1. Environmental Carcinogens
Over the past 100 years, there has been a significant rise in life expectancy, particularly in the developed portions of the world. Since 1950, there have been no appreciable changes in life expectancy in the United States. The two leading causes of death in the United States, heart disease and cancer, have increased since 1900 at a rate much higher than may be accounted for by population growth and by aging of the population. Environmental influences may already be factors in life expectancy and health in the United States.
Cancer has more than doubled since the turn of the century. Much of the increase may be attributed to cigarette smoking, but overall cancer rates exclusive of lung cancers are also rising.
Cancer has a long latency period. In humans there is usually a 15- to 40-year period through the progressive stages that eventually terminates in cancer. Increased levels of exposure to a carcinogen carry with them an increased risk of developing cancer, a single exposure to a chemical may also result in a cancer.
About two million chemical compounds are known, and each year thousands more are discovered by the U.S. chemical industry and hundreds are introduced commercially. We know very little about the possible health consequences of these new compounds. The sheer number of chemical compounds, the diversity of their use, and the adverse effects already encountered from some make it increasingly probable that chemical contaminants in our environment have become a significant determinant of human health and life expectancy.
It must be remembered that cancer is a disease of the whole organism. It possesses attributes ranging from unrestricted growth to invasiveness.
It is true that many agents that are mutagenic, that is, responsible for cellular mutation, are also carcinogenic, capable of inducing cancer, but a direct correlation between mutagenic and carcinogenic occurs within the cell and may initiate cancer. First, for a mutation to happen, an agent foreign to the cell constituents is needed. A number of agents are capable of resulting in cellular mutations and sometimes ultimately cancer.
We can thus distinguish three steps in the development of cancer:
- Initiation (or the first change).
- Promotion (from the dormant to the visible stage).
- Progression (leading to the irreversible state).
All normal cells have a finite life span. The normal cell divides in two at a constant rate, then the two daughter cells into four, and so on, but along the road of reproduction some of the cells die at a constant rate equal to the rate of reproduction. Balance is therefore maintained. But not so for cancer cells. They do not have a finite life span. Nor do they die in the same proportion as those that reproduce.
Uncontrolled growth, or geometric, not linear, reproduction occurs, the outstanding characteristic of cancer cells. The volume of the growing tissue continuously increases, and balance in number and volume is not maintained. In this first step toward cancer the transformed cells have escaped the controls and balances which govern normal cells.
Once a neoplastic cell has reached the stage of progression, the third, stage in carcinogenic growth, it is a cancer cell forever. But, before that stage, the neoplastic transformation may be reversible. The first step, initiation, or mutagenesis, does not necessarily lead to cancer; if the second stage (promotion) fails to take place, the third and definitive stage will not be attained. In other words, the initial change in the DNA of the cell may not be permanent or irreversible. In this case, if the cause of the cell irritation is removed, the cell will repair and resume normal function.
If we think of the process of neoplastic transformation in terms of the interaction in the cell of DNA, RNA, and protein, we could say that the primary change, or initiation, occurs in the base sequences of DNA; the second step, promotion, and the third step, progression, would be expressed in the perpetuation of the change as DNA replicates itself.
1.1 Chemical Carcinogenesis
It is probable that a high proportion of human cancer, perhaps 60 to 90% is due to environmental causes. Cigarette smoke, atmospheric pollution, and various other materials in our environment contain certain hydrocarbons which can produce cancer. For centuries some meats have been conserved in salt and it has been found that the nitrates present in meat cured in this way can also be carcinogenic.
Chemical carcinogenesis is generally a two-stage process as mentioned above consisting of, first, initial ion. and second, promotion. An example of this double process is to be found in the relationship of croton oil and polycyclic hydrocarbons to skin cancer. Croton oil alone rarely produces skin cancer but if a single dose of a polycyclic hydrocarbon is applied to the skin of an experimental animal and if this is then followed by an application of croton oil, skin cancer frequently occurs: the polycyclic hydrocarbon acts as an initiator and the croton oil as a promoter of the cancer process. Observations of this type are pertinent to human cancer since we know that our natural environment contains a number of chemical carcinogens. It is logical to assume that even a low level of these chemicals might serve as initiating agents, and that association with promoting agents could result in cancer.
