Web edition: May 26, 2009
Over the long Memorial Day weekend, legions of do-it-yourselfers plugged in power tools to tackle home projects. I was among the many. One clean-up project required brief use of a sander. And as its vibrations sent a tingling up to my elbows, I was reminded that these time-saving tools must be used with care. Overusing some of them — including my sander — has the potential to cause serious damage.
It’s something that’s not warned about on the label. But that may change in the not too distant future. Studies being conducted in labs at the National Institute of Occupational Safety and Health, and elsewhere, are quantifying cardiovascular risks. Not heart risks. But risks to small vessels in the extremities.
I learned about this, last month, at the Experimental Biology meeting in New Orleans. Kristine Krajnak at NIOSH’s facility in Morgantown, W. Va., sat down to discuss her animal studies with me. For many years, this research biologist notes, there had been an assumption that the whole-body, spine-rattling vibrations associated with jackhammers and other high-impact tools were the big safety concern. But epidemiological studies — and her new lab experiments — suggest that higher-frequency vibrations can trigger subtle damage that may actually kill tissue.
The chief symptom is something known as vibration-induced white finger, a condition where the tips of the fingers take on a frost-bitten pallor. Explains Krajnak, certain types of vibrations can induce “blood vessels to constrict so that people lose blood flow to the periphery, predominantly in the hands.” Over time, if this happens too often and for too long, cells in the blood-starved regions will die. A goal of her lab has been to identify which vibrations pose the biggest risk.
Her guinea pig is the rat. Or, more precisely, a rat’s tail, she explains, “because its response to vibrations is very, very similar to the physical response of the human finger.”
A caged animal’s tail is strapped to a platform that vibrates. At low frequencies, in the 30 hertz (cycles per second) range, the displacement (up-and-down and side-to-side movements) are greater than at higher frequencies, but “the tail tends to move in unison with the platform.” Once the frequency begins to approach the sander’s 100 Hz range, “the tissue can no longer move that fast,” she notes; “It begins to move out of phase with the platform.” Periodically, then, the tail begins smashing into the platform.
And that subtle hammering of the tail — or a woodworker’s fingers — is, presumably, what causes tissue damage, Krajnak says.
To quantify effects, rat’s tails were exposed to a range of frequencies for four hours a day, 5 days a week for two weeks. “Then we looked at markers that we knew were indicative of early changes in cardiovascular function,” she says. “And we found that certain markers tend to go up in all of the animals, regardless of what the vibration frequency is.” These are markers of things associated with cell repair, such as metallothionine. Their increases suggest, she says, that arteries are sustaining damage and attempting to patch up the problem.
At 63 Hz, which is sort of a medium vibration frequency, compounds indicative of inflammation begin to develop, such as interleukin-1-beta. At the upper range of test frequencies, such as 250 Hz, the inflammatory indicators are replaced by a huge spike in markers of oxidative stress, such as hydrogen peroxide and nitrotyrosine.
And where this oxidative stress occurs, the vessels become overly sensitive — or hyperresponsive — to a neurotransmitter that signals stress: noradrenaline. Affected vessels become especially prone to constricting.
“We knew in humans that at exposure to higher frequencies, blood flow goes down.” Moreover, she notes, that constriction tends to persist longer where the exposure to high-frequency vibrations is chronic. The new rat-tail experiments, she reported at the Experimental Biology meeting, “now are telling us why that’s happening.”
Another observation from the rat: After exposure to high-frequency vibrations, vessels lose their propensity to quickly dilate — or un-constrict. “Normally, if you get vessel constriction, dilating factors [in the body] will try to open that vessel back up again,” she explains. But the high-frequency vibrations appear to be “shifting the body over to a regime that tries to maintain a smaller vessel.”
Would a right-handed worker protect his or her right hand by using the left hand half of the time? Perhaps not, Krajnak says. “We’ve seen in experiments with people that the strongest [vasoconstricting] effect is in the exposed hand.” But researchers can measure a similar, albeit diminished, effect in the unexposed hand. In fact, people who develop vibration-induced white finger can even develop circulation problems in their feet, although these appendages were never exposed. So at some level, the cardiovascular damage can be systemic, at least in the extremities.
The Occupational Safety and Health Administration and regulatory agencies elsewhere around the world have been keeping an eye on these and related studies, Krajnak says. There’s been a strong suspicion that safety standards may need to be adjusted to account for the differential sensitivity of the body to different frequencies. But regulators can’t do much without the data to document which frequencies present what problems and at which intensities or over what duration of exposure.
One encouraging development for power-tool users: Krajnak notes that a new industry is developing to provide protective gear, such as gloves or vibration dampening handles for tools. So there’s hope that workers may not have to trade off cardiovascular health for a long and productive career.
Krajnak, K., et al. 2009. Vibration-induced changes in oxidative stress and expression of inflammatory factors are frequency dependent (Abst. #: 592.1). Experimental Biology '09: New Orleans (April 19).