Engineering Risk and Hazard Assessment

Volume I

Abraham Kandel

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315892603
• eBook ISBN: 9781351088602
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

The volumes deal with the newly emerging field ofRisk and Hazard Assessment and its application to science and engineering. These volumes deal with issues such as short-and long-term hazards, setting priorities in safety, fault analysis for process plants, hazard identification and safety assessment of human- robot systems, plant fault diagnoses expert systems, knowledge based diagnostic systems, fault tree analysis, modelling of computer security systems for risk and reliability analysis, risk analysis of fatigue failure, fault evaluation of complex system, probabilistic risk analysis, and expert systems for fault detection. This volume will provide the reader not only with valuable conceptual and technical information but also with a better view of the field, its problems, accomplishments, and future potentials

Chapter 1

Now or Later? A Numerical Comparison of Short- and Long-Term Hazards

Trevor A. Kletz

Table of Contents

• I. Introduction
• II. Coal Dust
• III. Radiation
• IV. Asbestos
• V. Chemicals
• VI. All Industry
• VII. Conclusions
• VIII. Further Work
• Acknowledgments
• References

I Introduction

In the Loss Prevention Symposium held in Newcastle in 1971, I¹ presented one of the first papers on the use of numerical methods for comparing the different risks to which employees in the process industries are exposed. Since then such methods have been used for setting priorities between different acute risks and the literature on the subject is now extensive.^2-10a In this chapter, I suggest an extension of these methods to hazards which take a long time, typically several decades, to produce their effects.

Consider a substance X, the product of an industrial process, which can cause harm in two distinct ways:

1. It may leak out of the plant, vaporize, mix with air, and be ignited, thus injuring or killing people by fire or explosion. Very small leaks do not matter: the hazard arises only if the leak exceeds several kilograms and is unlikely to be serious unless it exceeds 1 tonne (though smaller leads can be serious in confined spaces).
2. Exposure of employees to small quantities of the vapor for long periods for many years may cause industrial disease which may lead to premature death. In general, we are not concerned with the occasional large release but with the small quantities present in the atmosphere as the result of minute leaks from joints, glands, sample and drain points, maintenance operations, and so on. (However, for some materials occasional large doses may produce long-term effects, or may cause sensitization). The Threshold Limit Value (TLV) gives the time-weighted average (TWA) concentration for a normal 8-hr workday or 40-hr work week, to which nearly all workers may be repeatedly exposed daily, without adverse effect. It does not, of course, provide a sharp division between safe and unsafe conditions. In the U.K., TLVs are now being replaced by control or recommended limits. The concentration should be decreased below these when it is reasonably practicable to do so. (Control limits have greater force and are set when there is sufficient evidence to justify them; recommended limits [usually the old TLVs] are set for more substances.)

We try, of course, to prevent both sorts of leaks and we spend a great deal of money and effort in doing so. How does our success in overcoming hazard (1) compare with our success in overcoming hazard (2)? We do not know. Should we put more effort into preventing the occasional big leaks or more effort into preventing the continuous small leaks? We do not know. Usually different people, using different criteria and financed by different budgets are responsible for dealing with the two hazards. Finding a way of talking to them both at the same time is like finding a way of communicating, at the same time, to people who speak different languages. This is attempted in this chapter.

The problem is not just one of comparing risks. It is made more difficult by the fact that the risks are different and are felt to be different by those who are subjected to them. Suppose that the probabilities of an employee being killed by the acute hazard (1) and the chronic or long-term hazard (2) are equal. An employee might feel that hazard (2) leaves him with an extra 20 or more years of life and, therefore, further resources should be spent on reducing the risk from hazard (1). On the other hand, another employee might feel that a fire or explosion is soon over while industrial disease may mean years of worry, wondering whether or not he will contract it, possibly followed by many years of illness, and reduced quality of life.

From society’s point of view, long-term hazards, though they may kill many people, will kill them over a long period of time and will not produce the same trauma or public outcry as one that kills many people at a time.

In one important respect, the problem of long-term effects differs from all other industrial problems — often the size of the problem is not known. As we shall see in Section VI, we do not know how many people die or suffer from industrial disease (of all sorts, not just the legally prescribed diseases), because the same diseases usually also have nonoccupational causes. Before we spend resources on, for instance, reducing the usage of raw materials or energy, improving product quality, or preventing accidents, we start by asking ourselves what is the present usage, quality, accident rate, etc., and what is the scope for improvement. When dealing with toxicological hazards we do not seem able to do this.

A Personal Note

I have some experience of industrial accidents, but little knowledge of toxicology. In trying to compare the two I may be accused, like everyone who tries to compare two subjects normally considered apart, of dabbling in a field in which I am no expert. I admit the accusation, but if we want to knock a hole in a wall, we have to start from one side. Some may argue that immediate deaths and delayed deaths are so different that they cannot be compared; however, comparing different things is what management is about. Managers have to set priorities between different sorts of tasks. The question is not whether we compare short- and long-term hazards but whether we do so openly and explicitly or inwardly, using unknown criteria (unknown to ourselves as well as others).

Let us look at some examples, starting with those where the size of the problem is known.

II Coal Dust

This substance is not of great interest to the process industries, but it is the cause of serious industrial disease; in 1975, of 802 deaths from prescribed industrial disease in the U.K., 643 were due to pneumoconiosis, most of it caused by coal dust, and it is one of the few substances for which the dose-response relationship is known. The National Coal Board has shown¹¹ that

where x = mean respirable dust concentration in milligrams per cubic meter, and P = probability of developing category 2/1 pneumoconiosis (ILO classification) in 35 years. When calculating the sine, the expression in the brackets is assumed to be in radians.

This equation is based on a statistical extrapolation of observations over 10 years in 20 coal mines and mean coalface dust concentrations in those mines up to 8 mg/m³.

In 1970, the National Coal Board set standards which implied that long-term concentrations experienced by individuals would not exceed 4.3 mg/m³. The corresponding value of P is 9.42 × 10⁻⁴ per person per year. This would be equivalent to a fatal accident rate (FAR) of 50 or 100 deaths per 10⁵ men per year, if we assume that all cases of category 2/1 pneumoconiosis lead to premature death.

However, by no means do all cases of category 2/1 pneumoconiosis lead to premature death. According to the Institute of Occupational Medicine,¹² over 22 years, men in the 25 to 34 year age group with categories 1, 2, and 3 pneumoconiosis had survival rates of 90.1 % compared with 93.0% for those with category 0. The difference, they state, is statistically significant and suggests that the mortality risk for those with simple pneumoconiosis is about 60% higher than that experienced by young men with no radiological signs initially.

What happens after 22 years is not known. Let us th...

Table of contents

• Cover
• Title Page
• Copyright Page
• Contents
• 1 Now or Later? A Numerical Comparison of Short- and Long-Term Hazards
• 2 Setting Priorities in Safety
• 3 Fault Tree Analysis for Process Plants
• 4 Hazard Identification and Safety Assessment of Human-Robot Systems
• 5 Plant Fault Diagnosis Expert System Based on PC Data Manipulation Languages
• 6 Fuzzy Fault Tree Analysis: Theory And Application
• Index