Acceptable risk levels concept

The purpose of a risk management matrix is to (a) provide a logical framework for hazard analysis and risk assessment and (b) assist risk decision makers in arriving at their risk reduction and risk acceptance or declination conclusions. The implicit goal is to achieve acceptable risk levels. Several standards and guidelines now include the concepts of residual risk and acceptable or tolerable risk (e.g., ANSl/Bll TR3, ISO/IEC Guide 51, SEMI SIO—see references for full titles). 

ALARP is a sound concept. It promotes a management review that should result in achieving acceptable risk levels. Practical economic and risk trade-offs are frequent and necessary in the benefit/cost deliberations that take place when determining whether the costs to reduce risks further can be justified by the resulting decrement in risk. ... 

The concept of As Low as Reasonably Practicable (ALARP) was recognized as a valuable tool in determining acceptable risk levels. However, a word of caution was offered On occasion, achieving risk levels as low as reasonably practicable will not be acceptable. Prior to presenting the following definition, I said that a workable and sound definition of acceptable risk must encompass hazards, risks, probability, severity, and economics ... 

This Technical Report is an excellent resource for safety professionals who want to understand how the lean process and safety principles can be melded to serve waste reduction purposes while maintaining acceptable risk levels. It provides guidance from the initial concept stage for design and redesign and addresses operational waste reduction applications. 

However, in recent years the concept of acceptable risk has been interwoven into internationally apphed standards and guidehnes for a very broad range of types of equipment, products, processes and systems. That has occurred in recognition of the fact that risk-related decisions are made constantly in real-world applications and that society benefits if those decisions achieve acceptable risk levels. In this chapter ... 

The ALARP concept (as low as reasonably practicable) is discussed with an example of how the concept is applied in achieving an acceptable risk level. 

The goals of applying prevention through design concepts are to Achieve acceptable risk levels. 

Each application has revealed new aspects that had not been considered previously (Table I). Nevertheless, the examples share one characteristic common to toxic chemical risk analysis an acceptable exposure level must be combined with a relationship between source concentration and estimated degree of exposure. This concept has been published previously(1,2,3) ... 

Health organizations throughout the world utilize a safe dose concept in the dose-response assessment of noncancer toxicity. This safe dose has often been referred to by different names, such as acceptable daily intake (ADI), tolerable daily intake (TDI) or tolerable concentration (TC), minimal risk level (MRL), reference dose (RfD), and reference concentration (RfC). The approaches used by various health organizations share many of the same underlying assumptions, judgments on critical effect, and choices of uncertainty (or safety) factors. 

Many risks people are subjected to can cause health problems or death. Precautions should be taken based on what is practical, logical, and useful. However, those involved in laws and regulations, as well as the public and, particularly, the news media, should recognize that there is Acceptable Risk. This is the concept that has developed in connection with toxic substances, food additives, air and water pollution, fire and related environmental concerns, and so on. It can be defined as a level of risk at which a seriously adverse result is highly unlikely to occur but it cannot be proven whether or not there is 100% safety. In these cases, it means living with the reasonable assurance of safety and acceptable uncertainty. This concept will always exist. Note the use of automobiles, aircrafts, boats, lawnmowers, food, medicine, water, and the air we breathe. Practically all elements around us encompass some level of uncertainty. Otherwise, life as we know it would not exist. Many products and environmental factors are not perfect and never will be perfect. 

Some risk analysts use the term as low as reasonably practical (ALARP) for setting a value for acceptable risk. The basic idea behind this concept is that risk should be reduced to a level that is as low as possible without requiring excessive investment. Boundaries of risk that are definitely acceptable or definitely not acceptable are established as shown in Figure 1.16, which is an FN curve family. Between those boundaries, a balance between risk and benefit must be established. If a facility proposes to take a high level of risk, then the resulting benefit must be very high. 

The concept of acceptable risk has been discussed in Chapter 1. Given that risk is, at its root a subjective concept, terms such as acceptable risk and reasonable risk will necessarily mean different things to different people. Engineers and scientists are used to thinking of risk in terms of 90%—99% confidence levels. However, the public and the legal community may consider a risk to be reasonable if it is merely >50%. 

Individually and collectively we are risk acceptors. And risk acceptance is situational Variations in the risk levels that individuals and organizations accept in given situations are exceptionally broad. Nevertheless, in an attempt to promote an understanding of the acceptable risk concept, this chapter does the following ... 

The concepts inherent in the terms ALARA and ALARP serve as guidelines in determining whether a risk is acceptable, but cannot be used as absolutes in decision-making. It should be understood that in an exceptional situation, even though the risk level attained is as low as reasonably achievable or practicable, a decision will be made that the residual risk is not acceptable and that the operation should not proceed. 

Determining whether a risk is acceptable requires consideration of many variables. An additional excerpt from ISO/IEC Guide 51, Section 5, helps in understanding the concept of designing and operating for risk levels as low as reasonably practicable. 

Since early nineties of the last century, PSA methods have been used as a complementary approach to common deterministic analyses within the process of ensuring acceptable level of nuclear power plant operation safety in Czech Republic. More recently, PSA concept has become a fi eestanding decision-making tool for controlling both instantaneous and permanent plant risk level. Since human factor has been playing a fairly significant role in plant risk profile, special HRA methods had to be adopted as a part of worldwide know-how transfer and used to address human related specifics of Czech NPPs operation, in order to keep the corresponding PSA models as realistic as possible. 

Therefore, the question still remains as to the proper definition of safety. One possible improvement of the previously presented MIL-STD-882 definition might be that safety is a measure of the degree of freedom from risk in any environment (Leveson 1986). Hence, safety in a given system or process is not measured as much as is the level of risk associated with the operation of that system or process. This fundamental concept of acceptable risk is the very foimdation on which system safety has developed and is practiced today. 

The quotation from the foreword of B 155.1 previously given also includes this wording The introduction of hazard identification and risk assessment as the principal method for analyzing hazards to personnel and achieving a level of acceptable risk. That language presents an interesting and significant concept. 

An FSA requires the identification and evaluation of hazards over the life of the project from the initial feasibility study through the concept design stage, to construction and commissioning, then to operation, decommissioning, and abandonment of the facility. The FSA provides the technical basis for demonstrating that a design or operation is safe. The purpose of an FSA is to identify major hazards, assess the associated risks, and to show that those risks have been reduced to an acceptable level over the life of the installation. (The FSA needs to provide criteria for acceptable risk.)... 

It has long been recognized that a zero risk level is not attainable. If a facility exists or an activity proceeds, it is not possible to realistically conceive of a situation in which there is no probability of an adverse incident or exposure occurring. William W. Lowrance was one of the most significant and influential authors on the concept of acceptable risk. Lowrence (1976) writes in his previously dted book Of Acceptable Risk Science and the Determination of Safety that ... 

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