Hand-Arm Vibration (HAV) is an occupational vibration exposure that mainly affects people who regularly use all types of vibrating pneumatic, electric, hydraulic and gasoline-powered hand tools. Extensive research and numerous studies dating back to 1918 link HAV exposure to a generally irreversible condition of the fingers and hands called Hand-Arm Vibration Syndrome (HAVS).
HAVS is a cumulative trauma disorder first characterized by tingling and/or numbness in the finger(s). With continued HAV exposure, a single "white" or blanched fingertip will occur, usually in conjunction with exposure to cold temperatures. These "attacks" are often mistaken for frostbite by workers. Initial attacks last a few minutes and are widely spaced apart. As HAV exposure continues, especially in cold conditions, attacks increase in number, intensity, pain and duration. In the latter stages of HAVS, attacks occur in all seasons both on and off the job interfering in the work and non-work lives of afflicted individuals.
HAVS involves a combination of vascular and sensorineural damage. While blanching is a result of decreased blood flow, losses in touch perception and dexterity are also commonplace due to nerve damage. In extreme and rare cases, the loss of blood supply to the fingers can lead to gangrene, which can require amputation. Other medical conditions, medications and habits such as smoking, which affect blood flow, can cause an individual to be more susceptible to contracting HAVS. Presently, most doctors and scientists have found HAVS to be generally irreversible. Workers experiencing HAVS signs and symptoms are advised to stop all vibration exposure and seek medical attention immediately.
Preventing HAVS requires understanding and controlling the potential exposures created by the tools in use by the worker. While manufacturers of hand tools often publish vibration data for their products, this information is based on static bench tests of brand new equipment without any workload. Output characteristics of a hand tool in the workplace are dynamic and affected by the task being performed, the material being worked, the age and condition of the tool and the way in which the worker operates the tool. In-situ measurement of vibration output from hand tools provides the best assessment of risk.
Measurement Requirements
Vibration measurement is a description of motion expressed in terms of both magnitude and direction. The magnitude of hand-arm vibration is quantified as the acceleration rate in an engineering unit known as "meters per second squared" (m/s2). As example, a measurement of 4 m/s2 means that the motion of the vibration in the hand is accelerating at a rate of 4 meters per second, every second.
For hand-arm vibration, three perpendicular directions are simultaneously measured. These measurements are referred to individually as the Xh, Yh and Zh axes with the x-axis on a plane passing through the back of the hand, the y-axis crossing the knuckles and the z-axis running parallel to the plane of the bones in the back of the hand.
HAV exposures are of concern when the vibration occurs within a certain frequency range. This range is identified by ISO 5349 as nominally 5.6 Hz to 1,400 Hz. Band limiting filters are required in the monitoring instrument to restrict measurements to these frequencies of concern. The susceptibility of injury to the hand also varies with frequency within this range. Frequency-weighting filters are required in the monitoring instrument to reflect the importance of different frequencies in causing injury to the hand. ISO 5349 identifies the requirements of the HAV weighting filter.
Performing Measurements
Quantifying hand tool vibration involves monitoring the vibration acceleration rates in all three axes (directions) simultaneously while the tool is in use. Static bench tests of hand tools are not representative of the performance of the tool under load. To the extent the tool is used in different ways or for different tasks, tests should be conducted for each scenario. Using a hand sander as an example, if different types of sanding paper are used on different types of materials, tests should be conducted for each respective scenario. It is important to note that what is being measured is the output of the tool, not the exposure of the employee. Transmissibility determines actual employee exposure. Because transmissibility can be highly variable even under the most controlled situations, output of the tool is the focus of all current exposure standards and guidelines.
Proper measurement requires the use of a tri-axial accelerometer connected to a datalogging three-channel vibration monitor that meets the requirements of ISO-8041 for hand-arm vibration measurements. Meeting ISO-8041 assures the meter has the proper weighting and band-limiting circuits and measurement tolerances to correctly and accurately measure tool output in accordance with HAV concerns. Machine vibration monitors typically found in plant maintenance and operations departments are not suitable for the HAV application.
A tri-axial accelerometer is actually three accelerometers packaged in one compact, lightweight housing. Three independent accelerometers can be used, although tri-axial accelerometers present several advantages. Tri-axial accelerometers allow a single mount to the hand tool. Mounting three individual accelerometers can prove to be labor intensive and challenging in terms of finding adequate space on the tool for each one. Tri-axial accelerometers require only one cable between the accelerometer and monitoring instrument. Using independent accelerometers creates the need for three cables. In-situ monitoring becomes inconvenient and possibly unsafe due to all the cables that need to be kept away from the moving parts of the tool.
The method used to mount the accelerometer to the tool is critically important. The objective is to mount the accelerometer on the tool as closely as possible to where the operator holds the tool. Care should be taken not to interfere with the safe operation of the tool. A rigid mount is necessary in order to obtain accurate and repeatable results. Two examples of suitable rigid mounting methods are hose clamp adapters and stud mounting adapters. Loosely coupled mounting methods such as hand-held adapters are not recommended. Hand-held adapters are prone to producing inconsistent results due to variances in the amount of pressure applied by the tool operator holding the adapter against the tool.
