ARCHIVED - Chronic Diseases in Canada

Volume 29 · Supplement 1 · 2010

Cancer and the environment: Ten topics in environmental cancer epidemiology in Canada

Shirley A. Huchcroft, Yang Mao and Robert Semenciw, Editors

https://doi.org/10.24095/hpcdp.29.S1.01

Preface

The Public Health Agency of Canada’s Canada’s Centre for Chronic Disease Prevention and Control produces and communicates cancer surveillance findings and their implications for cancer control policies and programs. In 1998, a review committee recommended that the Centre (then housed within Health Canada) compile and disseminate results of cancer-related work funded under the Government of Canada Green Plan program. Initiated in December 1991, the Green Plan program was the federal government’s environmental agenda in the 1990s. In addition, the Federal-Provincial-Territorial Advisory Committee on Environmental and Occupational Health recommended improving the identification and assessment of factors in the environment that influence health.

Researchers with expertise in specific subject areas were asked to provide material for a supplement that would review the epidemiological literature on the risk of cancer, if any, associated with some Canadian exposures. The text was received, reviewed for scientific accuracy and edited over the next few years. Each section retained its own style and can stand alone as an essay on its topic. Introductory and summary sections were written to present important concepts and to tie the document together, and a glossary was added. However, the views expressed remain those of the authors rather than the Public Health Agency of Canada. This supplement to Chronic Diseases in Canada is the result of the process.

This is a technical document most suited to the information needs of the many parties interested in environmental health and members of the cancer control community, especially health professionals, policy makers and researchers. In particular, students and teachers in the health sciences may find this a useful introduction to the area of environmental cancer epidemiology.

Finally, a work of this magnitude and complexity takes a long time to produce.

Introduction

This supplement assesses the epidemiological evidence relating ten environmental exposures to cancer causation. Environmental exposures are defined broadly as those that are not a lifestyle choice (such as diet and smoking), are ubiquitous (e.g., ultraviolet radiation and air pollution) and/or involve involuntary exposure (e.g., occupational exposures, industrial pollution and environmental tobacco smoke). Although biological agents (e.g., bacteria, protozoa, viruses, fungi, algae, dust mites, and allergens, such as pollen) are a class of environmental exposures/hazards, they are not dealt with here.

The material in this supplement is organized into four main sections: radiation, chemicals, air pollution and industry. The radiation section includes chapters on radon, ultraviolet radiation and electromagnetic fields. The chemicals section includes organochlorines, disinfection by-products and pesticides. The air section includes environmental tobacco smoke and air pollution. The industry section includes chapters on pulp and paper mills and gold, nickel and copper mining and processing. Occupational exposures are included for three reasons: First, the pulp and paper and metal mining industries are prominent in Canada. Second, occupational epidemiology is closely related to environmental epidemiology in that the exposure and disease experiences of workers can be extrapolated to give an estimate of the degree of risk associated with non-occupational exposures to the same substances. Third, occupational studies are particularly valuable in instances where too few community studies are available to provide an estimate of the degree of risk to the general population. For example, most of the information on the hazards of radiation from exposure to radon gas derives from occupational studies.

The exposures discussed arise from both the “natural” and “built” (i.e., constructed or human-made) environments.^3a Ultraviolet radiation and radon are examples of the former, whereas by-products of the disinfection of water, electromagnetic fields, pesticides, air pollution and environmental tobacco smoke arise primarily from the built environment. However, it is acknowledged that exposures are often the interaction of factors in both environments.

Also, several of the topics are not mutually exclusive. For example, the disinfection of water as well as the pulp and paper industry both produce organochlorines, and several pesticides are organochlorines. As well, workers in metal mining are not infrequently exposed to radiation from radon decay products and environmental tobacco smoke is a component of air pollution.

