Safe level concept

The Safe Level Concept and the Rapid Field Method... 

The worst hazard scenarios (excessive temperature and pressure rise accompanied by emission of toxic substances) must be worked out based upon calorimetric measurements (e.g. means to reduce hazards by using the inherent safety concept or Differential Scanning Calorimetry, DSC) and protection measures must be considered. If handling hazardous materials is considered too risky, procedures for generation of the hazardous reactants in situ in the reactor might be developed. Micro-reactor technology could also be an option. Completeness of the data on flammability, explosivity, (auto)ignition, static electricity, safe levels of exposure, environmental protection, transportation, etc. must be checked. Incompatibility of materials to be treated in a plant must be determined. 

Moreover, reactor safety also requires fulfilling a more ambitious objective, that is, to design a reactor that will remain stable in case of mal-operation. The result will be a robust process towards deviations from normal operating conditions. This goal can be reached if the accumulation of non-converted reactants is controlled and maintained at a safe level during the course of reaction. The concept of maximum temperature of synthesis reaction (MTSR) was introduced for this purpose. This point will be described in the second section. In the... 

Although dose-response assessments for deterministic and stochastic effects are discussed separately in this Report, it should be appreciated that many of the concepts discussed in Section 3.2.1.2 for substances that cause deterministic effects apply to substances that cause stochastic effects as well. The processes of hazard identification, including identification of the critical response, and development of data on dose-response based on studies in humans or animals are common to both types of substances. Based on the dose-response data, a NOAEL or a LOAEL can be established based on the limited ability of any study to detect statistically significant increases in responses in exposed populations compared with controls, even though the dose-response relationship is assumed not to have a threshold. Because of the assumed form of the dose-response relationship, however, NOAEL or LOAEL is not normally used as a point of departure to establish safe levels of exposure to substances causing stochastic effects. This is in contrast to the common practice for substances causing deterministic effects of establishing safe levels of exposure, such as RfDs, based on NOAEL or LOAEL (or the benchmark dose) and the use of safety and uncertainty factors. 

REACH introduces the concept of adequate control in EU chemical law. Traditionally, the term adequate control has been used to refer to good practice in the workplace. REACH now redefines adequate control in the form of risk management measures detailed in an exposure scenario necessary for the control of hazardous properties. Through a set of systematic procedures, risk management measures must be selected to reduce exposure below which adverse effects to human health or the environment are likely to occur (i.e., a DNEL, DMEL or PNEC). There is debate as to whether a concept of a safe level of exposure reduction, similar to adequate control, can apply to non-threshold carcinogens and mutagens, endocrine disruptors, persistent, bioaccumulative and toxic (PBT) or VPVB substances (e.g., [270]). Industry may need to demonstrate that exposure to these substances is always avoided or minimised, as specified in Annex I of the REACH Regulation. 

A relatively new concept is presented for protection to the area surrounding hazardous work with explosives. Suppressive shields are vented composite steel structures which are designed to confine all fragments from an accidental detonation and to suppress hazardous blast and flame effects to a safe level. 

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. 

For nongenotoxic chemicals, risk assessment is based on the concept of threshold doses, below which no adverse effect results from exposure. From human or experimental animal data, one tries to establish the no observable adverse effect level (NOAEL) and the lowest observed adverse effect level (LOAEL). In order to establish safe levels of exposure to potentially toxic agents, the NOAEL is divided by a safety factor (often named uncertainty factor). When the risk assessment is based on data from experimental animals, a default safety factor of 100 is usually applied. The safety factor constitutes a factor of 10 for potential differences in susceptibility between animals and man, and another factor of 10 for interindividual differences among humans. The factors are combinations of differences in toxicokinetics and toxicodynamics, both in animals and man. If true factors are known, the size of the safety factor may be changed accordingly. When risk assessment is based on human data, a safety factor of 10 is applied in most cases, for instance, for food additives. However, for natural toxins in food, smaller factors are usually applied. This is a risk management decision, often based on information on the absence of adverse health effects at intake levels close to the estimated LOAELs. 

After these introductory remarks, I shall attempt to give you an account of the current recommendations of the World Health Organization in this field. In particular, I will stress the "safe" levels of intake based on concepts such as Acceptable and Admissible Daily Intake (ADI), respectively, for intentional food additives and pesticide residues in food Provisional Tolerable Weekly Intake (PTWI) for cumulative toxic metals figures for body burdens and the corresponding "safe" limits recommended for foodstuffs, including the relatively recent definition of irreducible limits to deal with the problem of trace contaminants in food. 

While thresholds are accepted for acute toxicity measmements, there is some controversy around the concept of thresholds for chronic toxicity measurements, especially for carcinogenicity detection. Many scientists believe there is no safe level for carcinogens but other scientists believe there are thresholds for carcinogens just as there are for acute effects. We will present below the complications in applying this threshold concept to chronic toxicants. Another phrase to describe this threshold is the No Observed Adverse Effect Level , or NOAEL. A closer examination of this simple concept reveals many underlying complexities. 

Threshold of Toxicological Concern The threshold of toxicological concern (TTC) is a principle that refers to the establishment of human exposure threshold value for all chemicals below which there would be no appreciable risk to human health. The concept that safe exposure thresholds or levels can be identified for individual chemicals is already widely used for toxicology profiles. However, the TTC concept goes further in proposing that de minimis values can be identified for many new chemicals including those of unknown toxicity base on their similarity to known chemicals. The TTC concept can be used to assess and set a safe level of exposure for a... 

The field watchdog provides an electric fallback position using intrinsic safety. The fallback position is controlled with a probabilistic level of safely (a concept of 2002 on the organ conunands) however, the failnre of a channel (conunanding the fallback position), could result in multiple failure scenarios that would armihilate (offset) the order to the fallback positioa Considering the temporal aspect of these scenarios, it is necessary to lock the fallback state. This lock can be effective (and its efBciency easy to prove) only if executed with the concepts of intrinsic safety. 

Many times, rather than selecting a specific effect, maximum concentrations that cause no effects are used. In this way, it can be ensured that toxic effects will not occur as long as allowable concentrations remain below this threshold. These issues are further discussed in the next chapter, which explains how such concepts are used to establish safe levels of exposure in humans. 

The high levels of functional safety needed from essential systems are usually achieved by some form of fail-safe design. The fail-safe design concept considers the effects of failures and combinations of failure in defining a safe design. The application of the fail-safe concept is probably the most important discipline involved in the design of systems and operations. It has evolved over many years. The definition first appeared in the dictionary in the mid-1950s after the final reports on the Comet disasters were published. 

The fail-safe concept implies the acceptance of the notion that there is no single element or process in any part of a system that can ever have a sufficient level of reliability to be relied upon without some manner of alternative back-up or protection. The CS/JAR/FAR25.1309 airworthiness standards are based on the fail-safe design concept. The following basic objectives pertaining to failures apply AMJ25A 309,2000) ... 

The basic approach is to direct the system to the safest operating level relative to people or the environment when any emergency condition is detected, including power loss. An important concept of process control safety is to have adequate redundancy to reduce unwanted shutdowns and maintain an adequate level of certainty that a safe state will result if a real emergency does occur. As far as possible, instruments should be of the fail-safe type. 

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