1. Introduction to Toxicology
Environmental Health Institute - July, 2006 University of Rochester Medical Center
Introduction to Toxicology
Dina Markowitz, Ph.D.
Director, Center for Science Education & Outreach
Department of Environmental Medicine
University of Rochester

2. What is Toxicology?
Toxicology is the study of how toxicants cause adverse effects on living organisms.
Toxicology (from the Greek words toxicon and logos) is the study of the adverse effects of chemicals on living organisms.
It is the study of symptoms, mechanisms, treatments and detection of poisoning, especially the poisoning of people.

3. Toxicology Terms to Know
Toxicant (Poison):
A chemical capable of producing a harmful reaction in a living organism.
Adverse effect:
Any change that interferes with an organism’s normal functioning.

4. What is a Poison? All substances are poisons;
there is none that is not a poison.
The right dose
differentiates a poison and a remedy.
Paracelsus ( )
Paracelsus was a scientist and physician who was born in Switzerland in Paracelsus pioneered the use of chemicals and minerals in medicine. He is sometimes called the "father" of toxicology.
A popular short version of this quote is: "The dose makes the poison." In other words, the amount of a substance a person is exposed to is as important as the nature of the substance. For example, small doses of aspirin can be beneficial to a person, but at very high doses, this common medicine can be deadly. In some individuals, even at very low doses, aspirin may be deadly.

5. Examples of Toxicants: Chemicals that can Cause Harm
Prenatal alcohol abuse
→ fetal alcohol syndrome
If a woman drinks alcohol during her pregnancy, her baby can be born with fetal alcohol syndrome, a lifelong condition that causes physical and mental disabilities. FAS is characterized by abnormal facial features, growth deficiencies, and central nervous system problems. People with FAS might have problems with learning, memory, attention span, communication, vision, hearing, or a combination of these.
Nearly all fish and shellfish contain traces of mercury. For most people, the risk from mercury by eating fish and shellfish is not a health concern. Yet, some fish and shellfish contain higher levels of mercury that may harm an unborn baby or young child's developing nervous system. Women who may become pregnant, pregnant women, nursing mothers, and young children to avoid some types of fish (such as Shark, Swordfish, King Mackerel, or Tilefish) and should only eat fish and shellfish that are lower in mercury. This slide shows a photo of people in Minamata Japan who were severely poisoned in the 1950s by very high levels of mercury in fish that they ate. The mercury was released into the water by improper treatment of industrial waste from factories.
Mercury in fish
→ brain damage

6. Examples of Toxicants: Chemicals that can Cause Harm
Lead in paint
→ brain damage
Dioxins are byproducts of many chemical manufacturing processes involving chlorine. They acquired notoriety in 1976 following an accidental release from a chemical plant at Seveso in Italy, which caused local residents to develop a severe form of facial scarring called chloracne. Dioxin poisoning made the news more recently with the poisoning of Ukrainian opposition leader Viktor Yushchenko. Yushchenko's face and torso became disfigured after he fell ill during a bitter election campaign in the fall of He was poisoned, probably by being given food laced with deadly dioxins by his political enemies.
Lead-based paint is a major source of lead poisoning for children and can also affect adults. In children, lead poisoning can cause irreversible brain damage. It can retard mental and physical development and reduce attention span. It can also retard fetal development even at extremely low levels of lead. In adults, it can cause irritability, poor muscle coordination, and nerve damage to the sense organs and nerves controlling the body. Eating paint chips is one way young children are exposed to lead. Ingesting and inhaling lead dust that is created as lead-based paint chips or peels from deteriorated surfaces can expose people to lead. Walking on small paint chips found on the floor, or opening and closing a painted frame window, can also create lead dust. As pictured here, lead dust can also be generated by sanding lead-based paint or by scraping or heating lead-based paint.
Dioxin poisoning
→ facial scarring (chloracne)

7. What amount causes harm?
Some chemicals are good in small amounts, but toxic in large amounts Example: botulinum toxin Small amount → Large amount →

