Non-Cancer Effects in Humans

As discussed in chapter 3, non-cancer effects do not lead to uncontrolled cell division. Rather, they cause specific effects in organs. This is the basis for drug therapy the neurotoxic effects (e.g., paralysis and death) discussed in chapter 2 regarding tetrodotoxin are non-cancer effects. Non-cancer effects are typically divided into the following categories  

For example, arsenic exposure can lead to lung cancer, skin cancer, or non-cancerous effects to the skin. Arsenic is a naturally occurring element, and historically was used in insecticides. Absorbed arsenic will reach the liver. Once in the liver, the arsenic is converted to a nontoxic form by an enzyme, known as methyltransferase, and is then excreted from the body. 

In order for this enzyme to work, a chemical known as S-adenosylmethionine (SAM) must be present in the liver. SAM is a derivative of the aunino acid methionine, which is essential to our health. There is only a certain amount of SAM in our liver that is available to combine with arsenic and render it nontoxic. If more au senic enters the liver than the amount of available SAM can handle, then excess non-metabolized au -senic may accumulate in the liver and be distributed to other orgams, causing toxic effects. This is known ais metabolic saturation. As long as the aunount of arsenic (i.e., the dose) is below this saturation level, our liver can detoxify it and no toxic effects will result. It is only when the amount of arsenic overwhelms the metabolic pathway that toxic effects can develop. This illustration of a threshold below which toxic effects do not occur is a key concept in toxicology (see chapter 3 for a graphical illustration of the threshold concept). 

A well-known chemical that can impact the nervous system is cocaine. The nervous system is composed of two parts peripheral and central. The peripheral nerv- 

The kidney, which filters unwanted things out of our bloodstrecim cind excretes them in urine, is also the target of unwanted chemicals because of this action. Many metals impact the kidney because they are particles that can block the filter, much as dust clogs air filters in your home. Examples of such metals include cadmium and mercury. 

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