Further evidence of a relation between noise-induced permanent threshold shift and vibration-induced digital vasospasms

The Combined Impact of Hand-Arm Vibration and Noise Exposure on Hearing Sensitivity of Agricultural/Forestry Workers-A Systematic Literature Review

Hand-arm vibration (HAV), which potentially causes vibration white finger (VWF), and occupational noise are serious issues in the agricultural and forestry industries. Generally, agricultural workers operate as single-family/small businesses and thus are exempted from Occupational Safety and Health Administration (OSHA) regulations/laws for noise and HAV otherwise applicable to other industries in general. The agricultural/forestry sectors are at increased risk as working hours are longer than a typical 8-h work shift putting them at greater risk of hearing loss. The study was conducted to assess the possible association between hearing sensitivity on combined exposure to noise and hand-arm vibration. A systematic literature review was conducted on exposure to noise and HAV in the agricultural/forestry sector and the resulting impacts on hearing. The peer-reviewed articles in English were searched with 14 search words in three databases of PubMed, Ergo Abstracts, and Web of Science without any filter for the year for fully available article text. The database literature search resulted in 72 articles. Forty-seven (47) articles met the search criteria based on the title. Abstracts were then reviewed for any relationship between hearing loss and hand-arm vibration/Raynaud's phenomenon/VWF. This left 18 articles. It was found that most agricultural workers and chainsaw workers are exposed to noise and VWF. Hearing is impacted by both noise and aging. The workers exposed to HAV and noise had greater hearing loss than non-exposed workers, possibly due to the additive effect on temporary threshold shift (TTS). It was found that VWF might be associated with vasospasm in the cochlea through autonomous vascular reflexes, digital arteries narrowing, vasoconstriction in the inner ear by noise, ischemic damage to the hair cells and increased oxygen demand, which significantly affects the correlation between VWF and hearing loss.

Vibrations transmitted from human hands to upper arm, shoulder, back, neck, and head

Some powered hand tools can generate significant vibration at frequencies below 25 Hz. It is not clear whether such vibration can be effectively transmitted to the upper arm, shoulder, neck, and head and cause adverse effects in these substructures. The objective of this study is to investigate the vibration transmission from the human hands to these substructures. Eight human subjects participated in the experiment, which was conducted on a 1-D vibration test system. Unlike many vibration transmission studies, both the right and left hand-arm systems were simultaneously exposed to the vibration to simulate a working posture in the experiment. A laser vibrometer and three accelerometers were used to measure the vibration transmitted to the substructures. The apparent mass at the palm of each hand was also measured to help in understanding the transmitted vibration and biodynamic response. This study found that the upper arm resonance frequency was 7-12 Hz, the shoulder resonance was 7-9 Hz, and the back and neck resonances were 6-7 Hz. The responses were affected by the hand-arm posture, applied hand force, and vibration magnitude. The transmissibility measured on the upper arm had a trend similar to that of the apparent mass measured at the palm in their major resonant frequency ranges. The implications of the results are discussed.

RELEVANCE TO INDUSTRY

Musculoskeletal disorders (MSDs) of the shoulder and neck are important issues among many workers. Many of these workers use heavy-duty powered hand tools. The combined mechanical loads and vibration exposures are among the major factors contributing to the development of MSDs. The vibration characteristics of the body segments examined in this study can be used to help understand MSDs and to help develop more effective intervention methods.

Occupational noise exposure and hearing: a systematic review

Purpose

To give a systematic review of the development of noise-induced hearing loss (NIHL) in working life.

Methods

A literature search in MEDLINE, Embase, Web of Science, Scopus, and Health and Safety Abstracts, with appropriate keywords on noise in the workplace and health, revealed 22,413 articles which were screened by six researchers. A total of 698 articles were reviewed in full text and scored with a checklist, and 187 articles were found to be relevant and of sufficient quality for further analysis.

Results

Occupational noise exposure causes between 7 and 21 % of the hearing loss among workers, lowest in the industrialized countries, where the incidence is going down, and highest in the developing countries. It is difficult to distinguish between NIHL and age-related hearing loss at an individual level. Most of the hearing loss is age related. Men lose hearing more than women do. Heredity also plays a part. Socioeconomic position, ethnicity and other factors, such as smoking, high blood pressure, diabetes, vibration and chemical substances, may also affect hearing. The use of firearms may be harmful to hearing, whereas most other sources of leisure-time noise seem to be less important. Impulse noise seems to be more deleterious to hearing than continuous noise. Occupational groups at high risk of NIHL are the military, construction workers, agriculture and others with high noise exposure.