The naturally-occurring fibrous silicates classified as “asbestos” now are believed to be the most deadly cancer-causing agents in the workplace. There has been a thousandfold increase in output of asbestos during the past 50 years, and although it has been known for more than half a century that persons who inhaled large amounts of it in the course of their work sometimes developed disabling or fatal fibrosis of the lungs, it has only been within the last 30 years that it has been found to cause cancer. It is estimated that as many as 3,000 different products in daily use throughout the world contain some asbestos
The first suggestion that it could cause cancer was raised in 1935 by two physicians who noticed a correlation between asbestosis, a condition caused by fibers remaining in the lungs, and lung cancer. The link was definitely established around 1960 when Mt. Sinai School of Medicine researchers found that asbestos was the cause of mesothelioma, a rare and always fatal cancer of the membranes surrounding the lungs and lining of the abdominal cavity. Cancer of the stomach has also been found among American insulation workers and among the Japanese who like to eat rice coated with talc, a substance usually contaminated with asbestos. Asbestos has also been linked with cancer of the larynx, and a study by the American Cancer Society showed that asbestos workers who smoke may be 92 times more likely than the average nonsmoker to develop lung cancer. Even relatively brief exposure can increase the risk of cancer 20 to 40 years later.
Asbestos has been found in the air we breathe. An estimated 158,000 pounds of this mineral is released from automobile brake linings every year, and thousands of schools and other buildings have been constructed with asbestos insulation and flame proofing.
Dr. Irving Selikoff of Mt. Sinai School of Medicine, one of the world’s leading experts on asbestos, says that a worker could be exposed heavily to asbestos for one day and develop cancer much later in life as a result, because his lungs continue to be exposed to the asbestos fibers deposited there.
Altogether, 2.5 million Americans are now working in trades involving asbestos, including asbestos mining and processing, insulation work, building demolition, and brake-clutch repair.
The modern home has asbestos from roof to basement—asbestos roofing or roof tiles, gutters, rainwater pipes, ceiling and floor tiles. The tiles, even if polished, are subject to wear and tear and presumably the liberation of asbestos. The instances of plural disease, one of them mesothelioma, have been found in workers who sanded asbestos floor tiles. Central-heating furnaces and pipes insulated by asbestos are common. Insulation of electrical equipment in the home, such as electric irons and stoves, is almost always compounded with asbestos. Ironing-board covers are made from the substance.
The latency period for asbestos-caused cancer is from four to fifty years. It primarily appears in the lungs, pleural cavity, gastrointestinal tract, ovary, skin, liver, and larynx.
1.1.2 Air Pollution
Polluted air affects the health of human beings and of animal and plants. Emitted pollutants are diluted in the atmosphere and swept away by winds, except during an inversion (when surface air cannot rise). Then, for a period that varies from a few hours to a week or more, pollutants are trapped and the dilution process is impeded. When an inversion persists for a week or more, pollution intensifies
substantially, and there is an accompanying increase in the death rate. Scientists are convinced that air pollution is very definitely a factor contributing to the three major types of diseases that result in sickness and death in our country—heart disease, lung and respiratory diseases, and cancer. Studies have shown that air pollution can actually be hazardous to people who live 50 to 100 miles away from the pollution source. This is because some common pollutants while moving through the atmosphere are transformed by chemical reactions with sunlight into more hazardous pollutants, such as photochemical oxidants that poison the lungs and respiratory system. Sulfur oxides aggravate asthma, lung and heart diseases, and lung functions of children. The amount of suspended particles in the air is
related to injury to the surfaces of the respiratory system, that is, to the linings of the lungs and throat. Chemicals carried into the lungs by particulates may cause cancer to develop on the lung lining. Carbon monoxide is harmful to persons who have lung disease, anemia, or cerebral-vascular disease. Photochemical oxidants may cause respiratory irritation and even changes in lung function. Nitrogen oxides in high concentrations can be fatal, and in lower concentrations they can cause acute bronchitis and pneumonia.
