Episodic events

Estimating the risk to workers or the public from episodic events involving a one-time exposure to potentially harmful substances or activities... 

In this guide, we will focus on the use of QRA in the safety assessment of acute hazards and episodic events only. 

The acronym for chemical process quantitative risk analysis. It is the process of hazard identification followed by numerical evaluation of incident consequences and frequencies, and their combination into an overall measure of risk when applied to the chemical process industry. It is particularly applied to episodic events. It differs from, but is related to, a probabilistic risk analysis (PRA), a quantitative tool used in the nuclear industry... 

When spills and releases of hazardous gases or liquids occur, the concentration of the hazardous material in the vicinity of the release is often the greatest concern, since potential health effects on those nearby will be determined by the concentration of the substance at the time of the acute exposure. There are many models of routine continuous discharges (e.g., discharges arising from leaky valves in chemical plants), but these carmot be applied to single episodic events. Research on the ambient behavior of short-term environmental releases and the development of models for concentration profiles in episodic releases are cmcial if we are to plan appropriate safety and abatement measures. 

Episodic events occur periodically and may be restricted to given small areas. These events may result from gradual erosion which ultimately reaches the slope instability of the canyon walls... 

Episodic Event— An event of limited duration, typically an incident. For example, release of hazardous materials, a spill, or an explosion. 

Sulfur dioxide is produced by both natural and anthropogenic sources. The most important of the natural sources are volcanic eruptions, which account for about 40 percent of all natural emissions of the gas. Since volcanic eruptions are episodic events, the amount of sulfur dioxide attributable to this source in any one year varies widely. Other natural sources of the gas are forest fires and other natural burns, biological decay, and certain metabolic processes carried out by living organisms, especially marine plankton and bacteria. Natural sources release about 27.5 million short tons (25 million metric tons) of sulfur dioxide per year. 

In addition to these factors, which produce the chemical conditions characteristic of episodic events, the likelihood of an acidic episode is also influenced by the chemical conditions prevailing before the episode begins. Episodes are more likely to be acidic, for example, if the baseflow ANC of the stream or lake is low. In this way, acidic anions, especially S042-, can contribute to the severity of an acidic episode by lowering the baseflow ANC of the stream or lake, even though they do not increase during the event (97). 

Reed, D.C., Laur, D.R., and Ebeling, A.W., Variation in algal dispersal and recruitment the importance of episodic events, Ecol. Monogr., 58, 321, 1988. 

Long-term AA-EQSs are intended to protect against long-term continuous exposure, and MAC-EQSs are intended to protect against short-term episodic events. It should be noted that the use of acute toxicity data for deriving AA-EQSs is considered permissible by some jurisdictions, for example, when suitable chronic toxicity data are not available (e.g., CCME, 2007), although the use of chronic data for this purpose is always preferable. 

The treatment of inhalant abuse faces some serious problems. Some substance abuse clinics often do not even know how to treat inhalant abusers. It is shocking that in spite of the fact that inhalant abuse is a widespread problem, inhalant abusers are often faced with having nowhere to turn to for help. Since inhalant abuse often arises in episodic events in communities, towns facing major problems with youth abuse often find that there is nowhere to accommodate the abusers for treatment and no nearby treatment facilities that can handle detoxification specific to inhalant abuse. Some communities have faced these problems by setting up their own treatment facilities, but this takes time and money. Indeed, with time being critical for treatment of abusers who need it, the lack of adequate treatment... 

Because NH4+ contains N at the oxidation level of proteins, it is readily assimilated by both phytoplankton and bacteria, and is a preferred N source. Ammonia oxidizers may be in competition for NH4+ with other planktonic organisms. The different physiological requirements of phytoplankton and nitrifiers probably play a role in determining exacdy where in the water column NH4+ assimilation and NH4+ oxidation occur. As explained below, most nitrification occurs within or near the base of the euphotic zone in the upper 100 or so meters of the ocean. However, there is usually very little N03 in the surface ocean, due to utilization by phytoplankton, except in high nutrient low chlorophyll regions and when supplied by episodic events such as regional upweUing. The N03 in the deep water of the oceans has accumulated from nitrification because phytoplankton assimilation is essentially zero below the euphotic zone. It is because of nitrifiers... 

Crutzen et al. (1975) noted that penetration of large amounts of protons into the middle atmosphere during solar proton events would also lead to an intense production of atomic nitrogen. These episodic events can produce enough nitrogen oxide to influence the ozone content,... 

Wassmann, P., Andreassen, I., Reigstad, M. and Slagstad, D. (1996) Pelagic-benthic coupling in the Nordic Seas the role of episodic events. Marine Ecology, 17, 447-471. 

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