Conclusion

The prevalence of NIHL is declining in most industrialized countries, probably due to preventive measures. Hearing loss is mainly related to increasing age.

Electronic supplementary material

The online version of this article (doi:10.1007/s00420-015-1083-5) contains supplementary material, which is available to authorized users.

Vibration-induced hearing loss: mechanical and physiological aspects

HYPOTHESIS

The sensorineural hearing loss (HL) after middle ear surgery has been explained by the noise generated by drilling, without considering the vibration generated by the burr.

BACKGROUND

The role of temporal bone vibration in the etiology of the HL was evaluated.

METHODS

An electromagnetic shaker was used to vibrate the bony external ear canal of guinea pigs at different frequencies ranging from 32 to 1,000 Hz and at intensities ranging from 4.2 to 18.8 m/s for 15 minutes. The hearing threshold was measured with auditory evoked responses. A total of 30 animals were tested.

RESULTS

After vibration, 60% of the guinea pigs developed a threshold shift (TS) exceeding 10 dB at two frequencies, with average TS of 8.8 dB across all frequencies and animals. The exposure to vibration at higher frequencies (range, 500-1,000 Hz) produced stronger TS than did the exposure to lower frequencies (range, 32-250 Hz). The vibration-induced TS showed prominent recovery so that after 7 days, TS was 2.4 dB on average and 27 of 30 animals had recovered. After 14 days, the TS was 1.3 dB. The vibration excitation measurements showed that at lower frequencies, the vibration transmission into the skull was significantly greater than at higher frequencies, at which the transmission was heavily attenuated. There were no acoustic resonances detected in the skull. The frequency of vibration and the hearing frequency in auditory brainstem response were significant determinants in the model explaining the vulnerability of vibration on hearing. Hearing loss primarily occurred at higher frequencies. The HL was mostly reversible, consistent with the results observed after human temporal bone surgery.

CONCLUSION

We conclude that in the guinea pig model, the temporal bone vibration at higher frequencies produced a more severe HL than did the vibration at lower frequencies, although the vibration at higher frequencies caused less efficient transmission from the vibrating probe to the temporal bone. The guinea pig model may be useful in the development of surgical techniques and in the understanding of temporal bone pathology.

[Effects of exposure to occupational hand-arm vibration on maintenance of postural balance]

In order to determine the relationship between exposure to hand-arm vibration through the use of vibration tools and dysfunction in the maintenance of postural balance, 106 male forestry workers were examined by stabilometry for deflection in the center of gravity and in the air conduction hearing levels. A questionnaire survey was also conducted among the workers in order to obtain details regarding their age, the types of tools used by them, and the duration for which they had used a chain saw. The vibration acceleration of a chain saw has been limited to a level of 3 G or less since 1976 in accordance with the notification from the Japanese Forestry Agency and the Ministry of Labor. In fact, chain saws with significantly reduced vibration acceleration in comparison with those used before 1976 have been available. Therefore, in 2000, we conducted a test on forestry workers who were divided into two groups-workers who had used a chain saw for 25 years or more (25-yr-or-more group) and workers who had used a chain saw for 24 yr or less (24-yr-or-less group). Compared to the 24-yr-or-less group, the 25-yr-or-more group exhibited significantly higher levels of average deflection in the center of gravity, expressed as the enveloped (aENV) and rectangular (aREC) areas, and in the hearing levels at 500, 1000, 2000, 4000 and 8000 Hz. For the aENV, the correlation coefficients revealed significant relationships between the hearing levels at 4,000 Hz, the duration of use of a chain saw, and age. Since the duration of use of a chain saw exhibited a significant relationship with age, it was necessary to eliminate the effect of age on the aENV. Subsequently, we divided all the workers into age groups spanning ten years each (from 20 to 70 yr) and compared the aENV among the same age groups in both the 25-yr-or-more and the 24-yr-or-less groups. The averages of the aENV for each age group were higher in the 25-yr-or-more group than in the 24-yr-or-less group. In particular, a significant difference was observed in the group of workers in their 40s. Moreover, the average of aENV value was significantly higher in the 25-yr-pr-more group than in the 24-yr-or-less group among the workers between 46 and 68 yr, with their ages being congruent in both groups. While investigating the impact on the workers who had used chain saws, it might be difficult to examine the effect of occupational vibration independent of the noise load. Nevertheless, this study suggests the possibility that a decrease in the maintenance of postural balance has a stronger relationship with exposure to thumping vibration in the past than with the age factor.