Hose clamp adapters are commonly used because they can be rapidly attached and detached from the tool. The hose clamp actually threads through a slot in an adapter that in turn mounts to the accelerometer itself. Tie wraps may also be used in place of the hose clamp, further speeding up attachment and removal of the accelerometer. Hose clamps are ideal where barrel-style handles exist on the tool.
Accelerometers for HAV applications will typically incorporate a threaded female fitting designed to accept a short, threaded, male union stud and the stud is usually furnished with the accelerometer. In some applications, it is possible to drill and tap a mounting hole in the hand tool. The short union stud is then threaded half-way into the accelerometer and half-way into the tapped hole in the tool. Safety of the tool is of the utmost importance. Modification of tools as described can introduce hazards that will result in injury or death. Modifications should never be made without first consulting and obtaining approval from the manufacturer of the tool.
Finally, proper orientation of the accelerometer must be confirmed. Tri-axial accelerometers will be labeled to indicate the x, y and z axes of the device. The x-axis shall pass through the back of the hand, the y-axis shall pass across the knuckles of the hand, and the z axis shall run parallel to the plane created by the bones in the back of the hand. Proper accelerometer positioning on the tool may prove to be a function of the task being performed or the worker using the tool. Failure to assure consistent and proper alignment of the accelerometer will result in a virtually useless database of results.
Determining Exposures
Once again, what truly is measured is the output of the tool and not necessarily the exposure of the worker. However, current exposure standards and guidelines are based on years of tool output data and corresponding observed worker responses and do not distinguish transmissibility rates. So for discussion purposes, tool output and worker exposure will be treated as one in the same.
Much like a sound level meter measures noise levels, the vibration monitoring instrument is capable of measuring and indicating the current acceleration level as well as minimum, maximum and average levels over time. Table 1 demonstrates some analogies between noise and vibration measurement.
Each of the values should be measured and recorded for all three axes. It is likely that one of the three axis measurements will be significantly greater than the other two due to the balancing characteristics of the tool. Some guidelines like the 2004 ACGIH TLVs prescribe exposure limits based on the most-dominant-axis reading. Whichever axis measures the highest acceleration rate is the one that should be used for determining exposure limits. ACGIH prescribes exposure time limits based on the magnitude of acceleration levels output by the tool.
An additional common HAV measurement is referred to as the "vibration total value." Vibration total value is a single-number summation of the three individual axis measurements. Vibration total value is calculated as the square root of the sum of the squares of each axis measurement:
Asum = ? Ax2 + Ay2 + Az2
Vibration total values, sometimes called "sum channel" measurements, are typically determined automatically by the monitoring instrument. The very latest HAV exposure laws like the new European Directive on hand-arm vibration exposure prescribe single number limits based on the vibration total value. The European Directive establishes 2.5 m/s2 as the action limit and 5.0 m/s2 as the maximum exposure limit for all EU member countries. Similar to the action level in OSHA's noise standard, the EU's action limit for HAV is the trigger for commencement of medical surveillance and other activities. The exposure limit for vibration similarly corresponds to OSHA's PEL for noise.
Who Prescribes Exposure Boundaries
Two examples of sources for exposure boundaries have already been discussed. At present, there are no exposure standards in place by any organization having enforcement authority within the United States. ACGIH's guidelines do serve as best practice standards. Europe has been most active in researching and promulgating exposure standards enforceable by their many regional health & safety agencies. The absence of OSHA standards should not be viewed as justification for inaction. Considerable expert research exists to substantiate the relationship between exposure and illness. OSHA's General Duty clause provides OSHA with enforcement authority. Settlements for HAVS are growing in frequency and value. Table 2 summarizes some settlements in recent years within the UK. This presents a challenge for some multinational companies with operations inside Europe. Responding with prevention programs within their European operations without addressing like operations in unregulated regions may prove negligent in years to come.
What to Do if Exposures Exceed Boundaries
Discovering that hand tools are operating outside prescribed boundaries does not automatically mean the tools must be replaced. Several actions are possible to avoid immediate and costly replacement of tools as a first step. These include:
1. Observe the tool in use very carefully. Is it being used correctly? Misuse can result in adverse vibration levels.
2. Check the tool maintenance records. When was it last serviced or overhauled? Normal wear and tear on operating components can lead to adverse vibration levels.
3. Based on measured vibration levels, how long can the tool be used and still be within the prescribed exposure boundaries? How does this compare to the time required per day? Perhaps the tools can be labeled or color-coded as to their safe usage time.
In summary, a great deal of valuable information and the tools to apply that information exists today to prevent HAVS. While specific exposure regulations do not exist in the U.S. at this time, the health risks associated with exposure and the business risks of not managing those exposure are each substantial. To avoid these risks, a careful examination of the usage of powered hand tools is required.
Cliff Wolcott is vice president, Marketing, for Quest Technologies Inc., Oconomowoc, Wis. Quest Technologies manufactures monitoring instrumentation and software for occupational and environmental health & safety applications. He can be reached at [email protected].