Evolutionary Health
Co-Evolution of Disease & Living Conditions
Health Effects
What is Risk?
Environmental Risk
Risk Assessment
Risk Abatement
Risk Perception
Risk Management
Uncertainty & Other Features of Risk Assessment
Precautionary Principle
Appendix 1: Contaminants
Appendix 2: Environmnet & Reproductive Health
Internet Links
Other Resources
Health & Risk System PDF
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Risk is the measure of the effect of an event, such as that of an effect taking place at a given level of exposure. Although the term "risk" is generally associated with negative outcomes, that is not always the case. An example of a risk with a possible positive or negative outcome is the risk you take when buying a lottery ticket. With a very small initial investment, you risk a (large) gain or (small) loss. The University of Virginia Center for Risk Management of Engineering Systems has a nice introduction to the topic (

A risk situation basically includes two components:

  • exposure processes - how exposure occurs and to what degree
  • effects processes - possible changes and how they may occur as a result of that exposure

In the lottery example given above, the ticket is the exposure. The more tickets you purchase, the higher your exposure and risk. The effects processes include winning a large amount of money, or losing your investment (money for purchase of tickets).

We talk about exposure and effect in terms of whether they are definite or occur by chance. Any given risk situation has four possible combinations of effect and exposure:

  1. definite exposure and definite effect
  2. definite exposure and chance effect
  3. chance exposure and definite effect
  4. chance exposure and chance effect

In formal discussions of risk, the word chance is replaced by "probabilistic," and the words "chosen" or "definite" are replaced by "deterministic."
Which combination of exposure and effect is most appropriate for the lottery example? Since people who play lottery choose to purchase their tickets, exposure is deterministic. Since lottery numbers are drawn randomly (or tickets distributed randomly), the effect is probabilistic.

Determine whether the four following situations have deterministic or probabilistic exposures and effects. Click the link for the answers.

Routine dental X-ray Exposure?
Being on top of a large gas-main when an explosion occurs Exposure?
Spending December on an arctic ice-floe in a tennis outfit Exposure?
Getting stung by a bee Exposure?
(Exercise adapted from M.Granger Morgan's "Probing the Question of Technology-
Induced Risk," IEEE Spectrum, vol. 18, no. 11, pp. 58-64. November 1981.)


An example that highlights these two components is the risk of getting lung cancer from smoking. What is the change - a change in state of health i.e., getting cancer. What is the chance? Not everyone who smokes gets cancer since your health also depends on genetics and several unknown variables. However, you can look at the percentage of smokers who got lung cancer over a period of time to come up with a probability of a smoker's chances of getting cancer.

Risk is formally defined in terms of the chance of exposure and the magnitude of effect:

risk R (from an agent)= (probability of exposure P)*(the severity of the effect or consequence C)

Note that while we often talk of risk as chance in everyday speech, the formal definition of risk is a combination of chance and consequence. Situations of exposure can be thought of in terms of P and C. Failure of technologies is a big source of risk in modern life. For example, when we consider a technology (anything ranging from a simple light bulb to an airplane or a power plant), the risk of using the technology is composed of the probability of its failure causing an “exposure” and the consequences of that exposure. Technologies are often designed to reduce both P and C as much as possible. But some technologies have severe consequences when they fail. So great attention is paid in the design and implementation of the technology to minimize P, the probability of failure (which causes the “exposure”). But if it fails, the consequences are sever. The space shuttle is an example.

The failure of complex technologies such as the shuttle or an airplane is catastrophic and the effect is quickly manifest. Such catastrophic events include exposure (usually accidental, but sometimes intentional in unusual situations such as war) that is acute. The effects are also acute and more visible, and therefore the cause-effect relationship is obvious. In the case of many environmental exposures, the “failure” leading to exposure (as well as the consequences) happens over a period of time and so the risk is hard to calculate. An example is air pollution or smoking leading to lung disease such as cancer. In this case, both the exposure and effect are chronic and probabilistic. The cause-effect relationship is hard to establish because other exposures may be occurring over the same time.

In general, we may think of technological risk as having high (H) or low (L) P and C. Thus the following matrix classifies technology in terms of P and C.


Examples of all but HP-HC technologies are easy to find. A HP-HC technology never becomes widespread because engineering design works to minimize both P and C and manages to lower both, or at least one. Thus technologies that are HP-HC early in the evolution are redesigned to reduce the risk of failure.



Name a technology that would exemplify each of the squares. Can you think of a HP-HC technology?

2. Think of two potential technology failures that pose a personal risk to you. Classify them as low or high P and low or high C.

The entire risk process in society may be represented in a diagram similar to Figure 2.


Figure 4: Risk process in society.



Select an activity you engage in routinely that results in a health effect, positive or negative. Draw the risk process diagram.

Which of the steps are definite (deterministic) and which of the steps are probabilistic? How can you affect (change) the risk involved.




  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.