System Safety and the Assessment of Risk

The idea, concept, or process of system safety has been defined in many ways, by a wide variety of scientific and technical professionals. However, since its inception. 

This problem is further complicated by the fact that attempts to eliminate risk result instead in the often unfortunate displacement of risk (Malasky 1982). For example, some approved (by the US Food and Drug Administration) preservatives currently utilized in the food processing industry to prevent bacteria growth and spoilage are, themselves, a suspected cause of cancer (e.g., sodium nitrates). Likewise, there is a risk trade-off between the known benefits of improved medical diagnosis and treatment which result from the use of radiation (e.g.. X-rays, radiation therapy), against the known risks of human exposure to radiation. Hence, safety is really more of a relative issue in that nothing is completely safe under all circumstances or all conditions. There is always some example in which a relatively safe material or 

Unfortunately, the question How safe is safe enough has no simple answer. For example, it is not uncommon to hear the term 99.9% risk-free used to signify high assurance or low-risk assessments, especially in the advertising industry. In fact, it would be safe to say that this terminology is somewhat overused in our society. However, consider the following statistical facts (Larson and Hann 1990). 

In the United States today, 99.9% safe would mean  

Clearly, a 99.9% assurance level is not really safe enough in today s society. If the percentage were increase by a factor of ten to 99.99%, the following information indicates that this level of risk is still unacceptable in certain instances. A 99.99% risk-free assurance level would mean  

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