It was stated in an earlier section (What is arsenic?) that the different forms of arsenic have different toxicities. This is probably due to a number of different factors: how well they get in to the body (absorption), where they are distributed, how they are modified in the body, and how fast they are flushed out.
|This is my lame attempt to show how arsenic and other
chemicals get in to the body and what happens to them.
- Sometimes the chemical passes right through the body as though it were never there.
- Sometimes the chemical is absorbed and must be changed somehow (metabolized) before it can be excreted.
- Sometimes it is absorbed, but is then excreted unchanged.
When the arsenic reacts inside the body with important biological molecules, it can disrupt their function. That's when you begin to see toxic effects.
In general terms, the organic forms of arsenic are less toxic than inorganic arsenic.
Of the inorganic species, As(III) is more toxic than As(V). The two different forms may exert their effects in completely different ways. As(V) forms arsenic acid, or arsenate. If you look on the periodic table, you can see that arsenic is in the position just below phosphorus. Phosphorus is a very important element in biological tissues. Phosphate is used to build DNA, the molecule that acts as the genetic blueprint for the cell, RNA which is needed to translate the genetic code into functional proteins and it is also used as the energy currency of the cell. That means that phosphate is crucial for life. Since the arsenate looks a lot like the phosphate molecule used in DNA and RNA synthesis, it can sometimes trick the transport proteins for phosphate into importing arsenate from outside the cell. Once inside, arsenate may interfere with normal processing of phosphate or act in other ways to prevent normal cell functions.
As(III) or arsenious acid works in a different way. Since it is missing the extra oxygen, it doesn’t look much like phosphate to the cell, but because it is uncharged, it can pass through the lipid membrane to the inside of the cell, or hitch a ride in transport proteins that move small, uncharged molecules around. Once inside, it will combine with sulfur groups on proteins. Sulfur groups are often used to form cross-linkages between protein sections to hold them in a specific 3-D arrangement (proteins are made of long strings of amino acids, but they have to be folded in a specific way to be functional). Preventing these linkages can cause a wide variety of problems in the cell since it can affect many different kinds of proteins. That may help to account for the wide range of toxic effects of arsenic.
The most obvious and immediate cases of arsenic poisoning have occurred when a large dose of arsenic is taken (or given!) in a short period of time. This kind of exposure is called acute arsenic poisoning, and some of the symptoms are:
Luckily this kind of poisoning is rare. It could occur through intentional poisoning, or maybe through occupational exposure, but many more people are affected by arsenic poisoning at much lower levels, and over longer periods of time. This kind of exposure is called chronic arsenic poisoning or exposure.
The more people look, the more negative health effects they find associated with chronic (long-term/low concentration) exposure to arsenic. Some of the health problems that have have been seen when people are exposed to high As in drinking water over a long period (such as in Bangladesh) include:
Low birth weight
The trouble is that the effects are not always the same for different populations of people. Different populations may be exposed to different forms of arsenic, and they may have different sensitivities and exposures (your daily dose of arsenic will depend on the arsenic concentration in your food and water, and how much you eat and drink). Another important factor in any health effect is nutritional status. If you eat a nutritious diet, you may be more resistant to negative health effects than if you were already weakened by poor nutrition. All the more reason to eat your veggies!
Generally speaking, the main routes of contamination for people who are not exposed to arsenic in their work (occupational exposure) are drinking water first, followed by food. Absorption through the skin seems to be minimal, so arsenic exposure through hand washing, laundry, bathing, etc. is not considered to be a problem.
You can see that the toxic effects of arsenic exposure actually depend on at least four different things: (1) how much arsenic a person, animal or plant is exposed to (dose); (2) how long the person (or animal or plant) is exposed (if you are exposed to a particular dose or concentration for a short time, you are less likely to be affected than if you are exposed to that same dose for a longer time); (3) the form of the arsenic (how toxic that forms is); and (4) the sensitivity of the individual (some people are more sensitive than others, and some plants or animals are, on the whole, more sensitive than others). All of these factors make a very complicated picture when you’re trying to sort out how “dangerous” it is to have certain levels of arsenic in the environment!
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