8. What amount causes harm?
Some chemicals are good in small amounts, but toxic in large amounts Example: botulinum toxin Small amount → prevents wrinkles (BOTOX) Large amount → paralysis, death
Botulinum toxin (BOTOX) is a purified substance derived from the bacterum, Clostridium botulinum. Is acts to block muscular nerve signals.  Botulinum toxin is the first biological toxin to become licensed for treatment of human disease and it is also considered a biological weapon.
Injecting very small amounts of BOTOX into specific facial muscles blocks the muscle’s impulse.  This temporarily weakens the muscle and diminishes the unwanted facial lines. 
Botulinum toxin is the most poisonous substance known. A single gram of crystalline toxin, evenly dispersed and inhaled, would kill more than 1 million people. Terrorists have already attempted to use botulinum toxin as a bioweapon. Aerosols were dispersed at multiple sites in downtown Tokyo, Japan, and at US military installations in Japan on at least 3 occasions between 1990 and 1995 by terrorists from a Japanese cult group.

9. Dose Dose refers to the amount of a toxicant entering the body
Dose is measured as milligrams of toxicant per kilogram of body weight = mg/kg
Example: 100 mg caffeine
50 kg adult (110 pounds)
dose = 100 mg/50kg = 2 mg/kg
10 kg baby (22 pounds)
dose = 100 mg/10 kg = 10 mg/kg
Since individual people have different body weights, the amount of a toxicant that will effect them will be different. In the science of toxicology, we use the term “dose” to refer to the amount of a toxicant (which is a chemical or pother poison) that enters the body.
Dose is measured as the amount of toxicant (in milligrams) per weight of an individual (in kilograms), or milligrams per killogram.
For example, if an adult consumed 100 mg of caffeine (about the amount in a cup of coffee) and if they weighed 50 kg (about 110 pounds), the dose would be 100mf/50 kg of body weight, or 2 mg/kg. If a baby weighing only 10 kg (about 22 pounds) consumed the same 100 mg of caffeine, the dose would be 10 mg./kg, which is 5 time as large, because the body weight is five times smaller. This principal can be extremely important in exposure to lead or pesticides, because the dose that a child received is far greater than the adult dose.

10. Effects of Amount on Dose
Increasing the amount of chemical for the same size of organism
This graphic shows what happens to the dose if we keep the size of an organism (for example, a person) constant but we increase the amount of chemical that the organism is exposed to.
In this example, the test tubes of water represent an organism (each test tube is the same size and is filled with the same amount of water). If we increase the amount of chemical in each test tube, the dose will increase.
Dose increases

11. Effects of Size on Dose
A smaller size of organism with the same amount of chemical
This graphic shows what happens to the dose if we keep the amount of chemical constant, but we decrease the size of the organism.
The test tubes of water represent an organism. The tube on the left is larger (it has more water) that the tube on the right. If we add the same amount of chemical to each tube, the tube on the right will have a increased dose - more of the chemical for the size of the organism.
Dose increases

12. ? What factors determine the dose of a toxicant that causes harm?
Not all toxicants are harmful at the same dose. There are a number of factors that determine what the harmful dose is of a specific toxicant.

13. What factors determine the dose of a toxicant that causes harm?
The concentration of the toxicant
The chemical properties of the toxicant
The number of times of exposure (frequency)
The length of time of exposure (duration)
How it gets into the body (exposure pathway)

14. Response: an abnormal change in an organism
Depending on the toxicant, dose, and route
of exposure, the response can be:
local (effects part of the organism) or systemic (effects the whole organism)
reversible or irreversible
immediate or delayed
Chemicals can have local effects, such as damaging the skin, but may also be absorbed into the body and affect other, distant parts of the body (systemic effects).

15. Dose-Response Relationship: As the dose increases, the percent of individuals who respond increases
100 75
% of Individuals Responding 50
The dose-response relationship is a fundamental and essential concept in toxicology.  Generally, the higher the dose, the more severe the response.
A graph can be made with the dose along the bottom (horizontal, X-axis) and response up the side (vertical, Y-axis) to create a dose-response curve.
For example, if you wanted to graph the toxic effects of a certain type of chemical that causes liver damage in mice. The dose would be milligrams of chemical per kilogram of body weight (mg/kg) and the response could be the percentage of animals that suffered a certain degree of liver damage.
The dose-response curve normally takes the form of a sigmoid curve.  As the dose of the toxicant is increased, the percent of individuals who respond (show ill effects) will increase. 25 10 20 30 40 50 60 70 80 90
100
Dose (mg/kg body weight)

