Environmental Carcinogenesis

Environmental carcinogens include outdoor and indoor air pollutants as well as soil and drinking water contaminants. Epidemiological studies have shown that 70-90% of all cancers are environmental.  Environmental factors such as lifestyle, personal habits, diet, chemicals and radiation and infectious diseases account for about three quarters of all cancers. The China admits that there has been an 80% rise in the mortality rate from cancers over the past 30 years. The United States has one of the world’s highest incidences of cancer associated with environmental pollution. The most recent data suggests there were 223,000 deaths from lung cancer caused by air pollution around the world (International Agency for Research on Cancer, IARC).The IARC classified ambient air pollution as cancer causing agent (carcinogen). Studies also show that nearly 30% of the total mortality in several industrialized countries is due to cancer.

Cancer

A tumour or cancer is an abnormal mass of tissue whose cells undergo rapid and uncontrolled growth at the cost of remaining cells. The tumours are classified as benign or malignant. Benign tumours remain localized in a specific area at the site of origin, forming a single mass enclosed in a capsule. They slow growing and can be removed effectively with surgery. Malignant tumours are cancerous with rapidly growing and actively moving cells.  The cancer cells migrate through the blood and lodge at distant sites which are called metastasis. Cancers or malignant tumours are uncapsulated and invasive.

Carcinogens

Carcinogens are agents that induce cancer. Primary or direct – acting carcinogens are those that do not require metabolic activation e.g., mustard gas. Some carcinogenic chemicals are inactive and require metabolic activation. They are secondary carcinogens e.g., carbon tetrachloride. The parent compound is called a procarcinogen  and is converted to a reactive metabolite called proximate carcinogen and then to a highly reactive species termed as ultimate carcinogen, which are covalently bind to macromolecules like DNA. Co-carcinogens (promoters) are substances that potentiate or promote the effects of carcinogens e.g., cyclopropenoid fatty acids. Carcinogenic agents can further classified as genotoxic or non-genotoxic, based on the ability to alter the genetic systems in cells. Asbestos increase the incidence of cancer, but do not possess genotoxic effects.

 3- stage model of chemical carcinogenesis

 The chemical carcinogenesis comprises 3- sequential and successive steps: initiation, promotion and progression. Tumour initiators can be defined as carcinogens capable to induce a first driver mutation in a dividing cell so that an initial clone of mutated cells emerge. Tumour promoters can be defined as non-genotoxic carcinogens capable of causing clonal expansion of initiated cells i.e., able to induce proliferation of mutated cells. Tumour progressors are carcinogens that advance mutated cells from promotion to progression  and transform a mass of fully malignant cells. So carcinogenesis is a multiple step process.

 Cancer develops over a peroid of several years (latent period) and has many causes. There are more than 100 types of cancers. Scientists have identified more than 300 altered genes called oncogenes that can signal the cell to divide out of control. One of the characteristics of chemical or physical carcinogenesis is the usually long latent periods (years to several decades) between the contact with the carcinogen and appearance of a tumour. The degree of cancer risk from pollutants depends on the concentration, intensity and duration of exposure. E.g., saccharin is carcinogenic only at higher doses.

Environmental carcinogens

Arsenic, asbestos and radon are three prominent human carcinogens strongly associated with lung cancer.  Benzene is known to cause leukemia (blood cancer) in human beings. Benzene has widespread use as solvent in the chemical and drug industries and a gasoline component. Drinking water that is contaminated with a high level of arsenic over a long period of time is known to increase the risk of lung, bladder and certain types of skin cancers. Exposure to arsenic caused 3,700 lung, bladder and skin cancer deaths in Bangladesh alone. An increased risk of stomach cancer has been reported in areas with high nitrate levels in drinking water. There are more than 75,000 chemical compounds in contaminated waters come from industry, agriculture and consumers/homes. Bisphenol A (BPA), a building block of polycarbonate plastic is an endocrine disruptor linked to breast and prostate cancer. Exposure to vinyl chloride (PVC) is linked to the development of liver and brain cancer. Polycyclic aromatic hydrocarbons (PAHs), a product of incomplete combustion of organic compounds is possibly carcinogenic. The chemical compounds with carcinogenic potential include benzopyrene, benzene, organic solvents, pesticides, dioxins, several heavy metals (arsenic, cadmium, lead, mercury) and others.

Cancer is a preventable disease

According to the National Cancer Institute, 80% of the cancers are due to factors that have been identified and can potentially be prevented. About 20 years ago, 1 out of 10 people were diagnosed with cancers and other debilitating diseases.  Now we are faced with 1 out of 2 people being diagnosed with cancer. The environment, which sustains the life of all living organisms, can also be a significant contributor of ill health. The natural environment is crucially a ‘commons’ a public good. Respect the ecosystems and keep them healthy.