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Vol. 5 No. 4 - October 1999

Atmospheric Carcinogens: How Safe We Are!

By: H.S. Srivastava

Although tremendous amount of  funds and labour have been invested in finding out the cure, the cancer continues to be one of the major killer diseases of the ninetees. Five major types of cancer, lung, colon, rectum, breast and prostate account for about 55% of the total deaths caused by cancer. Lung cancer is the major type of cancer and significant progress in reducing lung cancer has not occurred despite decades of intensive clinical research efforts. In U.S.A. alone, over 160,000 deaths are reported annually. Cancer is a multistep process, which is identified as three stages: initiation, promotion and progression. The carcinogens may activate one or more of these steps. Epidemiological studies indicate that approximately 80% of human cancer is caused by exposure to chemical carcinogens in tobacco smoke, in the diet, and in the work place.

The most important carcinogenic factor of environmental importance is the smoking, which may be either active or passive. Smoking which causes primarily lung cancer, contributes to about one third of the total incidence of cancer and about four fifth of the lung cancer. Other types of cancer are also induced by smoking. In a study conducted by Bogdan Prokopczyk, Head Bio-organic Chemistry at American Health Foundation in Valhall, New York and his co-workers, which was reported in 18 June 1997 issue of the journal of National Cancer Institute, evidence was provided that smoking may cause cervical cancer not just in smokers but in non-smoking women also, who are exposed to cigarette smoke. Cervical cancer is quite prevalent in both developed as well as developing countries. According to a National Cancer Institute (U.S.A.) study, cervical cancer is the top cause of death from cancer in the developing countries. Smoke contains a wide variety of mutagens and rodent carcinogens. It contains at least 40 known carcinogens, some of which are polynuclear aromatic hydrocarbons, aza-arenes, nitrosamines, aromatic amines, aldehydes etc. The tobacco smoke specific carcinogen identified in Prokopczyk's study, as mentioned above, was identified to be 4-(methyl nitrosamino) -1-(3-pyridyl)-1-butanone (NNK), an N-nitrosamine that is formed during the processing of tobacco products. Further, smoking is a severe oxidative stress and causes inflammation in the lung. A smoker inhales gas phase smoke as well as particulates that penetrate the lung cells, whatever filter is used. Both of these phases are highly oxidising. One of the suspected mechanisms of the smoking induced cancer or for that matter any other type of stress induced cancer is through the generation of oxidative free radicals, which can bind to DNA and cause carcinogenesis. Primary free radicals are reactive oxygen species (ROS) such as superoxide anion and hydroxyl radical. Excessive generation of the ROS can damage not only DNA but also the lipids and proteins. Often the endogenous levels of ROS or the damage caused by them to lipid membranes is used 3s a biomarker in the organism exposed to genotoxic compounds present in the air.

Smokers have lower concentrations of ascorbate and vitamin E, the natural antioxidants which help in the scavanging of free radicals, in their lungs, than the nonsmokers. A number of other antioxidants are also known to protect cells or animals from tumor formation. Smokers must have a higher intake of ascorbate in diet because ascorbate acts as an antioxidant and is able to neutralize the oxidative stress.

In India, air pollution, specially the indoor pollution is of great concern as far as the incidence of cancer is concerned. Radon is likely to be the most important carcinogenic air pollutant. It occurs naturally as a radioactive gas as a decay product of the radium present in trace quantities in earth's crust. It is the sixth decay product of uranium-238. The short lived decay products of radon are two alpha emitters, polonium-218 and polonium-214. Alpha particles are considered to be the most prominent environmental cause of lung cancer. This has synergistic effect along with the smoking for the incidence of lung cancer. Health related studies have demonstrated that exposure to radon can cause chromosomal aberrations and mutations in lymphocytes, chromosomal instability in bone marrow cells and free radical formation. As mentioned, the free radicals may induce changes in DNA leading to carcinogenesis.

Other components of a polluted atmosphere are also injurious to human health and a variety of atmospheric chemicals have been linked with the incidence of cancers also. The pollutants may be either in the gaseous form or bound to particulate matters. Many of these compounds have been proved to be carcinogenic in experimental animals. These carcinogens, may be either initiators or promoters of the cancer. In vitro experiments show that chemicals in the environment can cause genetic damage, modulate cell proliferation, bind to hormone receptors and regulate enzyme activity. As many as 160 chemicals have been identified to cause breast cancer in rodents. Dimethylbenzanthracene (DMBA) and dimethyldinitrosourea (DNU) are classic examples. For human population, many agro-chemicals and daily use house-hold chemicals, have been associated with the carcinogenesis. In U.S.A. studies have indicated that the incidence of cancer, specially of Hodgkin's disease, multiple myeloma, leukemia, melanoma, and some other types of cancers, have increased in agricultural workers, who use lots of pesticides, herbicides, fungicides and fertilizers. Increased mortality from the cancer of the nose and nasal cavity has been reported for workers exposed to chlorophenoxy herbicides. One such chemical is the synthetic pyrethroids, which are analogs of a natural chemical moiety pyrethrin, isolated from the Chrysanthemum. Word-wide they are used as insecticides against ticks, mites, mosquitoes (found in mosquito repellent essence sticks and coils) and as treatment for human head lice and scabies. By using rodents as models, and also by using human breast carcinoma cell lines, it has been demonstrated that pyrethroids are serious hormone disruptors and thus may indirectly affect the processes such as atrazine, simazine and cyanazine (chlorotriazine derivatives), which have been widely used for controlling the weeds in Canada, USA and some parts of Europe are known to cause breast cancer. This has been concluded from the study of the incidence of cancer in populations living near the agricultural farms. Many of the environmental chemicals which are primarily known as inducers of allergic contact dermatitis have also been found to be carcinogen in experimental rats. These include some common compounds such as DDT, formaldehyde, thiourea, benzidine, coal tar etc.

One of the important aspects of the prevention of environmental pollutant induced cancer is to prevent exposure to these carcinogens. Of course, it is not possible to have a complete insulation, but the exposure should be as minimum as possible. It is also advisable to have protective clothings while working in an area where chances of exposure to carcinogens are high. The other important approach is to regulate the diet. Often it has been demonstrated that the consumptions of fruits and vegetables reduce the incidence of different types of cancer. Several factors may contribute to this association. The most important of course is that some of the nutrients present in fruits and vegetables can inhibit carcinogensis and may also act as an antioxidant. Studies in China have indicated that supplementing the diet with selenium tablets significantly lowered the mortality rate due to gastric cancer. Selenium, δ-tocopherol and (β-carotene act as anti-oxidants.

Professor H.S. Srivastava is Head, Department of Plant Science, M.J.P. Rohilkhand University, Bareilly - 243 006 (India)

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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