16. Dose-Response Relationship: As the dose increases, the percent of individuals who respond increases
All individuals respond at a dose of 100 mg/kg
100 75
% of Individuals Responding 50
Half of individuals respond at a dose of 43 mg/kg
For this example of the response to a hypothetical toxicant, half (50%) of the individuals in a population will respond to the toxicant at a dose of 43 milligrams per kilogram. All of the individuals (100%) will respond to a dose of 100 mg per kilogram. 25 10 20 30 40 50 60 70 80 90
100
Dose (mg/kg body weight)

17. Glasses of Wine: Dose-Response
100 75
% of people who have difficulty walking 50
Half of people have difficulty walking after 4.5 glasses of wine
This graph shows an example of the dose-response curve of the effects of wine on the ability to walk.
The “dose” (X-axis) is the number of glasses of wine. The “response” (Y-axis) is difficulty in walking.
As seen in this graph, half (50%) of people will have difficulty walking if they drink about 4 ½ glasses of wine. 25 1 2 3 4 5 6 7 8
Glasses of Wine

18. Glasses of Wine: Dose-Response
100 75
% of people who have difficulty walking 50
Half of people have difficulty walking after 4.5 glasses of wine
Why don’t all people have difficulty walking after drinking 4 ½ glasses of wine? 25
Why don’t all people respond the same? 1 2 3 4 5 6 7 8
Glasses of Wine

19. Different individuals can show a greater or lesser response to the same toxicant
What factors can cause a difference in response? ?
Different individuals have more or less response to the same amount of toxicant.

20. Different individuals can show a greater or lesser response to the same toxicant
What factors can cause a difference in response?
Age - young or old
Gender - male or female
Genetic differences – different genes
Nutrition
Health – previous or current diseases
Exposure to other toxicants – previous or current
Different people can respond very differently to the same dose of a chemical. This variation is due to many factors, such as age, gender, and genetics. People can also be more at risk from toxic chemicals if they are taking certain drugs, if they have certain diseases, and if they are exposed to mixtures of chemicals.
Very young and very old people are usually more susceptible to toxic chemicals than the rest of the population. Young children can absorb relatively large doses of chemicals, and the systems break down and get rid of chemicals have not yet matured, and parts of their body, such as their brains, are more easily damaged. Older people are more susceptible because their bodies may stored up larger amounts of certain chemicals. Also older people may have other diseases which could put them at greater risk from toxic chemicals.
Men and women may differ in their response to chemicals in other ways. Women usually have lower body weight than men, and they will be more strongly affected by the same dose of chemical. Different genetic make-up and different proportions of body fat may also have an influence. For example, as women have a higher proportion of body fat than men, they are more prone than men to retain fat soluble chemicals such as pesticides.

21. The dose of toxicant which is deadly to 50% of the population
LD50
The dose of toxicant which is deadly to 50% of the population
Toxicologist often measure the toxicity of a substance in terms of the dose (or amount) of the substance that it takes to kill an organism (such as a lab mouse). the term LD50 (or lethal dose 50) refers to the dose of a toxicant that will kill 50% of the population – or half of the mice, in the case of this graphic.
LD50 is not the lethal dose for all individuals; some may be killed by much less, while others survive doses far higher than the LD50.

22. Which has the highest LD50? Which has the lowest LD50?
Toxicant LD50 (mg/kg)
Ethyl alcohol
Salt (sodium chloride)
Iron (Ferrous sulfate)
Morphine
Mothballs (paradichlorobenzene)
Aspirin
DDT
Cyanide
Nicotine
Black Widow Spider venom
Rattle Snake venom
Tetrodotoxin (from fish)
Dioxin (TCDD)
Botulinum Toxin

23. Which has the highest LD50? Which has the lowest LD50?
Toxicant LD50 (mg/kg)
Ethyl alcohol 10,000
Salt (sodium chloride) 4,000
Iron (Ferrous sulfate) 1,500
Morphine
Mothballs (paradichlorobenzene) 500
Aspirin
DDT
Cyanide 10
Nicotine 1
Black Widow Spider venom 0.55
Rattle Snake venom
Tetrodotoxin (from fish) 0.01
Dioxin (TCDD)
Botulinum Toxin
Notice that Botulinum Toxin (BOTOX) has the lowest LD50 – it is the most toxic of these substances.

