Current legal occupational health and safety threshold values and suggested curves for the rating of environmental stress are dominated by integral approaches, in particular by the physical principle of “dose.” This is rather understandable given the need to easily characterize work stress and work-environmental influences with simple, straightforward characteristic values in everyday work situations. Such a quest for simplicity often results in the use of single-value or summary measures such as the rating level for noise, the rated vibration intensity associated with vehicles and handheld tools, the effective temperature for climate, and the dose for toxic substances and stress due to radiation (e.g., ultraviolet immissions or radioactive radiation). Conversions that are based on the principle of energy equivalence equate singular high intensities of short duration with an accordingly lower intensity “leveled” over an 8-hour day. Such a rating – which is solely stress-oriented, i.e., based on the combination of intensity and time – does not do the human body's characteristics justice (e.g., for the evaluation of impulse noise). A single noise exposure with a high level of 160 dB of 1 ms duration or 100 impulses of 140 dB each and a duration of 1 ms are identical to a (still permissible) continuous sound exposure of 85 dB for 8 hours in a physical sense, i.e., in terms of energy. From an ergonomics perspective, however, continuous and peak stress cannot be assumed to have the same effect on the human body. On the other hand, restitution periods after exposure to short-term high noise levels can be filled with additional noise without a numerical change in the rating level as long as the additional noise is only 10 dB below the peak levels. It should be evident that the effect on the human body cannot remain the same.

Thus, threshold values or suggested values in ordinances, guidelines, and regulations concerning occupational health and safety – which are based on the dose maxim – can be associated with substantial risks from an ergonomics perspective. Blindly following these laws and rules without knowledge of the underlying compromises can result in substantial misjudgments of the effects on the human body. Inevitably, it becomes increasingly difficult to draw conclusions about strain or acute and potentially long-term effects or damage based on stress data the more integral characteristic values are formed to summarize the dimensions “intensity,” “frequency,” and “exposure time” of physical environmental stress. It may only be seemingly safe when a – mathematically easily accomplished – equilibration of peak levels or mutual compensation of stress level and duration based on an 8-hour workday takes place. This is especially relevant because the compressed acting of a noxa over time, i.e., the growing energy or pollutant concentration, makes it increasingly likely that physiological thresholds are exceeded since the human body does not have sufficient “buffering capabilities.”

This book extensively addresses this topic. It is an attempt to increase the transparency in existing rating methods and – in the interest of pertinent disclosure of risks associated with common procedures of occupational health and safety – to work towards the elimination of unacceptable simplifications and faulty ratings. The emphasis is on a discussion of rating methods of acoustic stress since partial loss of hearing due to noise is still the leading occupational hazard in practically all industrialized nations even though herewith only the tip of an iceberg of aural and other extra-aural effects of noise is visible.

The introductory Chapter 1 demonstrates the conventional method of measuring, evaluating, and rating of physical environmental stress using the example of noise exposures. For example, the fact that noise exposures are measured, evaluated, and – using complicated formulas – rated with impressive precision should not suggest that the procedure is sufficiently “tailored” to the human body. The problems begin with the acoustic measuring systems, which exhibit a lack in compatibility with the hearing's physiological characteristics. Among other things, the obvious differences between the stress specifications during physical work according to the principle of equal work and the various strain reactions of the circulatory system already suggest that the hearing can hardly be capable of handling extremely high, short noise exposures equally well as energy-equivalent lower, but accordingly longer stress. Consequently, there are risks associated with the use of the dose maxim or the energy equivalence in the context of occupational health and safety. Similarly, individual hearing protectors do not always deliver the level of effective protection suggested by common rules and regulations – especially against exposures to impulse noise.

