Risk is commonly defined as a result of multiplication of likelihood (frequency of an event happening) and undesired consequence.

In mathematical language:

Risk = Frequency X Undesired consequences.

Acceptable risk criteria

People active in process safety business are familiar with the risk triangle depiction and borders showing the intolerable region, tolerable region and broadly acceptable region (picture below):

Figure 1: Risk triangle/categories

This picture simply shows our effort to minimize the risk and more importantly shows that we can’t eliminate risk completely. Human activities are risk related; however, we need to draw a line (border) where the risk acceptable is.

One might think that the tolerable risk border line shall be not greater than normal daily life activities risk.

This means that we try to design a plant as safe as daily activities. It is very difficult if not impossible to design a plant to have such a low risk comparable to risk of daily life activities.

If we set the acceptable risk level very low, it will be difficult to reach it and moreover fulfill the required safety aspects of the design. However, as an engineer, it is our moral and legal obligations to design a safe plant, protecting people, environment as well as assets.

Are you aware of how these border’s values are defined?

Fatality Rates

A good start for defining the borders is to find out what are the day-to-day activities risks:

There are a number of sources like Frank P. Lees, Loss Prevention in the Process Industries that provides such a list.

We can divide the activities into two categories: voluntarily and involuntarily. Examples of voluntary activities are staying at home, traveling by car or bicycle or airplane or train, or sports activity such as climbing.

Examples of involuntarily activities are struck by lightening or run over by a vehicle.

Below table is a general fatality rates per activities, of course these numbers might be slightly different from source to source, based on the geographical area and the data that was gathered. Such a table shall be used to have the magnitude of fatality rates.

Voluntary activityFatality rate (deaths per person per year)
Staying at home2,63E-04
Traveling by: 
Rock climbing4,0E-05
Table 1: Voluntary activity’s risk
Involuntary activityFatality rate (deaths per person per year)
Earthquake, California2,0E-06
Struck by lightening1,0E-07
Run over by vehicle6,0E-05
Natural disasters (general)2,0E-06
All accidents5,0E-04
Table 2: Involuntary activity’s risk

Normally selected acceptable risk level

In general, the lower limit of intolerable risk per year for public is considered as (1E-4) and for workers (1E-3). The lower limit of tolerable region is considered as (1E-6).  These limits are shown in the figure 1 above.

The risk equal or greater than 1E-04 is intolerable. (Risk≥1E-4)

The risk higher than 1E-6 and less than 1E-4 is considered as tolerable. (1E-6<Risk<1E-4)

The risk less than 1E-6 is broadly acceptable region. As you can see the limit of acceptable risk or broadly acceptable risk is similar to the risk of fatalities because of natural disasters (general), one might say that it is 1/100 of fatality rate of staying at home.

A risk in tolerable region must be reduced to arrive at the tolerable region, provided that cost is reasonably low compared to risk reduction achieved (ALARP).

Suggestion for people involved in Risk assessment

As someone active in risk assessment, one needs to assess the risk without barrier and compare it to the acceptable risk level. This is done via hazard analysis methods like (LOPA). For information on LOPA see here.

Then it is always a good idea to discuss with the client and define the limits for unacceptable and acceptable risk.  After that any kind of risk assessment studies can be carried out.

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