24. Frequency of Exposure Number of times of exposure
(Number of glasses of wine)
Time in between exposure
(Time between each glass of wine)
The response to a toxicant can also be effected by the frequency of exposure to that substance.
Frequency can be determined by the number of times a person is exposed (for example the number of glasses of wine that are consumed) or by the time between exposure (for example the number of minutes a person waits between drinking each glass of wine). or or

25. Duration of Exposure: How long the exposure lasted
Acute < 24hr high dose
Subacute 1 month repeated exposures
Subchronic 1-3months repeated low dose
Chronic > 3months repeated low dose
The amount of toxicant can build up in body over time and:
Can move to different organs (example - lead)
Can overwhelm the bodies’ ability to repair damage and remove the toxicant (example - radiation)
The response to a toxicant can also be effected by the duration of the exposure.
Duration of exposure can be acute, subchronic, or chronic.
Acute exposure is once or twice in a short period of time, such as a week or less.
Chronic exposure is long-term or lifetime exposure. Subchronic exposure is somewhere in between acute and chronic
Certain toxicants can build up in the body over time – for example lead, mercury and fat soluble chemicals such as dioxins and PCBs – and cause damage to the nervous system and other organs.

26. Routes of Exposure Skin (dermal) Oral (gut) Lung (inhalation)
The route of exposure refers to how the chemical gets into the body. For humans, chemicals enter the body by three main routes: through the lungs (inhalation); through the skin (dermal absorption); and by being swallowed (ingestion). They can also enter through the eyes or by injection.
Inhalation exposure can be acute, for example breathing a chemical during short-term use, or chronic, for example longer-term inhalation of chemicals in an indoor environment .
Oral exposure can be direct (eating or drinking) or indirect such as from hand to mouth contact after touching a chemical. It can also be either acute or chronic .
Dermal exposure is usually short-term from splashing or spilling the chemical during use or from contact with treated surfaces. It can result in damage to the skin or absorption through the skin into the body. Dermal exposure can also be chronic if it occurs repeatedly over a long period of time.
A minor route of exposure is ocular (through the eye). Ocular exposure is also usually short term and results from splashing or spilling the chemical during use or from rubbing the eye with contaminated hands after touching treated surfaces.
Injection

27. Routes of Exposure Skin (dermal) Oral (gut) Lung (inhalation)
Which is the worst?
Oral (gut)
Lung (inhalation)
The job of the lungs is to exchange gases, allowing oxygen to be absorbed into the blood stream from the air we breathe, and get rid of waste carbon dioxide from the body. In the same way, the lungs will readily absorb other chemicals found in air.
Some chemicals can penetrate the skin and enter the blood stream. Whether or not a chemical is absorbed through the skin depends on its structure: chemicals need to be able to dissolve in both water and fat (lipids) to get through the skin. Those that are insoluble, or dissolve only in fats or water, and chemicals made up of very large molecules, tend not to penetrate the skin. Chemicals are more easily absorbed where the skin is thin, such as on the forearms, than through the thick skin covering the palms of the hands and soles of the feet. Chemicals are also more easily absorbed if skin is moist or damaged.
Chemicals can also enter the body if swallowed. In the workplace, this can occur if areas used to eat, drink or smoke are contaminated with chemicals, or if workers do not wash their hands or remove their gloves before eating or smoking.
Injection

28. Distribution: Where the toxicant accumulates in the body
Fat soluble
Water soluble
Bone
Muscle
Once absorbed into the blood, the chemical is carried around the body in the blood stream, and where it ends up is influenced by its structure and properties. However, some barriers exist in the body which can keep out some (but not other) chemicals, such as the blood-brain barrier which helps protect the brain, and the placenta which helps protect a fetus.
Inside the body, some chemicals are stored in certain tissues, such as fat or bone, and while they remain bound up there, they may do little damage. However, under certain conditions such as rapid weight loss, large amounts of the chemical are released into the blood. How long such chemicals remain in the body varies, but some, like the pesticide DDT, remain for years. One of the reasons why DDT stopped being used in the developed world was because of this persistence in the environment, and even though it has not been used for years in the developed world, most of us have DDT in our bodies.

