Methods for Measuring Toxicity
Steps in Risk Assessment
1. Hazard identification 2. Dose-response assessment 3. Exposure hazard 4. Risk characterization
Dose-Response Curve to Cyanide in Mice
*G, H, and I* all have a star next to them which indicate that those responses are "statistically significantly" different from 0 ➢ At these levels there is *definitely a toxic response* ➢ *G: LOAEL* → Lowest does where you do see biological response ❖ *F* is the highest level of exposure that does *not* lead to a toxic response. no adverse effect → *NOAEL* ❖ The orange line that goes down to a point that's labeled "T" on the X-axis ➢ The threshold is set at a higher level than it would be set at if the curves where used to set threshold ➢ The threshold set this way *may be higher than the level at which no biological response is observed* ➢ This is the *least conservative way to set threshold* ▬▬ *The extrapolation overestimates the threshold and underestimates risk levels* ▬▬ This is *not* an overly cautious approach but potentially places threshold higher than it should be ▬▬ In other words, a biological response might occur below this threshold ➢ If the curved part of the lines was used to set threshold, in the case of A it would not be possible to set a threshold as the curve never goes to "0" response ➢ In the cases of B and C, clearly the threshold would be less than D ➢ The problem with using curves A, B or C is that there were no measured responses used to determine the shapes of these curves, probably only mathematical curve fitting. For this reason the extrapolation method, extending the line defined by actually measured responses is used to establish threshold
Some Toxicology Term Definitions: Threshold
Dose below which no change occurs in the biological response (zero on curve)
Some Toxicology Term Definitions: ED₅₀
Effective dose that produces a 50% response
Some Toxicology Term Definitions: ED₁₀
Effective dose, that produces a 10% response
Some Toxicology Term Definitions: LC₅₀
Lethal concentration, that produces 50% lethality
Some Toxicology Term Definitions: LOAEL
Lowest observed adverse effect level
Some Toxicology Term Definitions: NOAEL
No observed adverse effect level
Exposure Limits: LD₅₀
What exposure level causes death in half of populations
Risk Assessment: Assessment of Exposure
What types, levels and duration of exposures are experienced or anticipated?
The Extremes of LD₅₀ of Selected Chemical
fyi
Risk Assessment
❖ *Estimating the risk associated with a toxicant at an ascertained level of exposure using an accepted mathematical model* ➢ *Systematic scientific characterization* ➢ *Qualitative information* ➢ *Quantitative assessment (e.g. dose-response curves, extrapolation)* ➢ *Description of uncertainties* ▬▬ We can't know everything, so sometimes risk assessment involves making an educated guess and identifying those things that we don't know and we just have to try and approximate ❖ You think there is a risk, and you know there's some probability, but you don't know how frequently the exposure has to take place for there to be a risk, don't know at what level, so have to collect this information through risk assessment ➢ Normally involves some time of mathematical model ➢ ex. I have this plant that produces this type of chemical, what's the level in air? Do i worry if it's exposed to my skin? do i have to worry about eyes because interface between liquid surface of eyes and toxicant? ➢ ex. Hydrogen sulfide gas on skin → If not sweating isn't that big of an issue ▬▬ When this gas mixes with liquid surface of your eye → Becomes sulfuric acid ▬▬ Same thing happens when it hits mucus membrane in nose or lungs
Exposure Limits: Acceptable Daily Intake (ADI):
❖ Composite of NOAEL ➢ Through response data you determine NOAEL (no observable adverse effect), then you take other factors into consideration (ex: something taken orally, contaminant in food) ➢ Level of NOAEL is taken and then looked at in terms of daily amounts taken in with toxicological and pharmacological data; you take in amount a day, and pharmacokinetics say you remove certain amount a day, so *how much can you take without having observable adverse effects*
Dose-Response Assessment
❖ Definition: *It is a quantitative assessment between chemical exposure and adverse effects produced by that chemical* ❖ All chemicals can be toxic ➢ Difference between pharmacologist and toxicologist is dose ❖ Most have threshold doses ➢ There is a debate in toxicology → Is there a threshold or not? ➢ Carcinogenicity → Whether or not there is a threshold level for risk of exposure to carcinogens ▬▬ Some argue that it only takes 1 hit of 1 DNA molecule to cause abnormality that could be a predecessor of eventual development of unregulated cell growth & development of tumor→ these people believe there's no threshold ❖ Limited absorption and distribution ❖ Prompt elimination ❖ Metabolic detoxication ❖ Repair and regeneration ❖ Functional capacity ❖ *All of these determine what the dose-response of an organism is to a toxin* ➢ Can have toxin that is toxic in vitro to a cell/group of cells, but if these mechanisms limit the exposure of target, the actual exposure to organism and toxicity will be less than what is expected in vitro studies ➢ Can't always translate toxicity to cell to toxicity to human because all these factor come into play
Risk Management
❖ Development of regulator options ➢ Control ➢ Substitute ➢ Inform ❖ Evaluation of public health, economic, social, political context for risk management options ❖ Do you control it? Do you look for a substitute or do you simply inform? Can you abate the risk by simply informing people about how they handle something?