Estimating cancer risk from the environment in Canada

Incidence, contaminant sources and exposure variation

Excluding non-melanoma skin cancer, approximately 160,000 Canadians are diagnosed with cancer each year and half that many die from it.¹ Cancers of the lung, breast, prostate, colon and rectum account for over half of all cancers diagnosed in Canada. Lung cancer is the most common cancer cause of death in either sex, accounting for over a quarter of cancer deaths. The most common cancer among women is breast cancer, accounting for 30 percent of new cases, while the most common cancer among men is prostate cancer, accounting for one quarter of new cases. Although cancer is primarily a disease of older Canadians (with 69 percent of new cancer cases and 82 percent of deaths occurring among those who are at least 60 years old), it attacks all ages, including infants.²

Public opinion polling conducted at the outset of this publication showed that a major concern of Canadians in relation to their health in general—and to cancer in particular—is environmental pollution. Although most Canadians believe that air is the primary route by which environmental contaminants reach us, Canadians are exposed to environmental contaminants primarily through food. Food accounts for 80 to 95 percent of our total daily intake of persistent organic pollutants, air for between 10 and 15 percent and, for most persistent substances, soil and drinking water contribute less than five percent.^3b

The nature and degree of exposures to environmental hazards vary significantly from region to region across Canada and by many factors such as age, sex, occupation and eating habits. Also, the potential of a specific exposure to cause harm depends on a variety of factors, including exposure levels (duration and concentration), inherent toxicity, the route by which individuals take in the contaminant (e.g., ingestion versus inhalation) and the susceptibility of different groups. The very young, the elderly, people with weakened immune systems and native populations are particularly susceptible.^3c Children can be more vulnerable to environmental contaminants because of their rapid growth and metabolic immaturity, as well as their greater food, air and fluid intake relative to body weight.⁴ Native populations are particularly susceptible because of the tendency of many organic pollutants to concentrate in colder northern climates and contaminate fish and other wildlife on which these populations depend for sustenance.

Risk investigations

Estimates of the degree of risk associated with exposure to environmental contaminants depend upon various types of investigation. Since environmental exposures are low for both radiological and chemical hazards, risk levels are rarely detectable from direct observational studies of human populations.^5a For effects such as cancer, it is often difficult to estimate exposure or demonstrate cause and effect in the general population because cancer takes a long time to develop and multiple factors may be involved in its onset. Sources of variability in epidemiological studies include physiological parameters (such as body weight, respiratory rate and cardiac output, which can vary among individuals), routes of exposure, uncertainties in exposure estimates, errors in disease diagnosis and the effects of confounding factors.

Toxicology experiments, typically performed in laboratories on non-human models, are used widely to identify possible human health hazards (especially for chemicals) and to determine, for specific substances, the levels of exposure that present little or no risk to humans. Highly sensitive tests are available to examine a variety of deleterious effects, including tests of acute and chronic toxicity in animals, metabolism of chemicals, reproductive and developmental effects, and long-term carcinogenic effects.^5b

Biological markers are useful in the study of chemical hazards. These are biochemical changes that indicate that an exposure has occurred, but that are not necessarily linked to a clinically harmful effect. These markers may be studied to evaluate exposure, health effects or susceptibility, to assess intra- and inter-subject variability, to clarify mechanisms, or to identify dose-response relationships. Their ultimate usefulness is the extent to which they can predict disease occurrence.^5c

Structure/activity relationship studies use the chemical structure of a compound to predict toxic or carcinogenic effects. Predictions are often based on the known behaviour of similar compounds, considering specific properties and attributes. However, while such classification rules are useful, they are not perfect predictors of health effects.^5d

Extrapolations are used to relate the results of tests involving high doses of substances in different species to relatively low doses of substances in humans. Similar models for quantitative risk assessment are used for both radiation and chemicals.^5e For genotoxic carcinogens, such as ionizing radiation and certain types of chemicals that cause cancer by damaging DNA, it is assumed that there is a probability of harm at any level of exposure (in other words, it is assumed that there is no threshold for effects). Although dose-response curves may be non-linear at high doses, it is generally assumed that the dose-response curve for ionizing radiation and genotoxic chemicals is linear at low doses. Risks at low doses are therefore predicted from effects observed at high doses using what is known as the linear, non-threshold hypothesis (LNTH). This assumption has been widely used in cancer risk assessment in the absence of convincing evidence to the contrary. For other substances, including chemicals that cause cancer but do not damage DNA (non-genotoxic carcinogens), it is assumed that there is a threshold dose below which adverse effects are unlikely to occur.