Chapter 2 provides an overview of occurrences and characteristics of impulse noise, which, in addition to posing a particular threat to the hearing (e.g., as rebound on the shooter) associated with the use of firearms both in the civil and military sector, also occurs more often than typically assumed during various work processes. Using bolt setting tools as an example, it is demonstrated that it would be unwise to rely “blindly” on a tool's advertised “acoustic quality” which is based on standardized measuring procedures. Similarly, the level of expected noise emissions and the resulting tolerable work cycles per day should not be taken “at face value.” The use of steel profiles instead of concrete (the standard material), for example, results in noise emissions of a different, substantially more dangerous nature, which means that an employee's protection cannot be guaranteed under real-life working conditions.

Chapter 3 presents field studies on the use of bolt setting tools as advantageous, mobile tools for roofing and paneling of industrial buildings. The studies show that in addition to the noise caused by the tool's operator, extraneous noise, which occurs with at least equal frequency, must be taken into consideration as well despite its slightly reduced volume. While such extraneous noise and the general noise level at a construction site do not substantially increase the rating level, a substantially increased risk of damage to the hearing results.

The bulk of the book consists of more than a dozen chapters, which present comprehensive statistically secured results of studies on audiometrically determined hearing threshold shifts and their restitution behavior after various sound exposures.

Chapter 4 describes specifically developed measuring methods and statistical evaluation procedures for the determination of hearing threshold shifts (with precision to 1 dB) at the frequency of maximum threshold shift immediately after the exposure (TTS₂), the restitution course, and the time t(0 dB) after which all threshold shifts have subsided. The integral over the restitution function, the so-called Integrated Restitution Temporary Threshold Shifts (IRTTS), is a global characteristic value for the “physiological costs” that must be “paid” by the hearing for the sound exposure.

The results presented in Chapter 5 demonstrate via experiments that measurements of threshold shifts at a single frequency capture the majority of metabolic fatigue in the inner ear, thus permitting the use of such a procedure for the remaining studies.

The quantitative study in Chapter 6 shows that energetically equivalent stress from continuous and impulse noise with a legally permitted rating level of 85 dB(A)/8 h results in extremely different physiological costs. The already substantial threshold shifts of more than 20 dB after exposure to continuous noise of 94 dB(A)/1 h (equivalent to 85 dB(A)/8 h) only increase by a few dB as a result of exposure to impulse noise (e.g., after 9,000 impulses with a level of 113 dB and a duration of 5 ms, administered at 3-s intervals). However, the restitution times increase from approximately 2 h (after continuous noise) to more than 10 h (after impulse noise) which is associated with a substantially higher risk of permanent hearing threshold shifts.

Among other things, the studies concerning threshold shifts in Chapter 7 examine the effect of additional continuous noise (which increased the rating level by only 0.1 dB and was thus of no energetic relevance) after a preceding exposure to continuous noise. The documented results clearly show that there is a “price to be paid” if restitution periods are “filled” with additional noise. The marginal increase in the rating level caused the hearing's physiological costs to more than double relative to the costs that were associated with the “initial” stress of 94 dB/1 h. Without the determination of the restitution course and the IRTTS-values, it would not have been possible to show such an effect.

Chapters 8 and 9 investigate the effects of variations in the number and duration of noise impulses on the threshold shift. Again, the effects on the hearing when duration and number of impulses were “swapped” against each other (while maintaining energy equivalence) showed differences that were of statistical and practical significance.

In addition to noise, music can pose a threat to the human hearing. Thus, the following three Chapters 10 through 12 present studies which examine stress from various styles of music (heavy metal, techno, and classical music) by comparing their aural effects to those caused by energy-equivalent industrial noise and “white noise.” The results suggest that heavy metal has effects similar to industrial noise. Furthermore, the human hearing seems best suited to tolerate harmonic and sine-shaped sounds.

On the one hand, noise exposures rarely occur in isolation. In the workplace, they are often compounded by physical stress. On the other hand, stress from noise or music often coincides with alcohol or cigarette consumption during leisure activities. Therefore, Chapter 13 analyzes the effects of such combined stress on the hearing and the circulatory system. It was found that such “double stress” is not necessarily negative: For example, restitution processes of the hearing can be accelerated by limited physical work, and “reasonable” amounts of alcohol also exhibit positive effects. Exposure to nicotine and carbon monoxide from cigarette smoke, however, has a negative impact on the restitution processes of the hearing.

