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The woman in this photo may not know she risks harming her unborn baby’s health by smoking while pregnant. Or she understands that, but she is taking that risk because she is addicted to smoking cigarettes.  Risk assessments are applied to many things we encounter in our daily lives including life-saving drugs, food additives, cosmetics, household cleaning products, pesticide residues in foods, and countless other environmental agents.   This Tox Topic briefly explains some basics of human health risk assessment.   TEF’s “Is It Safe? Evaluating Chemical Risk” primer also offers tips on how an individual can undertake their own informal assessment of risk using the principles explained in “RITE”: Risk Is dependent on Toxicity and Exposure.  This discussion focuses on chemicals, although human health risk assessments can be applied to any agent–for example even physical hazards such as handling a sharp object or certain forms of radiation.

What is a human health risk assessment?

Risk assessment is an approach used to help understand the human health effects an agent can have.  Just because something can cause cancer or some other effect doesn’t mean it will.  Keep in mind that risk is the probability or likelihood that something will happen, which is a different concept from the possibility that it will happen.  We can’t emphasize enough the importance of understanding this difference between the terms ‘hazard’ and ‘risk’: Hazard describes the potential to cause harm while risk describes the probability of causing harm under defined circumstances.

People may be exposed to chemicals in different ways; for example, through the air we breathe, through what we consume by mouth and through skin contact (see Basics of Dose and Exposure).  A health risk assessment requires the evaluation of toxicity, or hazard of an agent and the exposure to the agent.


Many risk assessments are done to establish a safe level of exposure or to estimate how likely it is that a group of people will be harmed by exposure.

One of the US EPA definitions of risk assessment is the “qualitative or quantitative evaluation of the risk posed to human health and/or the environment by the actual or potential presence and/or use of specific pollutants.”  Although risk assessments are also conducted on agents that are not pollutants, we offer this particular definition from the US EPA because it introduces the idea of risks to the environment as well as to human health.  Evaluating risks to the environment is commonly referred to as ecological risk assessment.  Our focus here is on human health risk assessments (see more details describing the two at

Any chemical can have a risk assessment done on it to determine its safety.  Take water, for example.  Water is a chemical (H2O).  A risk assessment for water might ask the following: How many glasses of water are safe to drink in a day, every day for a lifetime? 6? 16? 24? And does it depend on your health, how active you are and other factors? (See Water Intoxication).  Most risk assessors spend a lot of time thinking about the different factors the answer depends on and on how different individuals may react differently simply due to their genetics or their lifestyle.

How are risk assessments conducted?

Risk assessments can be very simple or very complex.  Risk assessors consider many things when evaluating risks, and yet there are just four basic steps to every risk assessment:

• hazard identification (what health effects does the chemical have?)
• dose response assessment (at what levels are these harmful health effects observed?)
• exposure assessment (how much are people exposed to?), and
• risk characterization (what is the risk? which takes into consideration information on both exposure and dose-response)Slide1

The four basic steps depicted in the simple diagram above are explained in more detail in this US EPA link ( and in other resources offered below for anyone wanting much more in depth coverage of this topic.

If information for a chemical is considered sufficiently detailed and reliable, then determining a safe level begins.  This involves identifying the critical effect (i.e., the most sensitive effect) or the effect that occurs at the lowest level as part of dose response assessment.  Then, combining that with typical exposure levels (i.e., exposure assessment), extrapolation methods are used to derive a safe level for human exposure.

In setting a safe level of exposure, all of the reliable, high quality information describing the biological effects of the chemical is evaluated, noting the doses or concentrations at which harmful effects occur.  Any information that is lacking is also noted; for example if there is no information on whether a chemical could cause harm to the unborn child, or could be transferred to a newborn child in breast milk.  These data gaps are taken into account when deriving safe exposure levels.

Where does the information for risk assessments come from?

