8 Safety Factors and Exposure Limits

8 Safety Factors and Exposure Limits Sven Ove Hansson 8.1 Numerical Decision Tools Numerical decision tools are abundantly employed in safety engineering. Two of the most commonly used tools are safety factors and exposure limits. A safety factor is the ratio of the maximal burden on a system not believed to cause damage to the highest allowed burden. An exposure limit is the highest allowed level of some potentially damaging exposure. 8.2 Safety Factors Humans have made use of safety reserves since prehistoric times. Builders and tool-makers have added extra strength to their constructions to be on the safe side. Nevertheless, the explicit use of safety factors in calculations seems to be of much later origin, probably the latter half of the nineteenth century. In the 1860s, the German railroad engineer A. Wohler recommended a factor of 2 for tension. In the early 1880s, the term “factor of safety” was in use, hence Rankine’s A Manual of Civil Engineering defined it as the ratio of the breaking load to the working load, and recommended different factors of safety for different materials (Randall, 1976). In structural engineering, the use of safety factors is now well established, and design criteria employing safety factors can be found in many engineering norms and standards. Most commonly, a safety factor is defined as the ratio of a measure of the maximum load not inducing failure to a corresponding measure of the load that is actually applied. In order to cover all the major integrity-threatening mechanisms that can occur, several safety factors may be needed. For instance, one safety factor may be required for resistance to plastic deformation and another for fatigue resistance. The other major application area for safety factors is toxicology. Here, the use of explicit safety factors is more recent. Apart from some precursors, it dates from the middle of the twentieth century (Dourson and Stara, 1983). The first proposal 114 S.O. Hansson for a safety factor for toxicity was Lehman’s and Fitzhugh's proposal in 1954 that an ADI (Acceptable Daily Intake) be calculated for a food additive by dividing the chronic animal NEL (maximum No Effect Level) in mg/kg of diet by 100. They thus defined a safety factor as the ratio of an experimentally determined dose to a dose to be accepted in humans in a particular regulatory context. If the NEL is 0.5 mg/kg body weight, the application of a safety factor of 100 will then result in a maximum allowed dose of 0.005 mg/kg body weight. This definition is still in use. Their value of 100 is also still widely used, but higher factors such as 1 000, 2 000, and even 5 000 are employed in the regulation of substances believed to induce severe toxic effects in humans. Compare with Figure 8.1. Toxicological safety factors are often based on products of subfactors, each of which relates to a particular “extrapolation.” The factor 100, for example, is described as composed of two factors of 10, one for the extrapolation from animals to humans and the other for the extrapolation from the average human to the most sensitive members of the human population (Weil, 1972). For ecotoxicity, factors below 100, such as 10, 20, and 50, are widely in use. Lower factors are, of course, associated with a higher degree of risk-taking. Figure 8.1. Japanese factory workers inspect packages of processed foods containing roasted peanuts imported from China at a confectionary factory in Niigata city, Japan, March 28, 2008. Following news reports of lethal chemicals found in imported Chinese food products, Japanese companies are facing tighter scrutiny from consumers and health ministry inspectors to insure the quality of their food products. (Photo: Everett Kennedy Brown/EPA/Scanpix) In addition to safety factors, the closely related concept of a safety margin is used in many applications. The essential difference is that, whereas safety factors are multiplicative, safety margins are additive. Airplanes are kept apart in the air; a Safety Factors and Exposure Limits 115 safety margin in the form of a minimum distance is required. Surgeons removing a tumor also remove the tissue closest to the tumor. Their safety margin (surgical margin) is defined as the distance between the tumor and the lesion. Typical values are 1–2 cm (Kawaguchi, 1995). The notion of a safety margin is also sometimes used in structural engineering, and it is then defined as capacity minus load. Independently of the area of application, safety factors and safety margins can be divided into three categories (Clausen et al., 2006): 1. Explicitly chosen safety factors and margins. Safety factors in this category are used, e.g., by the engineer who multiplies the foreseen load on a structure by a standard value of, say, 3 and uses this larger value in his or her construction work. Similarly, the regulatory toxicologist applies an explicitly chosen safety factor when she divides the dose believed to be harmless in animals by a previously decided factor such as 100, and uses the obtained value as a regulatory limit. Explicitly chosen safety factors are also used in, e.g., geotechnical engineering, ecotoxicology, and fusion research (for plasma containment). As already mentioned, explicitly chosen safety margins are used in air traffic and in surgery. They are also used in radiotherapy to cope with set-up errors and internal organ motion. 