be a risk factor among some miners, however, too few cases were observed among asbestos miners in the ODSS to be able to examine this association among miners with the highest risk of exposure.

    • Mining and quarrying including oil and gas field occupations: 1.18 times the risk
      • Blasting occupations: 1.79 times the risk
      • Occupations in labouring and other elemental work: 1.42 times the risk
      • Other rock and soil-drilling occupations: 1.41 times the risk
    • Mines (including milling), quarries and oil wells industries: 1.21 times the risk
      • Metal mines: 1.25 times the risk
        • Gold quartz mines: 1.25 times the risk
        • Miscellaneous metal mines*: 1.34 times the risk
        • Uranium mines: no excess risk observed
      • Non-metal mines: 1.25 times the risk
        • Asbestos mines: too few cases to report
      • Quarries and sand pits: 1.28 times the risk

*mostly nickel miners

 

Excavating, grading and paving

Workers in this group may be exposed to diesel engine exhaust, a possible risk factor for colorectal cancer.

    • Construction: Excavating, grading, paving and related occupations: 1.18 times the risk
      • Paving, surfacing, and related occupations: 1.56 times the risk
Police Officers and Firefighters

Night shift work may be a colorectal cancer risk factor for firefighters and police officers. Firefighters may be exposed to asbestos in older buildings [9,10], which may contribute to the increased colorectal cancer risk observed for this group.

    • Protective service occupations: 1.21 times the risk
      • Firefighting occupations: 1.33 times the risk
      • Police officers and detectives, government: 1.32 times the risk
Transportation

Occupational colorectal risk factors for transportation workers may include sedentary work, night shift work, and exposure to diesel engine exhaust.

    • Motor transport operating occupations: 1.19 times the risk
      • Bus drivers: 1.30 times the risk
      • Truck drivers: 1.20 times the risk
Other groups

Excess risks were observed among several additional occupational groups in the ODSS. The excess risk among service station attendants may be linked to diesel engine exhaust exposure, but occupational colorectal cancer risks among most of these groups is unclear. Occupational differences in smoking may be a factor. Exposure to ionizing radiation is the only known occupational risk factor for colorectal cancer, and a modest increased risk was observed among radiological technologists and technicians in the ODSS, a small group with only 10 cases.

    • Telephone systems industry: 1.68 times the risk
    • Foremen: fabricating, assembling and repairing occupations, textile, fur and leather products: 1.67 times the risk
    • Bartenders: 1.44 times the risk
    • Service station attendants: 1.38 times the risk
    • Radiological technologists and technicians: 1.13 times the risk 
Relative Risk by Industry and Occupation

Figure 1. Risk of colorectal cancer diagnosis among workers employed in each industry group relative to all others, Occupational Disease Surveillance System (ODSS), 2006-2016

The hazard ratio is an estimate of the average time to diagnosis among workers in each industry/occupation group divided by that in all others during the study period. Hazard ratios above 1.00 indicate a greater risk of disease in a given group compared to all others. Estimates are adjusted for birth year and sex. The width of the 95% Confidence Interval (CI) is based on the number of cases in each group (more cases narrows the interval).

Figure 2. Risk of colorectal cancer diagnosis among workers employed in each occupation group relative to all others, Occupational Disease Surveillance System (ODSS), 2006-2016 

The hazard ratio is an estimate of the average time to diagnosis among workers in each industry/occupation group divided by that in all others during the study period. Hazard ratios above 1.00 indicate a greater risk of disease in a given group compared to all others. Estimates are adjusted for birth year and sex. The width of the 95% Confidence Interval (CI) is based on the number of cases in each group (more cases narrows the interval).

Table of Results

Table 1. Surveillance of colorectal cancer: Number of cases, workers employed, and hazard ratios in each industry (SIC)

SIC Code * Industry Group Number of cases  Number of workers employed Hazard Ratio (95% CI) †
1 Agriculture 270 35000 0.92 (0.81-1.04)
2/3 Forestry, Fishing and
Trapping		 143 10695 1.00 (0.85-1.18)
4 Mines, Quarries and
Oil Wells		 469 23179 1.21 (1.10-1.33)***
5 Manufacturing 8485 693317 1.00 (0.97-1.03)
6 Construction 2346 210891 1.01 (0.97-1.06)
7 Transportation, Communication
and Other Utilities		 2350 197090 1.05 (1.01-1.10)*
8 Trade 3792 429093 1.03 (0.99-1.07)
9 Finance, Insurance and
Real Estate		 312 23972 1.02 (0.91-1.14)
10 Community, Business and
Personal Service		 4993 599273 0.94 (0.91-0.97)***
11 Public Administration and
Defense		 2459 190800 1.08 (1.04-1.13)***
         
         
* SIC: Standard Industrial Classification (1970)    
† Hazard rate in each group relative to all others    

 

Table 2. Surveillance of colorectal cancer: Number of cases, workers employed, and hazard ratios in each occupation (CCDO) group

CCDO Code * Occupation Group Number of cases  Number of workers employed Hazard Ratio (95% CI) †
11 Managerial, administrative
and related		 267 30976 0.88 (0.78-0.99)*
21 Natural sciences, engineering
and mathematics		 242 26332 0.89 (0.78-1.01)
23 Social sciences and
related fields		 161 30700 0.86 (0.74-1.01)
25 Religion <5 128 —
27 Teaching and related 391 48454 0.90 (0.81-0.99)*
31 Medicine and health 1076 135268 0.96 (0.90-1.02)
33 Artistic, literary,
recreational and related		 94 14979 0.95 (0.78-1.17)
41 Clerical and related 1951 197238 1.06 (1.01-1.11)*
51 Sales 1086 148052 1.00 (0.94-1.06)
61 Service 3639 370688 0.99 (0.96-1.03)
71 Farming, horticultural
and animal husbandry		 360 50126 0.86 (0.78-0.96)**
73 Fishing, hunting,
trapping and related		 <5 558 —
75 Forestry and logging 113 10681 0.85 (0.70-1.02)
77 Mining and quarrying,
including oil and gas field		 248 13025 1.18 (1.04-1.34)**
81 Processing
(mineral, metal, chemical)		 873 79195 1.04 (0.97-1.11)
82 Processing
(food, wood, textile)		 1007 99196 1.01 (0.94-1.07)
83 Machining and related 2313 189346 1.00 (0.96-1.04)
85 Product fabricating,
assembling and repairing 		 4087 328189 1.03 (1.00-1.07)
87 Construction trades 2784 215504 1.05 (1.01-1.09)*
91 Transport equipment
operating		 2306 168002 1.17 (1.12-1.22)***
93 Materials handling and related,
not elsewhere classified		 1552 153012 1.00 (0.95-1.05)
95 Other crafts and
equipment operating		 353 28290 0.98 (0.88-1.09)
99 Other occupations not elsewhere classified 2218 215187 1.02 (0.98-1.07)
         
         
* CCDO: Canadian Classification Dictionary of Occupations (1971)  
† Hazard rate in each group relative to all others    

Please note that ODSS results shown here may differ from those previously published or presented. This may occur due to changes in case definitions, methodological approaches, and the ongoing nature of the surveillance cohort.

 

References

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  8. Canadian Nuclear Safety Commission. Uranium mines and mills [Internet]. [cited 2020 Oct 29].
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