How to calculate daily hand-arm vibration exposure
Long hours on a breaker, grinder or impact wrench can damage the nerves and blood vessels of the hand, a condition known as hand-arm vibration syndrome. Risk depends not only on how rough the tool feels but on how many hours a day the trigger is actually pulled.
Exposure is therefore normalized to eight hours: A(8) = √(Σ vibration² × hours ÷ 8). A 5 m/s² tool used for four hours gives an A(8) of about 3.5 m/s², lower than the 5 m/s² it would produce over a full shift. When several tools are used, each contribution is added as a square.
| Total daily exposure | ACGIH TLV, dominant axis |
|---|---|
| 4 hours to less than 8 | 4 m/s² |
| 2 hours to less than 4 | 6 m/s² |
| 1 hour to less than 2 | 8 m/s² |
| Less than 1 hour | 12 m/s² |
Catalog vibration values come from standardized tests and real work often runs higher, so treat a result close to the limit as a result over it. Where exposure exceeds the TLV, rotate trigger time, move to lower vibration tools, add anti-vibration gloves and scheduled breaks, and refer workers reporting numbness or blanching for medical evaluation. A formal assessment should use measured data reviewed by an industrial hygienist.
Frequently asked questions
A(8) is the daily vibration exposure normalized to an 8-hour reference. For several tools, multiply each vibration value squared by its trigger time, add them, divide by 8 and take the square root.
Use the vibration acceleration listed on the tool data sheet or test report. Real job conditions often run higher than the test value, so leave margin when judging the result.
Rotate trigger time, switch to lower vibration tools, add anti-vibration gloves and scheduled breaks. Numbness or blanched fingers calls for medical evaluation as well as an exposure survey.