2. Implicit safety reserves. These are safety factors or margins that have not been specifically chosen, but can, after the fact, be described as such. They have their origin in human choice, but in choices that are not made in terms of safety factors or margins. As one example of this, occupational toxicology differs from food toxicology in that allowable doses are usually determined in a case-by-case negotiation-like process that does not involve the use of generalized (fixed) safety factors. However, it is possible to infer implicit safety factors; in other words, a regulatory decision can be shown to be the same as if certain safety factors had been used (Hansson, 1998). Another example can be found in traffic safety research. The behavior of drivers can be described as if they applied a certain safety margin to the distance between their car and the car nearest ahead. This margin is often measured as the time headway, i.e., the distance divided by the speed (Hulst et al., 1999). 3. Naturally occurring safety reserves. These are the safety reserves with regard to natural phenomena that can be calculated by comparing a structural or physiological capacity to the actually occurring load. These safety reserves have not been chosen by human beings, but are our way of describing properties that have developed through evolution. As in the case of implicit safety reserves, naturally occurring safety reserves can often be described in terms of safety factors or margins. Structural safety factors have been calculated for mammalian bones, crab claws, shells of limpets, and tree stems. Physiological safety factors have been calculated, e.g., for intestinal capacities such as glucose transport and lactose uptake, for hypoxia tolerance in insects, and for human speech recognition under conditions of speech distortion (Clausen et al., 2006). The reason why safety factors can be applied in descriptions of natural phenomena is that when we calculate loads – whether of natural or artificial origin – we do not 116 S.O. Hansson consider unusual loads. Resistance to unusual, unforeseen loads is as important for the survival of an organism as it is for the continued structural integrity of a man- made artifact. For example, the extra strength of tree stems enables them to withstand storms even if they have been damaged by insects. On the other hand, there is a limit to the evolutionary advantage of excessive safety reserves. Trees with large safety reserves are better able to resist storms but, in the competition for light reception, they may lose out to tender and high trees with smaller safety reserves. In general, the costs associated with excessive capacities result in their elimination by natural selection. There are at least two important lessons to learn from nature here. First, resistance to unusual loads that are sometimes difficult to foresee is essential for survival. Secondly, a balance must nevertheless always be struck between the danger of having too little reserve capacity and the cost of having a reserve capacity that is never or rarely used. 8.3 What Do Safety Factors Protect Against? In characterizing the sources of failure against which safety factors provide protection we need to consider the decision-theoretical distinction between risk and uncertainty. A decision is said to be made under risk if the probabilities of the relevant outcomes are known or are assumed to be known. Otherwise, it is made under uncertainty. Uncertainty comes in different forms. Sometimes it is due to a lack of reasonable probability estimates for identified outcomes. On other occasions, there may also be a considerable uncertainty about what outcomes are in fact possible (Hansson, 1996). In structural engineering, safety factors are intended to compensate for five major sources of failure: (1) higher loads than those foreseen, (2) worse properties of the material than foreseen, (3) imperfect theory of the failure mechanism in question, (4) possible unknown failure mechanisms, and (5) human error (e.g., in design) (Moses, 1997). The first two of these can possibly be described in terms of probabilities, whereas the last three concern uncertainty rather than risk (Hansson, 2007a). In toxicology, safety factors are typically presented as compensations for (1) various extrapolations such as that from animals to humans, (2) intraspecies variability, (3) lack of data, and (4) imperfection in the models used for interpreting the data (Gaylor and Kodell, 2002).

Load more