29. Not all organs are affected equally by a toxicant
Target organs:
higher concentration of toxicant
→ more adverse effects
Liver
Kidney
Lung
Neurons
Heart muscle
Bone marrow
Intestines
Sperm/eggs
When talking about toxicity, the easiest way to divide chemicals is by the organ or system they damage. These target organs include: the lungs, the skin, the gut, the liver, the kidneys, the nervous system, the blood, the cardiovascular system, the immune system, and the reproductive system. There are even chemicals which can affect hearing.
Chemicals causing liver damage are called 'hepatotoxins', those which damage the kidneys 'renal toxins', and those harming the nervous system 'neurotoxins'. Chemicals that cause cancer, although they may affect either one or several organs, are lumped together and described as 'carcinogens'. Those that cause birth defects are called 'teratogens'.
Some chemicals are known to make men less fertile by affecting the production of sperm. In women, exposure to certain chemicals can cause irregular periods, increased rates of miscarriage, or premature or low birth weight babies, or deformed babies.

30. Target Organs: Mechanisms of Action
Adverse effects can occur at the level of the:
Molecule
Cell
Organ
Organism
Toxicant can interact with:
Proteins
Lipids
DNA

31. Metabolism of Toxicants
How the body breaks down a toxicant
Using enzymes in the body
What the toxicant turns into
Water-soluble toxicants are easier to excrete
How fast does this occur
Can take hours, days, weeks or years
If chemicals are not stored, the body deals with them by metabolizing (changing their structure) and excreting them. This occurs mainly in the liver, but also the skin, lungs, gut and kidneys, by similar processes used by our bodies to metabolize the chemicals which make up our food. The products of metabolism are known as metabolites, and these can be more or less toxic than the original chemical. In fact, many of the adverse effects of chemical exposure are due to the effects of metabolites.
The pathways involved in metabolizing chemicals vary greatly between species, and also between individuals, which explains why some people are harmed by very low levels of chemicals that others seem able to tolerate.
Chemicals and their metabolites are excreted from the body, mainly through urine produced by the kidneys. Small amounts are also excreted by the lungs, and in sweat, semen, milk, saliva and bile. The amount of a chemical a person has been exposed to can sometimes be estimated by measuring how much of the chemical, or certain metabolites, is found in urine. This is known as biological monitoring.

32. Half-life: How long it takes for ½ to go away14 12 10 8
Concentration of toxicant in blood (microgram/ml) 6
Chemicals can be degraded and eliminated by the body or may be stored in tissues, depending on the biochemical properties of the chemical.
The 'Half-life' of a toxicant refers to the amount of time it takes for the body to eliminate half of the amount of the toxicant. Half-life is used as a measure of the lifespan of a chemical or toxic compound.
The half-life of a chemical measures how much of the chemical is still available in the body. Some chemicals are eliminated quickly, and have short half-lives. Other chemicals accumulate in the body, and are eliminated very slowly. These chemicals have long half-lives (they are often called “persistent chemicals”). Chemicals that have long half-lives and that accumulate in the body can cause more disease than chemicals which are rapidly degraded and eliminated before producing disease. 4 2 1 2 3 4 5 6 7 8 9 10 11
Time (hours)

33. Half-life: How long it takes for ½ to go away14 12 10 8
Concentration of toxicant in blood (microgram/ml)
Half life is 4 hours 6 4 2 1 2 3 4 5 6 7 8 9 10 11
Time (hours)

34. Risk Assessment
Risk: The probability or likelihood that exposure to a particular toxicant at a specific concentration or dose may cause an adverse effect.
Risk Assessment: The process used to estimate the likelihood that humans will be adversely affected by a chemical or physical agent under a specific set of conditions.
Risk assessment is the characterization of the potential adverse health effects of human exposures to a toxicant.
The toxicity of a substance is difficult to determine. In general, the toxicity of a chemical is determined through animal studies which are extrapolated to humans. The extrapolated values are calculated for the most sensitive humans (for example, children) because their immune systems are much weaker than the average healthy middle-aged person. Children are more susceptible to have an adverse effect through exposure to a hazardous chemical.
oxicology data helps estimate the risk of exposure to certain chemicals.