Risk Assessment: Hazard Identification
❖ Does the agent cause adverse effects? ➢ Structure activity analysis ➢ In vitro tests ➢ Animal bioassays ➢ Epidemiology ❖ If this is an environmental toxin, is there any evidence? Has there been harm done to anyone? ❖ Once this hazard identification has occurred, may have question that we don't have enough information ➢ Need to collect more information → research; laboratory and field exposures taken ❖ If there's concerns about production of toxins from plant → take water samples from plant to further identify hazard
Risk Assessment/Risk Management Framework: Example - Smoking
❖ Everyone smoked back in the 60s ❖ 1966: Warning: Cigarette smoking may be hazardous to your health ➢ This kinda scared people ❖ 1969: Changed → "Surgeon general says smoking causes lung cancer, heart disease, emphysema and may complicate pregnancy" ➢ This happened because this process of risk assessment, of collecting data of what the actual risk was, determining the frequency of the incident and reevaluating policy associated with that caused this to change ➢ Ex. Thalidomide→ It's been around for a long time, but when it was approved for use in pregnant women → Resulted in birth defects → Changed use because the policies change as you collect more information and have more details about the risk that lead to changes in policies ❖ 1985: Now we got regulations that say we *have to* have a label on the front and back of the cigarette pack that occupy more than 50% of the area with skulls or smoking kills ❖ 2015: UH made a smoke-free campus ➢ This happened because of the risk of secondhand smoke ▬▬ Started out with places like airplanes, restaurants, where people were exposed to higher level because of their occupation in an enclosed space, but then the risk assessment lead to the circumstances we have now ❖ This is how toxicology enters into a lot of these decisions. *Collection of data, assessment of risks, risk management, and policies to control the risk*
Risk Assessment/Risk Management Framework
❖ Keep in mind the fact that NIH and FDA go through the same process of risk assessment ➢ Should understand that same process regardless of nature of risk ➢ The same process is used for drugs ▬▬ This is a more controlled situation in that they know what dosage will be used, talk about dose limits (min and max dose can use), but all decision-making is the same ▬▬ Keep in mind that NIH and FDA when they go through decision-making process whether drug goes into market, they go through similar process as this ▬▬ Identify risk → make assessment of risk → managing risk → developing policies
Research
❖ Laboratory and field measurements of exposures ➢ Evaluation of exposed populations and observations of adverse effects ❖ New mechanistic understandings of toxicity ❖ New genomic information
Some Toxicology Term Definitions: LD₅₀
❖ Lethal dose, dose that produces 50% lethality ➢ 50% of population will be dead at this dose.
Exposure Limits: NIOSH & EPA
❖ More concerned about *environmental exposure* ❖ *IDLH*: Immediate danger to life and health ➢ Standard for assessing a risk
Exposure Limits: OSHA
❖ Occupational safety and hazard administration ➢ Their concern: what are you exposed to at *work*? ❖ *PEL*: Permissable exposure limit ➢ What is the limit given that you work 40-60 hours each week and are exposed daily
Objectives of Risk Assessment
❖ Once you've set the target levels and priority on how you're going to regulate things, you have to look at regulation at different levels ➢ Exposure to employees, environment, people, animals, etc. ➢ Once we assess all these things and estimate the risk and put in place all these plans for reducing risk, then we go back and see if our methods were effective or if they're underestimating the risk and should we be doing more to control the risk ▬▬ ex. Cigarette smoking: Primarily focused on smokers → Infants and pregnant woman → Second-hand exposure in confined spaces → Second-hand exposure in all places
Risk Management
❖ Policy Actions ❖ Evidence, risk estimates (economics, social, political)
Definitions of Risk
❖ Probability of an adverse outcome ❖ Expected frequency of undesirable effects arising from exposure to a chemical (toxicant)
Risk Assessment Framework
❖ Process is iterative; don't just do it once and stop ❖ You identify the *problem, context* ➢ Occupational hazard, environmental hazard to human/animals ❖ Look at *risks* ➢ What are the possible consequences ➢ Know quantitative/qualitative measures ❖ Now we know the risks, the quantitative and qualitative measures, we've done the risk assessments, now we look at *options* ➢ What can I do? ➢ Whether you like it or not, you can't eliminate risks so you have to find way to mitigate risks ❖ Reach series of *decisions* ❖ Look at *actions* ➢ Set certain standards, put in regulations about how people can be exposed and at what levels ❖ Once actions are put in place, you periodically *evaluate* ➢ Are we preventing risks? ➢ Are people being protected ➢ If not, do we need to go through process again and reevaluate risks now that we have more information?
