Using Hand-Arm Vibration Controls
Armed with a diagnosis, knowledge, and a recommendation to take steps to limit his exposure, Larry once again thought of his co-workers at the plant.
- By Rob Brauch
- Dec 01, 2008
Larry wakes up in the middle of the night with a prickly, tingling sensation in his hand. At first, he thinks nothing of it; maybe he slept in a bad position and his hand “went to sleep.”
But when it happens again some days later, and then begins happening more frequently and with greater severity, Larry becomes worried. Sometimes, he feels a sharp pain, or one of his fingers feels like “pins and needles.” Other times, one or more of his fingers appear to lose their natural coloration and feel cold. He tries to ignore it but, over time, it seems to happen more frequently— and when his grip strength and manual dexterity start to be affected, he seeks medical help. He’s worried it could some truly terrible disease such as ALS or multiple sclerosis. Not associating his symptoms with the fact that, by day, he is an employee at a company that fabricates large process vessels and other structures from steel and various metals, and his job requires frequent use of pneumatically powered grinders and similar tools in an outdoor environment, Larry thinks the worst.
“You have Vibration White Finger disease, medically known as Raynaud’s Syndrome of Occupational Origin,” says his specialist, “but sometimes it’s also called HAVS or Hand-Arm Vibration Syndrome. Your family doctor told me she thought it could be a ‘common’ case of Reynaud’s Syndrome, a medical condition that occurs in the natural population. We really don’t know why some people acquire this, but it typically afflicts more women than men— in fact, nine times as many women according to one study—so she was concerned that it might be caused by your work.”
Larry asks, “I’ve never heard of such a thing. Are you saying this condition is caused by something I am exposed to at work, like dangerous chemicals or welding fumes or dusts? I’d sure like to know why it picked me—what gives?”
“Actually, yes,” continues the doctor. “Your condition is caused by a workplace exposure—but it is caused by high levels of vibration. This medical problem has actually been around a long time; in fact, it was first reported by Dr. Alice Hamilton back in 1914, when she observed that many workers in her town were afflicted with the same symptoms as you.When she discovered the fact that all these patients were stonecutters using some new-fangled air-powered stone chippers, she connected the dots and realized it was caused by their work, although it wasn’t until 1985 that it was formally recognized as an occupational disease. Vibration White Finger Disease is a serious consequence of working with power tools, especially in a cold climate like ours. Tobacco smokers are more likely to have this problem, too. It says here you are a fabricator. What exactly do you do for a living?”
Larry immediately thinks about his coworkers and wonders how many of them also might be affected. “I am a welder-fabricator, and I do a lot of work using metal grinders, saws, and nibblers. You know, some of them vibrate a lot. And I pride myself on the fact that I work faster and push my tools harder than the other guys.”
Quantifying Vibration Risk
It is well known that exposure to high levels of vibration can cause injury. But how common is Larry’s condition in today’s workplace? Various estimates have been given, but in the United Kingdom alone, based on a survey done in 1997 and 1998 by the Medical Research Council, it is estimated more than 228,000 workers are already afflicted with VWF, mostly in the mining and construction trades. In the UK, typical compensation claims can be in the range of £17,000 (approximately $27,000).
According to NIOSH, a similar number of U.S. workers are currently at risk of acquiring this disease in the construction trades alone. Given that the sources of exposure are air-, electric-, and gasoline-powered devices such as grinders, impact drills, and chain saws that are found in so many workplaces, the risk of a worker’s developing VWF/HAVS is hardly insignificant.
How can an occupational health and safety professional determine whether the amount of vibration present in a work environment puts the workers at risk? First, it’s important to know exactly what we are dealing with and how to quantify it.
Vibration, defined as “a time varying motion (acceleration) of a body about an equilibrium position where the long term average of motion tends toward zero,” is measured in units of “g” (1g = the acceleration level exerted by Earth’s gravity).But for exposure measurements, it is more commonly expressed in units of m/s2.
Similar to noise, vibration can cause injury by repeated exposures over time, with the onset and severity of the resulting condition (noise-induced hearing loss and VWF/HAVS, respectively) being dependent on the duration of the exposure and the amplitude and frequency of the exposure source. As with noise, the frequency of the vibration, measured in Hertz (Hz), is important in determining the risk because the hand, like the ear, is more susceptible to physical damage from some specific frequency bands, or ranges, than others. Unlike noise, however, vibration is not transmitted omni-directionally, and thus it must be measured in each of three different directions (x, y, z) simultaneously.
Ascertaining the risk of injury from hand-arm vibration in the workplace is possible, given that the levels of vibration exposure that cause harm are already understood. These are published in the reference standards ANSI 2.70-2006 and ISO 5349. In both documents, the average RMS (root-mean-square) level of acceleration for an eight-hour work shift that is considered harmful and should not be exceeded is 5.0 m/s2.This is considered the Permissible Exposure Limit in many countries. A level of 2.5 m/s2 is considered the Action Level, even though at that lower level, according to the standards, fully 10 percent of workers will develop vascular symptoms similar to Larry’s finger discoloration within 12 years of daily exposure. Because many common power tools produce levels well beyond 5 m/s2 of vibration—and sometimes more than double that level—it is not hard to imagine risk of injury is more common than might have been previously thought.
Measuring Vibration Exposure Levels
Instruments are available that combine hand-held accelerometers, which measure the acceleration into the hand from three different axes, with special circuitry that filters the frequencies of interest to record vibration exposure levels accurately. These will produce a representative measurement of a given task performed with a power tool that can be used to determine whether or not the levels present are dangerous.
Because the permissible exposure limit is based on an average level of continuous exposure for an eight-hour work shift, the representative samples from different tasks and tools must be combined and extrapolated to build a projected exposure model. Knowing that, application of administrative or engineering controls could be reasonably considered.
More recently, personal dosimeters for hand-arm vibration have been developed that allow continuous measurements to be performed for the entire work shift.
Replacing tools that emit dangerous levels with those that perform better is one way to reduce risk, and tool manufacturers are now using the ISO8662 “Reference Condition” measurement technique to quantify and certify the levels of acceleration produced by their tools. It is important to note the levels resulting from this method represent the very best performance of that tool, in “as new” condition, with a test method that does not reproduce the most severe modes of use that might actually be encountered on the work site. And tool performance will vary greatly with maintenance (or lack thereof), abuse, and age.
This article originally appeared in the December 2008 issue of Occupational Health & Safety.