Defining and expressing risk
Risk is all about the likelihood of a given harmful outcome. Hazards are expressed as the potential for an event, but there is no quantification of their likelihood. Risk is the combination of a hazardous event and its likelihood or probability.
Refer back to Topic 1 Subject overview, hazards and risks . Read again the sections on risk and vulnerability to remind yourself of what risk is, the various definitions and how it fits within emergency management and what constitutes vulnerability.
There are a number of ways to express risk. Some examples of the use of this definition of risk are:
- 'This industry poses a fatality risk to residences in Forth Street of 1 in a million.'
- 'The average lifetime cancer risk in this city is 4 x 10-5.'
- 'A flood of 5 metres at this point is considered to be a 1 in a 100 year event.'
- 'We expect a serious bushfire through these mountains about once every 20 years.'
Before we get into the mechanics of risk, just a quick word about probability. A probability of 1 means that something will happen in a given time frame. A probability of 0 means that it won't happen. A probability of 0.5 means that there is 50% (or 1 in 2) chance of it happening.
You will find that many of the risks we will look at in this topic are close to zero. Because they are such small numbers, a special type of notation is used - negative powers of ten.
The following table shows some examples of this notation.
Table 4.1: Some mathematical notation
| Power of ten (negative) |
Fraction | Decimal | Read as |
1 x 10 - 3 |
 |
0.001 | 'one in a thousand' |
| 1 x 10 - 6 |  |
0.000001 | 'one in a million' |
| 3 x 10 - 4 |  |
0.0003 | 'three in ten thousand' |
Much of the risk we will be examining in this topic is expressed as multiples of 10-6 (i.e. in millionths).
Expressing risk
Risk is expressed in many different ways depending on the type of risk, but one way of expressing risk is the number of things likely to be affected, per the number of things, per year, for example, '380 people per 1 000 000 people die from lung cancer each year'.
This means that if you have a population of 1 000 000 people it is likely that 380 will die of lung cancer each year.
Another way of expressing risk is to look at it from the point of view of an individual. This way of expressing risk is to say that the individual risk level is such and such a probability per year, for example 'the individual fatality risk level from lung cancer is 380 '10-6'.
Consider this figure of individual risk for a moment. Can you see pitfalls in this approach?
If you said that some people are more exposed to atmospheric carcinogens, for example smoke or dust, than others and therefore their risk of cancer is higher, you were right. This is just a figure averaged over all of the population regardless of the degree of exposure, and regardless of individual susceptibility. This is something to be wary of with individual risk level figures.