Carcinogenicity is a term applied to anything that has a demonstrated tendency to cause cancer, particularly in humans. A carcinogen is something that has the property of carcinogenicity. Carcinogens exist in several different forms. Some are substances such as those found in cigarettes and grilled or barbecued meats while others exist in the forms of radiation, such as that emitted from atoms with unstable nuclei. Carcinogens cause cancer, which is often deadly, by causing uncontrolled cellular growth and division that leads to the formation of deadly tumors that can spread throughout the body.
There are many different substances, natural and artificial, that have carcinogenic properties. Various naturally occurring fungi, bacteria, and viruses, for instance, are able to cause cancer and are thus said to have carcinogenicity. Many different sources of carcinogenicity are either man-made or related to the human use of natural substances. Smoking cigarettes, for instances, exposes one's body to many different cancer-causing agents. Different food preparation methods, such as grilling, frying, and barbecuing, also can induce carcinogenicity in some foods.
Some studies have shown that human habits and behaviors can have an inherent level of carcinogenicity. Staying awake through the night, for instance, affects the chemical balance in the human body in a way that may lead to or promote cancer. Such behaviors or habits that promote cancer usually fall into the category of "circadian disruption," meaning that they are behaviors that disrupt the normal, healthy cycle of sleep and wakefulness that people are supposed to go through on a daily basis.
The degree to which something is regarded to be carcinogenic, or the "level of carcinogenicity," is measured based on a variety of different systems. Some systems have very few classifications and are based only on whether or not a substance can cause cancer. Others have more classifications, such as whether the substance is known to induce cancer in animals and whether it is possible to make a judgment about its cancer-inducing potential at all. It should be noted that "carcinogenicity" is generally used to refer to whether or not something can cause cancer and is not usually used to compare the cancer-causing potential of different substances.
Many carcinogens damage an animal's DNA, thereby causing the uncontrolled cell growth that leads to cancer. Some substances, while not able to cause cancer themselves, can promote the growth of cancerous tumors and are still sometimes classified as carcinogens. Such substances are often hormones or other substances that can promote cellular growth.