Occupational hearing loss

Hearing loss is a significant and unfortunately common occupational malady. Over the past several decades both the Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH) have initiated efforts to better understand and to limit the occurrence of occupational hearing loss, particularly as it relates to excessive noise exposure. This paper briefly addresses the pathophysiology of noise-induced hearing loss and then describes the occupational and non-occupational factors which influence a worker's risk of hearing loss. The primary foci of this discussion are the clinical evaluation, diagnosis, and management of occupational hearing loss. Issues of prevention, OSHA-mandated hearing conservation efforts and compensation are reviewed.

Hearing loss risk from exposure to shooting impulses in workers exposed to occupational noise

The effect of shooting impulses on hearing was analysed in 150 professional forest workers exposed to noise from chain saws. The exposure to shooting impulses (Lesi) was designed to take into account the peak levels of shooting impulses, their number and use of hearing protectors. Hearing loss was dependent on Lesi even after allowing for the age of subjects and their exposure to chain saw noise. Hearing threshold levels were compared between pairwise matched groups with high and low Lesi. Proper matching was achieved for age, chain saw noise, salicylate consumption, blood pressure, cholesterol, smoking and vibration-induced white finger symptoms. The workers with high Lesi had 9 dB greater hearing loss at 4 kHz and 10 dB greater hearing loss at 8 kHz than those with low Lesi, the difference being significant at P < 0.05 level. In evaluation of noise-induced hearing loss the exposure to shooting impulses from different calibre weapons should be determined since they increase the extent of hearing loss. The present study describes the Lesi method which more accurately evaluates the harmful effects of shooting noise impulses on hearing.

Vibration disease

Today, in this age of technology, vibration caused by machinery is an almost universal hazard. Vibration transferred from a machine to the human body may cause discomfort, a reduction of performance, and even injury. Vibratory manual tools may cause damage to the circulatory system of the upper extremities (Raynaud's syndrome), to the peripheral nerves (peripheral neuropathy), and to the bones and joints (aseptic necrosis, fatigue fractures, degenerative joint disease). Vehicles and machines causing floor vibration cause degenerative disc disease of the lumbar spine. The pathogenesis of vibration injuries is still not completely clear and there is no effective treatment. Some of the abnormalities are irreversible and may cause permanent decrease of working ability, and even unemployment. This is why prevention is so important. Prevention is complex, including technical and organizational measures, use of individual protective clothing and footwear, and medical supervision both before and during employment. Workers who are exposed to vibration should be protected against other aggravating factors such as cold and noise, etc. Vibration-induced injuries are recognized in law in many countries as grounds for financial compensation. Their cost to industry is rising and, unless a means of prevention or cure is found, will continue to do so in the foreseeable future.

Sensory-neural hearing loss during combined noise and vibration exposure. An analysis of risk factors

A detailed analysis of risk factors for the development of sensory-neural hearing loss (SNHL) was carried out on 122 forest workers. These forest workers were selected from a larger group (n = 217) by restricting the age range to 30-55 years. The hearing threshold of the left ear at 4000 Hz was measured and the effect of age, exposure, systolic and diastolic blood pressure (DBP), presence of vibration-induced white finger (VWF), tobacco smoking and use of earmuffs were evaluated in multiple linear regression analysis. Robinson's nonlinear model was used to evaluate the rate of hearing loss. Aging was the major risk factor and it explained 15.4% of the variance of the SNHL. The presence of VWF was the second most important single risk factor and explained a further 5.2% of the SNHL. Elevation of DBP correlated significantly with SNHL and explained an additional 4.1% of the SNHL. These main factors were able to explain about 26% of the spread of SNHL. Additional factors in the analysis, e.g. smoking, systolic blood pressure, did not significantly contribute to the genesis of SNHL. When Robinson's model was applied to the SNHL data, on a group basis, we did not observe any exaggerated risk of hearing loss due to combination of noise and vibration. In combined exposure subjects with VWF as well as subjects with enhanced DBP will run a higher risk for SNHL.