Risk assessments are based on the most reliable dose response and exposure information available.  In many cases there may be no information on humans clearly showing the harmful effects associated with exposure to a chemical.  This is because it is unethical to intentionally expose people to chemicals in order to determine the harmful effects that may be observed.  Also, in cases where human beings are exposed, such as unintentional exposure to an environmental chemical, the levels they experience are often unknown or low, and at levels where harmful effects may be difficult to measure.

To understand more fully what harmful effects a chemical can have, and the exposure levels at which these harmful effects occur, animal models have historically been used, and information is extrapolated to humans.  In some cases, there are no human or animal data on a chemical and a level is set using other experimentally derived information.  In these cases, some basic chemical data and data from cells are collected, and the limit is set acknowledging the uncertainty involved.  Today, modeling tools are often used to evaluate chemicals, thus decreasing the number of animal experiments that are needed.

A thorough and transparent risk assessment also explains assumptions made and where uncertainties lie.

See two health examples of health risk assessments for exposure to the inorganic form of arsenic shown in the blue box below the list of additional resources for further information.  Readers interested in knowing more about details of these risk assessments, including the assumptions made to arrive at conclusions, are encouraged to visit this — USEPA link.

Acceptable risk, risk management, and risk perception: How do these concepts relate to risk assessment?

What happens if it is impossible to completely eliminate the risk?  What is an “acceptable risk”?  Often this depends on the decision being made and the impacts of making a wrong decision.  Many different additional factors are involved in addressing this question, including who is exposed (general population? specifically children? workers?) and the population’s beliefs about what is an acceptable risk.

Risk management is all of the ways risk can be reduced.  Examples range from setting safe levels and warning the public of a potential risk to an outright ban on the use of a chemical, if warranted.

Risk perception is simply the degree of risk a person or population perceives regardless of the real risk.  Studies have been done revealing fears people tend to have that are not always supported by facts.  For example, an irrational fear of chemicals is sometimes referred to as “chemophobia.” Familiarity with and ability to control or manage the risk can sometimes reduce how a person perceives the level of risk and their fear of it, whereas the unfamiliar and the inability to control one’s exposure to the risk tend to heighten a fear even when the actual risk is low.

TEF’s main mission is to help people understand at least the basic concepts of toxicology and risk assessment and to help them learn how to apply them.  By we can all make better informed, rational decisions about choices we all make on a daily basis.  For example, some people may choose to buy organic products because they are concerned that they may be exceptionally sensitive to certain chemicals.  That’s a personal choice.  What’s important is that everyone has access to information that helps them understand how regulated consumer products like cosmetics, pool chemicals, and pesticides are evaluated to ensure that they do not pose significant risks if used as directed.  Remember, any agent that is considered hazardous does not necessarily pose a risk to you or anyone else if the exposure is low.  Always remember what the physician Paracelsus figured out as early as the 15th century:  It is the dose that makes the poison!

Resources for further information:

Definition links above are from the US EPA’s online vocabulary search tool found at
Check out that resource for other explanations of terms and acronyms that you may find helpful as you review this and our other Tox Topics.

TEF’s “Is it Safe?” Written Primer and Video
TEF’s written primer stands alone or works in conjunction with TEF’s “Is It Safe?” video. Whereas a comprehensive formal risk assessment is highly technical and involves people with a broad variety of expertise, the primer suggests how one can conduct an informal risk assessment in a non-technical way using “RITE”.

Office of Environmental Health Hazard Assessment, California EPA: A Guide to Health Risk Assessment
The purpose of this booklet is to provide a basic explanation of risk assessment for anyone interested knowing how the potential health effects of chemicals are evaluated.  Specifically written for a variety of people involved in environmental health issues, including policymakers, businesspeople, members of community groups, news reporters, and others.

The Risk Bites series is both entertaining and helpful for explaining risk and risk perception.  See in particular…

Hazard, Risk & Safety – Understanding Risk Assessment, Management and Perception
TEF finds the content of this video both useful and accurate. 

Inclusion of selected materials does not necessarily imply a general endorsement of organizations or other groups that produced them.

Related topics: Hazard vs. risk; Basics of dose and exposure



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