Title: Unit 9 Learning Module
Course: BIS 105 Environmental Biology > Section: BIS 105 Environmental Biology

Unit 9 Learning Module

What is Toxicology?
Copyright
Adapted from U of I Open Source Textbook. Sustainability: A Comprehensive Foundation. Connexions. 3 Mar 2013 .
Adapted from Wikipedia contributors. "Endocrine disruptor." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 8 Apr. 2013.
Adapted from Wikipedia contributors. "Bisphenol A." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 11 Apr. 2013.

Toxicology

Toxicology is the study of adverse effects of chemicals on humans. Scientists who study how hazardous a given substance is are called toxicologists. These researchers identify the toxicity of various chemicals, which is the degree of harm a substance poses to organisms. Substances that are the most toxic only require a small amount to be ingested to cause harm. There is a common saying that says, "The dose makes the poison." This refers to the fact that nearly any substance, even those needed to stay alive, can be harmful if the quantity is great enough. Therefore, not only does the type of chemical influence the toxicity, but so does the dose (amount) that an individual is exposed to.

According to the EPA, more than 85,000 chemicals are used in the United States with an additional 1,000-2,000 new chemicals developed each year. People are exposed to these chemicals through a variety of routes: the air we breathe, the water we drink, and the food we eat. In some cases we can control this exposure, such as the type of food we eat or the cleaning products we use, while other times we have little control, such as the air being polluted by power plants or vehicle emissions.

Types of Hazards

Substances can cause harm to an organism in a variety of ways. The following are categories of substances that cause a particular type of harm:

- Carcinogen - a substance that causes cancer

- Mutagen - a substance that causes DNA mutations (change in DNA sequence)

- Teratogen - a substance that birth defects in developing embryos

- Allergen - a substance that over-stimulates the immune system

- Neurotoxin - a substance that interferes with the nervous system

- Endocrine disruptors - a substance that interferes with the endocrine (hormones) system

Duration of Exposure

Some substances can be harmful immediately and are known as acute hazards. In contrast, other substance only pose a risk from long-term exposure and are known as chronic hazards. However, people do not always respond to a substance in the same manner. Factors such as age, sex, weight, race, etc can all influence they way each person reacts to a given substance. In addition, sometimes individuals can develop a tolerance to a toxicant in which the effects are reduced. Conversely, in some cases an individual can be hypersensitive to a substance, meaning they become extremely sensitive to very small doses. Formeldehyde is one such chemical that can cause hypersensitivity. People can be exposed to it from furniture, pressed-wood products, and cigarette smoke and can often tolerate high quantities initially, but later react when exposed to much lower amounts.

Chemical Interactions and Environmental Contaminants

In modern society where there are over 85,000 chemicals in use it can be very difficult to predict all the potential impacts. Not only is it challenging to predict how one chemical can impact an individual in many ways, but these chemicals can interact in the environment and alter the outcome. Some chemicals can result in an additive response, meaning the result is the sum of the individual impacts (e.g. 1+1=2). Alternatively, some chemicals increase the effects of each other resulting in an even large impact (e.g. 1+1=10). This is known as a synergistic response. Sometimes a chemical that itself is not harmful can increase the harm of another chemical (e.g. 0+1=10), a result known as potentiation. Antagonism is when certain chemicals counteract the impacts of one another (e.g. 3+5=4).

Persistence

How long a chemical is present in the environment, its persistence, can vary dramatically for different chemicals. Sunlight, reactions with other chemicals, and metabolism by bacteria are all natural examples of how chemicals can be degraded. Some chemicals are able to resist being altered, allowing them to accumulate in the soil, in aquatic habitats, or volatilize (evaporate) and move great distances in the atmosphere. Synthetic chemicals containing halogens (chlorine, fluorine, bromine) are often resistant to degradation. Also, the products of a chemical reaction can also remain problematic. For example, DDT is a hazardous chemical that can breakdown to form DDE, which also is a hazardous chemical. In addition, metals such as lead, mercury, and arsenic are persistent since they are fundamental elements that cannot be further broken down.

Bioaccumulation

When a chemical is not metabolized or eliminated from an organism, it ends up being stored within the organism, often in fatty tissue. As a result, over time the concentration increases (bioaccumulates). Mercury, lead, and DDT are examples of compounds that can bioaccumulate.

Biomagnification

When an organism eats another organism, it can be exposed to a contaminant that is stored within their prey. This can result in organisms at higher trophic levels often having much greater concentrations of bioaccumulated toxins than those who feed lower in food webs. Such a pattern is known as biomagnification.

In the figure below, the increased concentration of DDT as you move up trophic levels is evident. DDT was used as an insecticide until 1972 when it was banned for use in the United States. More than 1.35 billion pounds of DDT was applied in the US before the 1972 ban. Concerns over biomagnification, particularly in birds, and the resulting impact on reproduction spurred the restrictions on its use. As DDT concentrations increase with higher trophic levels, such as bald eagles and osprey, there were declines in raptor reproduction rates due to eggshell thinning. When the adult birds attempt to incubate the eggs, they crack open as a result of their reduced thickness.

DDT can also affect humans. Higher concentrations in blood have been linked with increased incidence of diabetes. DDE, a metabolite of DDT, has also been found to act as an antiandrogen. Androgens are hormones that cause the development of male characteristics. Therefore, DDE can disrupt the normal functioning of the endocrine system by affecting the balance of male and female sex hormones.