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Vibration Hazards in the Workplace: The Basics of Risk Assessment
Every day, much time and effort is spent on measuring vibration levels in factories vehicles, buildings, and on other structures and machines—even products as diverse as computer hard drives and spacecraft as they are being designed, developed, and tested. Thousands of engineers, technicians, consultants, and machine designers have become expert when it comes to measuring how physical objects are affected by vibration. Yet an astonishingly small percentage of this expertise is focused on how vibration in the workplace causes serious injury in humans—injuries that could have been prevented with the right amount of knowledge and the application of a few simple guidelines.
Let's examine how vibration in the workplace can be identified and its level of risk estimated or even quantified, in order to prevent potentially compensable injury from happening in the first place.
How Vibration Causes Injury
Repeated exposure to high levels of vibration is known to cause injury to workers over time. Based on exactly how these exposures intersect an individual's work environment, they are classified into two general types: hand-arm and whole-body vibration. Hand-arm vibration exposure (HAV), besides being a known contributing factor to carpal tunnel syndrome and other ergonomic-related injuries, causes direct injury to the fingers and hand, affecting feeling, dexterity, and grip. These injuries are debilitating and compensable. Whole-body vibration (WBV) is a consideration when dealing with higher than expected levels of low back pain and injury in the workforce and is one of the most pervasive causes of lost time and production output, according to the Journal of the American Medical Association.
Some of the earliest diagnosed cases of injury directly attributed to working with hand-held power tools were identified by Dr. Alice Hamilton in the early 20th century, and she led the way in correlating these cases of "Reynaud's Syndrome of Occupational Origin" (or what is often referred to as Vibration White Finger Disease, or VWF) to the use of pneumatic stone-chipping tools used in a local quarry. This opened the door to the study of the relationship between high levels of vibration entering the body and the resulting debilitating injuries that occur, some so severe that they have resulted in complete amputation of the fingertips. The study of vibration and its relationship to the prevalence and severity of injuries caused by chronic exposure in the workplace continues to this day.
So what exactly is going on that would cause the repetitive or cyclical mechanical motion of a tool (a/k/a vibration) to injure someone's hand and fingers to the point of possible amputation? The human body is a remarkable structure built of multiple interconnecting systems, including the vascular system, which distributes oxygen and nutrients to tissues throughout the body, and the nervous system, which provides sensory input to our brain, as well as muscle control to everything from our heart to our fingertips and toes. Excessive levels of vibration can cause localized disruption of these functions, and a simplified way to envision the very complex biomechanical and biochemical processes that are thought to be at the root of the problem is to imagine the workings of the smallest structures in the vascular system, those very small vessels and capillaries that allow oxygen-rich blood cells to transfer that O2 to the surrounding cells and tissue structure that need it to survive.
Research has shown that vibration levels similar to those found in hand-held power tools can decrease blood flow in the extremities; therefore, the body is less effective at transferring oxygen and other vital nutrients to those cells when they need it most. (Research in the medical field is ongoing and continues to uncover more understanding as to how this occupational disease occurs and progresses, and researchers such as those at CDC/NIOSH's Health Effects Laboratory Division in Morgantown, W.Va. are studying the exact cause-effect relationships between different levels of vibration and their effects on different parts of the body). When less oxygen and fewer nutrients are transferred to the cells and tissues that need them, they can and will die—with nerve cells being perhaps the most vulnerable and first to exhibit cell death or necrosis. Ironically, then, the very cells in the hand and fingers that allow us to feel and touch with great sensitivity and grip a tool for exacting control are the first to be "killed off" by excessive vibration exposure. In fact, when diagnosing HAV, doctors use a system that classifies the injury for both vascular and nerve damage components. Unfortunately, the physical effects of WBV on the lower back are more difficult to diagnose because the injury is not visible, as is the case with severe or even moderate VWF disease.
Common Sources of Vibration
Many of the tools that are used regularly throughout multiple industries can and do cause injury from repeated use. Grinders, chipping hammers, sanders, pavement breakers, impact drills, air-powered wrenches, saws of all types, and even dental tools can all be sources of vibration that, if used repeatedly for long periods of time, could cause hand-arm vibration injury. As a result, NIOSH has estimated that as many as 1 million workers may be at risk of developing symptoms. The latency period from continuous exposure to onset can be as little as two years or as long as 17 years, depending on the tools used and work performed. WBV becomes a concern when long periods of time are spent operating vehicles such as forklift trucks, off-road haulers, mining machinery and logging equipment, paving machines, and even ferryboats and other common conveyances.
Risk Assessment in the Workplace
Identifying the potential for injury from vibration at work is relatively simple, and estimating the level of risk is possible even though actual exposure levels on any given day are difficult to accurately quantify. Like many workplace hazards, the amount of risk is driven by time of exposure and magnitude of the vibration. The presence of vibration sources, typically hand-held, air- or electrically powered tools, is quickly determined through an inventory audit of equipment used on site, and a simple study of tool utilization gives a reasonable estimate of time of exposure.
