Safety factor definition

Traditional definitions of safety factors in terms of strength requirements, such as load-resistance factors or allowable stresses, are not applicable in blast resistant design. Safety factors arc more appropriately measured in terms of strain energy demand versus strain energy absorption capacity, Allowable deformations arc a practical method to quantify energy absorption capacity. 

TABLE I. DEFINITIONS OF TOLERANCES AND TOXICITY TESTS FOR REQUESTED TOLERANCE WITH SAFETY FACTORS... 

The 1997 consultation addressed the topic of safety factors, which is vitally important for die protection of public health. Setting MRLs is in fact based on a series of assumptions. One assumption is that humans are at least as sensitive as the most sensitive laboratory animal to a potentially toxic residue. Another assumption is diat all the residues covered by the MRLs are as toxic as the parent substance. A third assumption is that residues free from the human gastrointestinal tract are all totally bioavailable. A fourth assumption is the safety factor used to infer an ADI from a NOEL, including the additional safety factor, generally with a value of 2, to establish a provisional ADI until further information is available to convert this into a definite ADI. Other assumptions are the overestimation of consumer exposure to drug residues and the reduction of MRL values to take account of normal conditions under which the veterinary drugs are administered. 

Although the time to reach the IDLH levels of pyridine, -hexane, and methyl acrylate are substantially longer than bromine, the time to reach the TLV of all of these chemicals in the unventilated laboratory is only a few minutes. For each of these chemicals, the concentration in the laboratory could be kept below the TLV with reasonable air flowrates, even when increased by an appropriate safety factor. A spill or routine repeated use of these chemicals in an unventilated laboratory would definitely result in a significant health risk and should be avoided. If a large safety factor is necessary, requiring air flowrates that are impractical, other measures to reduce the concentration in the air, such as localized hoods, should be considered. 

The concept of the safety factor has caused some difficulty. In its simplest terms it may be thought of as the amount by which the capillary pressure has to be raised before oedema, i.e. swelling of the tissue spaces, occurs. This is a carefully constructed definition note it says nothing about the flow of fluid across the capillary wall - this may be substantially increased but as long as no swelling, i.e. no accumulation of water, in tissue spaces occurs, then there is no oedema. 

Application of this 1/100 safety factor to the NOAEL (1 ng/kg/day or 1000 pg/kg/day) defined in the lifetime rat study (30) that is considered most definitive by essentially all of the regulatory agencies was the basis for the Ontario MOE expert panel to recommend a Maximum Daily Intake for 2,3,7,8-TCDD (or its toxic equivalent of other chlorinated dioxins and furans) of 10 pg/kg B.W./day for humans (38). This more recent approach used by these four non-U.S. groups demonstrate the manner in which carcinogenicits based lifetime exposure control limit recommendations for humans can be realistically derived from the data available from the animal cancer bioassays when interpreted in concert with the data available regarding the likely mechanism of action by which the carcinogenic response occurred in the animal bioassays. 

The second and preferred method is to apply appropriate statistical analysis to the dataset, based on linear regression. Both EU and USFDA authorities assume log-linear decline of residue concentrations and apply least-squares regression to derive the fitted depletion line. Then the one-sided upper tolerance limit (95% in EU and 99% in USA) with a 95% confidence level is computed. The WhT is the time when this upper one-sided 95% tolerance limit for the residue is below the MRL with 95% confidence. In other words, this definition of the WhT says that at least 95% of the population in EU (or 99% in USA) is covered in an average of 95% of cases. It should be stressed that the nominal statistical risk that is fixed by regulatory authorities should be viewed as a statistical protection of farmers who actually observe the WhT and not a supplementary safety factor to protect the consumer even if consumers indirectly benefit from this rather conservative statistical approach. 

From the definition of t, it is seen that the value is almost equal to the maximum/threshold potential ratio, i.e., to the safety factor. Normally, the maximum potential is several times higher than the threshold potential (it may be 4 to 5 times as great). Therefore, if W is not too small the exponentially small terms may be neglected to give ... 

Such rules, or safety factors , should therefore only be used as a guide and must not be used as a definite basis of safety unless previous experience has shown that they are valid for the type of reaction being studied. 

For the landslides consists of loose unconsolidated sediment and are without definite sliding surface, circular slip method should be employed to search for the minimum safety factor. 

While mining continues, compared with the results of the landslide simulation, the minimum slope safety factors without the landslide simulation (black line) are significantly small. A landslide will definitely occur on a slope during an underground mining process. Therefore, to calculate slope safety factor, the approach using the landslide simulation should be used because it matches better to reality compared with that without the landslide simulation. 

Implicit in the definition of strucural bonding is that the adhesively bonded structure have good environmental resistance under service conditions. For most applications, therefore, the main consideration is that the adhesive be able to withstand design loads under service conditions for the planned lifetime of the structure with a reasonable safety factor. These topics will be discussed in greater detail in Chapters 8 and 9. 

Health and Safety Factors. The combustion products are toxic. The OSHA website www.osha-slc.gov/ChemSamp data/CH 275620.html provides the following information and full definitions of each classification code for environmental exposure and health factors as of year-end 2000. 

In the event of a complete break in the downstream hose or pipe, the force of the fluid flow, or the high difference in pressure between the fluid container and downstream piping, will cause the excess flow valve to close, as illustrated in Fig. 7-20(b). This action not only saves the loss of fluid but is a definite safety factor if the fluid is flammable. 

The reference dose (RfD) assumes there is a threshold of exposure below which a chemical does not produce a toxic effect because the body is able to detoxify and/or eliminate it. The reference dose is derived either from a no observed adverse effect level (NOAEL) or from a benchmark dose (BMD) determined in an animal toxicity study. The NOAEL or BMD is divided by at least two uncertainty factors or safety factors a factor of 10 to account for the uncertainty involved in extrapolating from animals to humans, and a second factor of 10 to account for variation in human sensitivity. If the animal toxicity data supporting the NOAEL or BMD are not definitive, a third safety factor of 10 is included. Thus, the RfD is set equal to the NOAEL or BMD divided by 100 alternatively, it is set equal to a number approximating the NOAEL or BMD divided by 1,000. The acceptable daily intake (ADI) is the same as the reference dose. The reference concentration (RfG) refers to the concentration of a pollutant in the air. It differs from the... 

Safety factors must definitely have an impact on overt behavior. For example, goals and purposes of an industrial organization may influence behavior, even under dangerous circumstances but what real effects this variable may have can hardly be determined. However, the effects of an intervention such as conducting a first aid course can be observed (Miller Agnew, 1973 Hale, 1983). 

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