Carcinogenicity of Substances and Factors in the Environment

Introduction
Environmental epidemiology of cancer aims at identifying risk factors in environmental and occupational settings and evaluating the risk. Many chemicals classified as carcinogens by the International Agency for Research on Cancer (IARC) were first evaluated at the work place. In the last decades, occupational exposure to carcinogens has actually decreased in many countries as the awareness of their hazards has increased. In recent years the field of occupational cancer epidemiology is balancing on a turning point, i.e. on one hand, it seems that major occupational carcinogens have already been identified and on the other hand we have a long list of substances from which epidemiological data remains inconclusive. Still many occupation carcinogens remain to be identified; this can be illustrated by the association between cancer and industrial occupations for which the specific agents have not been identified. This means that workers continue to be exposed to hazardous substances but are not yet recognized as such.

Carcinogen or Mutagen
A carcinogen is a substance that has the ability to cause cancer (somatic mutagen, but not necessarily), while a mutagen is a substance/agent (physical or chemical, may also be biological like oncoviruses {e.g. HPV} and helicobacter pylori) that can increase or induce the frequency of mutations.

Mutagen Classification

 * 1) Base analogues (become inserted into DNA strand during replication resulting in alteration of complementarity and base substitution mutations)
 * 2) Direct acting agents (directly react with DNA and cause structural changes that lead to miscopying of the template strand when DNA is replicated)
 * 3) Indirect acting agents (require the help of metabolism to produce direct acting agents)

Epigenetic Mutagens
This is the process of altering the activity of genes without changing their structure, i.e. altering the activity of DNA repair enzymes thus increasing the frequency of mutations.

Teratogens
These are agents that causes physical defects. Disruption of fetal development resulting in congenital defects which are Not hereditary. The first 8 weeks, is the most critical period for teratogenic action. Some teratogens include, Alcohol, tobacco, caffeine, PCB (polychlorinated biphenyls), dioxin, radiation, Herpes virus, CMV, Rubella, syphilis.

Xenobiotic Agent
This is a compound alien to humans but found in the body. Some are harmless, many are harmfull. They may be present in soil, food, air, water.
 * Natural: aflatoxins
 * Artificial: nitrates (smoked meat). PAH formed from incomplete combustion!
 * Metals: arsenic, lead, asbestos, mercury.
 * Chlorine in water
 * Air: traffic, industry, heating

Classification of substances
A list of substances slassified according to their dangerousness, based on the Ames Test (IARC) follows.

Group 1
[107 agents]: The agent (mixture) is definitely carcinogenic to humans. The exposure circumstance entails exposures that are carcinogenic to humans.

Group 2A
[58]: The agent (mixture) is probably carcinogenic to humans. The exposure circumstance entails exposures that are probably carcinogenic to humans.

Group 2B
[249]: The agent (mixture) is possibly carcinogenic to humans. The exposure circumstance entails exposures that are possibly carcinogenic to humans.

Group 3
[512]: The agent (mixture or exposure circumstance) is not classifiable as to its carcinogenicity to humans.

Group 4
[1]: The agent (mixture) is probably not carcinogenic to humans. Caprolactam