Understanding whether the level, or amplitude, of the vibration poses significant risk is more difficult, but recent development of Manufacturer’s Directives by the European Union (EU) has led to tool suppliers disclosing the vibration emissions of their tools, such that safety professionals can easily research whether or not the tools on their job sites pose significant risk or not by reading the operator’s manual for the tool or vehicle in question. (It should be noted that these stated levels are for tools that are in "as new" condition and have not been subjected to rough handling, abuse and wear of tool bits, etc., that would cause actual vibration levels to be as much as 2x higher, in which case, estimates should be adjusted to account for this.) If the levels of vibration are above the "action level" of 2.5 m/s2 (meters per second per second, a measure of acceleration) for HAV or above .5 m/s2 for WBV, and the expected duration of exposure is close to a full work shift, then it is recommended that an exposure model be constructed by taking the average time on task per day with that tool and computing the expected vibration "dose" contribution (sometimes called "exposure points") for that task. If more than one type or model of tool is used, then the contribution dose for each should be calculated and summed to see whether the total is considered above harmful levels. Spreadsheets are available to make this calculation and can easily be found on sites such as www.hse.gov.uk/vibration/hav/vibrationcalc.htm (HAV) and www.hse.gov.uk/vibration/wbv/calculator.htm (WBV).
Other risk factors that contribute to the probability of injury include working in cold, damp environments; poor tool maintenance; and even tobacco use. The nicotine found in tobacco products is a known vasoconstrictor that will reduce blood circulation in the extremities even further, which exacerbates the problem and increases risk of irreparable harm. Reports of workers suffering from a tingling sensation in the hands and fingers after using a piece of equipment can be another indicator of risk. If workers are complaining of whitening of the digits followed by a "flushing" effect, or pain and numbness occurring while off the job, the risk may be excessive and could have already resulted in some onset of injury, although it must be noted that these symptoms can be unrelated to their work, and instead is result of that person having Reynaud's Syndrome, a naturally occurring condition (the cause of which is still unknown), which is present to some degree in roughly 5 percent of the population. Medical evaluation is recommended, and the worker should be referred to a medical professional for diagnosis.
Exposure Assessment and Monitoring
In cases where the estimated levels may be above the recommended daily limits of 5m/s2 (HAV) and 1.15m/s2 (WBV) and where the work performed is ongoing and necessary, it may be useful to perform exposure assessment by obtaining one of the many vibration monitors that are commercially available; but it is absolutely critical that the monitor chosen is built specifically to measure human vibration (as opposed to a machinery condition tester) and that it meets the ISO 8041 vibration monitor performance standard, which in turn specifies that the meter also measures to the requirements of ISO 5349 for HAV and ISO 2631 for WBV; however, there is a significant amount of study that usually is needed before actually making measurements in the field with such devices. The placement, orientation, and mounting of the vibration sensor, or accelerometer, is important, as is the setup of the instrument itself to ensure the proper settings are applied. It may be best to consult with the instrument manufacturer and its local supplier before renting or purchasing such a device, or even to consider using a consultant who is experienced in making vibration measurements specifically for worker safety. They will have mastered the operation of these instruments and know how to reduce uncertainty in the result, as well as offer recommendations for risk reduction.
Reducing and Eliminating Risk
If workers are truly at risk from excessive vibration, steps can be taken to reduce or eliminate that risk altogether, including purchasing new tools that vibrate less and maintaining them to a high standard of performance. However, some operations will still require the use of tools that are going to produce levels that are considered harmful—it's virtually impossible to design certain types of tools to produce low vibration levels and still be capable of performing their intended task. When that is the case, limiting the amount of time a worker can perform the task is an acceptable alternative, so job rotation may be an option; another is to make sure frequent breaks are taken and hands are kept warm and dry.
Operator technique also can be used to reduce risk of injury, because the amount of grip force used and the way a tool is allowed to "do the work" can limit the amount of vibration energy entering the body (as opposed to "forcing" the tool through the work to attain faster results). For WBV, the type of tires and suspension used on a vehicle are important considerations, as are the seats and their adjustability. Let's not forget training; many workers may not be aware of the risks they face when working with tools and vehicles that produce vibration levels that seem harmless today but will over time, perhaps years or decades, severely impact their health and quality of life. Informing workers of the risks and how to avoid or reduce them, as well as empowering them to speak up when a tool becomes worn or damaged, becomes part of an injury prevention program that addresses the long-term risks posed by working around excessive vibration sources.
Some workplace risks are imminent and obvious; vibration exposure is not always considered a priority by those whose job it is to make sure their fellow employees stay safe and healthy on the job. Injury from exposure to too much vibration is progressive, permanent, and preventable. With a little knowledge and some keen observation, any safety and health professional can be cognizant of the risks from vibration and take steps to eliminate or mitigate them. When it comes to preventing vibration injury, it's never too late to shake things up!
This article originally appeared in the February 2015 issue of Occupational Health & Safety.