35. Risk Assessment
An estimate of the likelihood that exposure to a toxicant may cause harm
Toxicity Assessment
Exposure Assessment
Chemicals are tested for toxicity and the results of these tests are used to decide how chemicals are used, controlled, labeled, and regulated in the workplace.
Toxicity studies can be divided into laboratory studies using live animals (in vivo studies) or groups of cells (in vitro studies), and studies of human populations (epidemiological studies).
Animal studies are used to test chemicals for their acute and chronic toxicity by various routes of exposure. The standard way of measuring a chemical's acute toxicity is to feed it at a range of single doses to groups of laboratory animals, such as rats. The dose that kills 50% of the group (the LD50), is then recorded as well as the effects noticed in the animals.
The effects of chronic exposure are also tested in animals. In these tests, the animals are fed, inhale, or have the chemical painted on their skins throughout their lives. The type and amount of disease they develop are then compared with the effects in a control group. The controls should be the same as the exposed group in every respect except the chemical exposure. They should, for example, be the same strain of the same species, and be fed the same diet. The differences in rates of various diseases are then tested by various statistical means to see if they are significant. If a study is badly designed, its results will not be reliable.
For example, when testing chemicals for their cancer-causing effect on animals, the US National Cancer Institute required that the chemical should be tested at two doses in both sexes of two species of rodent, and each group should contain at least 50 animals. Trying to predict what will happen in humans is one of the major problems of animal testing.
Risk Assessment

36. Toxicity Assessment Toxicity testing:
Determines the hazard which a substance may present to humans
Exposure limits are established
If exposure to the substance is kept below the exposure limit, the risk from the substance is considered to be acceptable.
Risk assessment is the characterization of the potential adverse health effects of human exposures to a toxicant.
The toxicity of a substance is difficult to determine. In general, the toxicity of a chemical is determined through animal studies which are extrapolated to humans. The extrapolated values are calculated for the most sensitive humans (for example, children) because their immune systems are much weaker than the average healthy middle-aged person. Children are more susceptible to have an adverse effect through exposure to a hazardous chemical.
oxicology data helps estimate the risk of exposure to certain chemicals.

37. Exposure Assessment
Must evaluate potential for exposure to a substance:
Where do you encounter it?
How often will you encounter it?
How might it enter the body?
How long does it remain in the body?
Risk assessment is the characterization of the potential adverse health effects of human exposures to a toxicant.
The toxicity of a substance is difficult to determine. In general, the toxicity of a chemical is determined through animal studies which are extrapolated to humans. The extrapolated values are calculated for the most sensitive humans (for example, children) because their immune systems are much weaker than the average healthy middle-aged person. Children are more susceptible to have an adverse effect through exposure to a hazardous chemical.
oxicology data helps estimate the risk of exposure to certain chemicals.

38. Risk Assessment
Must take into account the possible harmful effects of the toxicant on many individual people
Risk assessment is the characterization of the potential adverse health effects of human exposures to a toxicant.
The toxicity of a substance is difficult to determine. In general, the toxicity of a chemical is determined through animal studies which are extrapolated to humans. The extrapolated values are calculated for the most sensitive humans (for example, children) because their immune systems are much weaker than the average healthy middle-aged person. Children are more susceptible to have an adverse effect through exposure to a hazardous chemical.
oxicology data helps estimate the risk of exposure to certain chemicals.

39. Risk is only part of the picture
Risks are only half of the story.

40. Choices
As part of our society, you must make decisions which assess risks, benefits, and potential trade-offs.
Thalidomide: Leprosy treatment vs. birth defects
Pesticides: Mosquito abatement vs. toxicity
Sunlight: Vitamin D and skin cancer
Each of us makes decisions at the voting booth, in what we purchase, in everything we do.

41. Tradeoffs
Plan to reduce risks to take advantage of the benefits offered by use of a particular ‘product.’
Sunlight: Vitamin D and skin cancer

42. Precautionary Principle
If the consequences of an action are unknown, but judged to have some potential for negative consequences, then it is better to avoid that action.
“Better safe than sorry.”