How is Risk Assessed
❖ Risk is usually described in terms of: ➢ Exposure level ➢ Duration ➢ Exposure period ❖ ex. Smoking 10 cigarettes a day → Risk of death is 1/400 from 1 year of exposure ❖ In UK: Incidence of lightning strikes in golf course is much lower than in US ➢ Deals with weather → The don't have a lot of lightning storms
Hazard Identification (assessing toxicity)
❖ Structure/activity relationships ➢ Different chemicals with similar structures are expected to produce similar activity ❖ In vitro and short-term studies ➢ Outside biological system ❖ In vivo animal bioassays ➢ Initially small animals are used such as mice, rats and rabbits. If the results are encouraging then the chemical is tested in more physiologic complex animals which are more close to humans such as non-human primates ➢ There is another term used in research ex vivo system. This does not involve testing in whole animal rather animal organs are isolated and used for testing or cell lines from different organs such as heart, liver and kidney are available which are used for testing ❖ Epidemiological data ➢ Human population study data are very helpful in identifying hazard. For example, a subset of population group in a community is more prone to develop diabetes. One starts to look for risk factors, their genetic make-up and their life style. Probably they are consuming food with additives that may be harmful for β cells inhibiting insulin production.
List of Tests
❖ Typical list that came out of toxicology textbooks of the kind of tests that one would conduct if one where assessing the toxicity of a potential toxicant (an environmental toxicant) ❖ Start looking at the physical and chemical properties of the toxicant ❖ Look at the exposure and environmental fates (degraded in water, soil or by light?) ❖ How does it move in the soil? Do we have to worry about if it's gonna stay where it is, or is it gonna move into the water and be a part of the water run-off? Will it accumulate somewhere? ➢ *Mercury*: A big problem in fish because fish eat other plants and organisms that accumulate the mercury. Not just a question of they'll get mercury once, but you'll get it every time you eat so it's constantly accumulated. → They have no way to get rid of it ▬▬ Fun fact: the bigger the fish, the more the mercury → little fish are eaten by bigger fish and etc. that's why tuna has high levels ❖ *In-vivo test*: Subchronic speciality test - chemical toxicant may be known on its structure ➢ Particular sensitivity test only because of the structure of that particular toxicant ❖ *In-vitro test*: Other tests to assess mutagenicity ➢ AMES test ➢ Subculture test: Look for incidents of mutation → Potential of a chemical to cause a mutation ❖ Concerns about wildlife adverse effects
Relative Substance Toxicity
❖ Water → Essentially safe ❖ Nicotine → Supertoxic ➢ Nicotine is normally a liquid ➢ LD₅₀ of nicotine: 1-2 drops ❖ Even an ounce of aspirin is potentially toxic
Risk Assessment: Characterization of Risk
❖ What is the nature and estimated incidence of adverse effects in a given populations? ❖ How robust is the evidence? ➢ Sometimes we have isn't good ➢ Sometimes the evidence we have is circumstantial ➢ Sometimes the models that are used are not acceptable/valid ❖ How certain is the evaluation? ❖ Are susceptible populations characterized ➢ Is this something we need to worry about an age group (young vs old, too old to adapt and compensate for risk?) ❖ Is there a relevant mode of action? ❖ Environment regulation: Big issue ➢ Endangered species living in the area of potential exposure
Risk Assessment: Dose-Response Assessment
❖ What is the relationship between dose and response? ➢ Susceptibility ▬▬ Age ▬▬ Gene-environment ▬▬ ex. Flame retardants: Exposure may be important to assess in younger individuals than older individuals ❖ Other Phrasing: What is the relationship between exposure vs. effects?