Although epidemiology and toxicology are useful for estimating risk, both have limitations that can result in considerable uncertainty. For example, when human risks are estimated using animal toxicology, some uncertainty is introduced from the extrapolation of effects seen at the high doses used in laboratory studies to potential effects at the lower exposure levels experienced by humans in everyday life.^3d Results from studies of people exposed to particular contaminants in the workplace may not apply to people exposed in other settings because the health effects observed at high levels of exposure may not occur at lower levels. Thus, when performing risk assessments, a range of possible risks is considered, as indicated by a careful analysis of all sources of uncertainty in the data, and conclusions are generally based on appropriately conservative interpretations. Such uncertainties are believed to be smaller for ionizing radiation than for genotoxic chemical hazards.

Information sources

It should be noted that the data cited in this supplement, including the toxicology data, were obtained from the published literature only. It is acknowledged that extensive additional databases for the chemicals cited exist within regulatory agencies, including Health Canada, and that their data may not support the conclusions in this supplement. However, current legislation restricts access to these databases as they contain proprietary data supplied by manufacturers.

Three sources that were relied upon heavily as background material for this introduction are recommended for further reading. The first is Health and Environment: Partners for Life,^3e a Health Canada publication that describes our current understanding of the relationship between human health and the Canadian environment. It focuses on contaminants that are of particular concern to the health of Canadians, notes the progress made in reducing levels of environmental contamination and describes the impact that either the human-made or built environments can have on our health. The report also provides practical suggestions for things that individuals can do to protect and enhance their own health, and notes some emerging issues and future challenges related to health and the environment.

A second major resource is Assessment and Management of Cancer Risks from Radiological and Chemical Hazards,^5f produced jointly by Health Canada and the Atomic Energy Control Board of Canada (now the Canadian Nuclear Safety Commission). This publication describes the risk assessment and management processes used to protect the public from radiation, chemicals and microbiological hazards.

Thirdly, It’s Your Health,⁶ a component of Health Canada’s Web site, provides periodic updates on topics relating to the health of Canadians, including the relationship between some environmental exposures and cancer. Readers interested in the topics discussed here may wish to look at the sections for dioxins and furans, electric and magnetic fields, occupational exposure to radiation and PCBs.

The following chapter discusses general principles and some of the methodological challenges in environmental cancer epidemiology.

References

1. ^ National Cancer Institute of Canada. Canadian Cancer Statistics 2007. Toronto: NCIC; 2007. Available from: http://www.cancer.ca
2. ^ Health Canada. This battle which I must fight. Cancer in Canada’s children and teenagers. Ottawa: Supply and Services Canada; 1996.
3. a,b,c,d,e Health Canada. Health and environment: partners for life. Ottawa: Minister of Public Works and Government Services Canada; 1997. Cat.: H49-112/1997E.
4. ^ Federal, Provincial and Territorial Advisory Committee on Population Health. Toward a healthy future. Second report on the health of Canadians. Ottawa: Minister of Public Works and Government Services Canada; 1999.
5. a,b,c,d,e,f Health Canada and Atomic Energy Control Board. Assessment and management of cancer risks from radiological and chemical hazards. Ottawa: Minister of Public Works and Government Services Canada; 1998.
6. ^{^} Health Canada. It’s your health. Available from: http://www.hc-sc.gc.ca/hl-vs/iyh-vsv/index-eng.php