A safety factor (also called an uncertainty factor or assessment factor) is a number by which some variable such as load or dose is multiplied or divided in order to increase safety. Safety factors are used in engineering design, toxicology, and other disciplines to avoid various types of failure.
The sources of failure that safety factors are intended to protect against can be divided into two major categories: (a) the variability of conditions that influence the risk of failure, such as variations in the strength of steel and in the sensitivity of humans to toxic substances, and (b) the uncertainty of human knowledge, including the possibility that the models used for risk assessment may be inaccurate.
Safety factors are used to obtain a safety reserve, a margin between actual conditions and those that would lead to failure. Safety reserves can also be obtained without the use of explicitly chosen safety factors.
At least since antiquity, builders have obtained safety reserves by adding extra strength to their constructions. The earliest known use of explicit safety factors in engineering dates from the 1860s. In modern engineering, safety factors are used to compensate for five types of failure:
- higher loads than those foreseen,
- worse properties of the material than foreseen,
- imperfect theory of the failure mechanism in question,
- possibly unknown failure mechanisms, and
- human error in design or calculations.
The first two of these can in general be classified as variabilities, whereas the last three belong to the category of (genuine) uncertainty.
In order to be an efficient guide for safe design, safety factors should be applied to all the integrity-threatening mechanisms that can occur. For instance, one safety factor may be required for resistance to plastic deformation and another for fatigue resistance. A safety factor is most commonly expressed as the ratio between a measure of the maximal load not leading to the specified type of failure and a corresponding measure of the applied load. In some cases it may be preferable to express the safety factor as the ratio between the estimated design life and the actual service life.
The use of explicit safety factors in regulatory toxicology dates from the middle of the twentieth century. In 1954 Arnold J. Lehman and O. Garth Fitzhugh, two U.S. Food and Drug Administration (FDA) toxicologists, proposed that ADIs (acceptable daily intakes) for food additives be obtained by dividing the lowest dose causing no harm in experimental animals (counted per kilogram body weight) by 100. This value of 100 is still widely used. It is now often accounted for as being the product of two subfactors: one factor of 10 for interspecies (animal to human) variability in response to the toxicity and another factor of 10 for intraspecies (human) variability in the same respect. Higher safety factors such as 1,000, 2,000, and even 5,000 can be used in the regulation of substances believed to induce severe toxic effects in humans.
The effect of a safety factor on the actual risk depends on the dose–response relationship. If the risk is proportionate to the dose (linear dose–response relationship), then the risk reduction will be proportionate to the safety factor. If the dose–response relationship is nonlinear, then the reduction in risk can be either more or less than proportionate. Because the dose–response relationship at very low doses is always unknown, the exact effect of using a safety factor cannot be known with certainty.
Natural organisms often have safety reserves that can be described in terms of safety factors. Structural safety factors have been calculated for mammalian bones, crab claws, shells of limpets, and tree stems. Natural safety reserves make the organism better able to survive unusual conditions. Hence, the extra strength of tree stems makes it possible for them to withstand storms even if they have been damaged by insects. But safety reserves also have their costs. 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.
At least two important lessons can learned from nature in this context. First, resistance to unusual loads is essential for survival. Second, a balance will nevertheless always have to be struck between the dangers of having too little reserve capacity and the costs of having an unused reserve capacity. Perfect safety cannot be obtained, but a chosen balance between safety and costs can be implemented with the help of safety factors and other regulation instruments.
SVEN OVE HANSSON
Dourson, Michael L., and Jerry F. Stara. (1983). "Regulatory History and Experimental Support of Uncertainty (Safety) Factors." Regulatory Toxicology and Pharmacology 3(3): 224–238. Safety factors in toxicology.
Randall, F. A. (1976). "The Safety Factor of Structures in History." Professional Safety, January: 12–28. Safety factors in engineering design.