Chapters 14 through 16 present experimental data on the objective determination of hearing protection devices' attenuation effectiveness via the artificial head measuring technique versus the subjective hearing threshold method. Additionally, 2 extensive test series establish that short time periods during which no hearing protection is worn does not lead to the drastic negative effects on the protection's effectiveness that mathematical models – on which national and international standards are based – predict.

The experimental results with respect to the physiological costs of various sound exposures, which have been accumulated over more than 10 years refute the concept of energy-equivalence along virtually all dimensions. On the one hand, the use of this paradigm – which is solely based on laws of physics – substantially underestimates the risk of impulse noise, and it legalizes the “filling” of resting periods with noise. It is certainly true, however, that such noise results in “physiological costs” as well as mental effects. On the other hand, the concept of energy equivalence ignores that short-term, high-level noise exposures are quite favorable for the human body. If such exposures remain below a threshold of approximately 120 dB, the human body can handle them quite easily, both from a mental and physiological perspective. The concept of energy equivalence even beats its supporters at their own game when – incorrectly – drastic losses in attenuation after short time periods of not wearing personal hearing protection devices are prognosticated, making them sound worse than they are.

All results regarding the “physiological costs” to the hearing are based on legally permissible acoustic stress, which is equivalent to a rating level of 85 dB(A)/8 h, since dangerously high levels must not be used in tests involving human test subjects for ethical reasons. According to the consistent experimental findings, but also based on plausible scientific-critical evaluations in Chapter 1 as well as the concluding Chapter 17, the dose principle or the concept of energy equivalence cannot be viewed as ergonomic paradigm for occupational health and safety and ergonomics with respect to assessment of environmental stress during an 8-h workday.

Supporters of the dose maxim like to cite Paracelsus who – following the spirit of the Renaissance – adopted this name in lieu of his original name (Philippus Aurelius Theophrastus Bombastus von Hohenheim) approximately 450 years ago. He is credited with the phrase “dosis facit venenum.” By no means does that imply, however, that alternatingly high and low immissions should be expressed as a single mean value to describe a workplace's typical amount of stress (an often-cited justification for the concept of energy equivalence or the dose maxim for the rating of physical environmental stress and toxic substances). A thorough study of Paracelsus' work reveals that his phrase reflects a medical doctor's experience and knowledge of toxicology that medication (the extract of a medicinal plant) in several smaller amounts (the “right dosage”) has healing effects while the same amount administered at once (the dose) could be fatal. This does not suggest that stress, which is repeated daily for years (as work dose) cannot correlate with noticeable effects on the human body, which may even include “wear and tear” on an organ. However, the dose maxim may certainly not be used – following Paracelsus – to legitimize the leveling of variable physical and toxic environmental stress during the course of a workday, whose effects are often hastily equated with those of energy-equivalent continuous stress.

In order to live up to the claim that work protection is based on the human body, the presented unambiguous, statistically significant experimental results regarding the vastly different “physiological costs” of, e.g., continuous noise and impulse noise must effect changes in the way they are rated. In the area of occupational health and safety, it would be irresponsible to take the convenient position of limiting the assessment of stress to the physical aspects while ignoring the fact that human beings react to exposures according to physiological and psychological characteristics rather than “function” according to the laws of physics as they apply to dead matter. Thus, ergonomics and occupational medicine must insist vehemently on the inclusion of current knowledge regarding short- and long-term effects of stress on the human body in rules and regulations of occupational health and safety.

I wish to extend my sincere thanks to Ms. Jenny Deter Gritsch who did a great job in translating large parts of this book from German into English.

Prof. Dr.-Ing. habil. Helmut Strasser, Siegen, 2005