Air pollution accounts for a doubling of the bronchitis mortality rate for urban as compared to rural areas. In 1958 the rate of death from lung cancer was 1.56 times as high in the urban areas as in the rural areas. Stomach cancer is related significantly to a deposit index and a smoke index. The mortality rate due to stomach cancer is more than twice as great in areas of high pollution as in areas of low pollution. The mortality of all cancers is 25% higher in polluted areas than in areas of relatively clean air.
Cigarette smoke and automotive exhaust, the most common environmental air problems, are composed of gases, liquid and solid aerosols. Personal pollution from smoke is the main cause of respiratory cancer. Municipal incinerators are major sources of several toxic elements in the air of many cities. Refuse incineration can account for major portions of zinc, cadmium, antimony, and possibly silver, tin, and indium observed in airborne particles. Many of the toxic elements from incinerators are associated with predominantly small particles that can be easily inhaled into the lungs, where these poisonous elements can be dissolved in body fluids and transported about the body.
The impact of air pollution on humans generally increases with age. This indicates that air pollution has a cumulative effect on health.
It has been estimated that 25% of the deaths from lung cancer could be saved by a 50% reduction in air pollution.
Drinking water that comes into your home may be the greatest source of cancer-causing agents to which you are exposed. There are thousands of organic chemicals potentially present in our water supplies due to industrial discharges and spills. The use of agricultural chemicals, industrial discharges and spills, and the runoff of rainwater from cities present a growing pollution problem. Groundwater—subsurface water supplied by springs, lakes, and rivers—can be polluted by surface waters, deep-well disposal, seepage from mines, landfills, septic tanks, feedlots, and pesticides. Groundwater supplies 20% of the freshwater used in the United States. It constitutes the entire water supply of more than 95% of the rural population and 20% of the 100 largest cities in the country; the semiarid Southwest is almost completely dependent upon groundwater. It is estimated that 10 million barrels of brine are injected into underground reservoirs by the gas and oil industry. While relatively little is known about the chemicals that pollute our water, it is known that many of them do cause cancer in test animals.
The NAS committee noted that chlorination results in the formation of suspected carcinogens for humans. Lead toxicity, the committee said, is a particular risk for inner-city children. Consequently, the interim drinking-water reputations for lead—established under the Safe Drinking Water Act in 1975—may provide an adequate margin of safety for adults but not for urban children. Similarly, they noted, present data support reexamination of the margins of safety provided by interim drinking-water limits for nitrate, arsenic and selenium.
1.1.4 Polycyclic Hydrocarbons
The polycyclic hydrocarbons are members of a broad class of chemicals produced by incomplete combustion of organic matter. Cigarette smoke, vehicle exhaust, forest fires, and even charcoal-broiled meat have been shown to contain substantial quantities of these potent carcinogens.
- Part I
- 1. Introduction
- 2. The History Of Cancer
- 3. What Cancer Is
- 4. Cancer Incidence
- 5. Normal Cells To Cancer Cells
- 6. A “Cure” For Cancer
- 7. The Seven Stages Of Disease
- 8. Can Cancer Be Prevented?
- 9. How Not To Develop Cancer
- 10. The Requirements For Health Will Fullfill The Needs Of The Sick
- 11. Habits
- 12. Cancer Treatment
- 13. Chemical Contaminants
- 14. Geographical Factors
- 15. Cocarcinogens
- Part II
- Part III
- Part IV
- 1. Environmental Carcinogens
- Part V
- Part VI
- Part VII
- Questions & Answers
- Article #1: Autolyzing Tumors By Dr. Herbert M. Shelton
- Article #2: Some Prefer Cancer By Lewis E. Machatka
- Article #3: Black Pepper Causes Cancer!
- Article #4: Ten Commandments of Cancer Prevention