Prevention of the hand-arm vibration syndrome.

In order to determine the prevalence of the hand-arm vibration syndrome before preventive countermeasures were taken, a special health examination was carried out among 417 national forestry workers operating chain saws in the northern area of Hokkaido, Japan. Thirty-two chain-saw workers (7.7%) were diagnosed as exhibiting the vibration syndrome. The highest prevalence rates were 15.8% among the workers who had operated the saws for 11 to 15 years and 20.3% for workers in their 50s. Improved work conditions for chain-saw workers have increasingly prevented the vibration syndrome in the state forests of Japan since 1978. The present report covered the six years since 1978, and evaluated the effects of these improved work conditions on chain-saw workers from data on the recovery rates of skin temperature and the vibration sense threshold after a cold provocation test. As a result, recovery rates of skin temperature and the vibration sense thresholds at the fifth and tenth minutes after the immersion of the hands in cold water were significantly better than those six years earlier. It is suggested that adequate restrictions on the operating time of the chain saw and on the age of workers can completely prevent the vibration syndrome even if the total operating time is appreciably lengthened.

operating cycle lasts 10 min only. The operating time is less than 4 d a week, not more than 2 d continuously, less than 32 h a month, and less than 120 d a year (table 3). Improvements in the design of the chain saw have also led to a decrease in its vibration. The effects of these alterations in the work conditions of forestry work on chain-saw workers were evaluated from data showing the recovery rates of skin temperature and the vibration sense threshold after a cold provocation test (immersion of the hand in cold water). The data were collected during special health examinations over a period of six years. 6 It is well known that the hand-arm vibration syndrome occurs after the long-term operation of many kinds of vibrating tools and that cold in the work environment and noise induced by vibration tools are associated factors (4,5,6,8,9,10,11,13,14,15,16).
Large numbers of forestry workers exhibiting the syndrome have been observed in mainland Japan since 1965. We have also experienced the same phenomenon in Hokkaido. In order to determine the incidence of the hand-arm vibration syndrome before preventive countermeasures were taken, a special health examination was carried out in 1978 among 417 national forestry workers operating chain saws in the northern area of Hokkaido. Thirty-two chain-saw operators (7.7 010) were diagnosed as suffering from the handarm vibration syndrome. The distribution of the cases, classified by the scheme of the Forestry Agency, Japan (3), was 50 010 in grade I, 31.2 % in grade II, and 18.8 % in grade III (table 1). Twenty-seven of the 32 authorized patients had been treated with periodic therapy and had been prohibited from chain-saw operation prior to this special health examination. The highest prevalence rate for workers with the hand-arm vibration syndrome was 20.3 010 for those in their 50s, and 15.8 % among those who had operated chain saws for 11 to 15 years (table 2).
The real work conditions of the chain-saw workers have been improved in the state forest of Japan since 1978, so that the operators now work 2 h in the morning or 2 h in the afternoon per day, while one Reprint requests to: Dr K Saito, Department of Hygieneand Preventive Medicine, Hokkaido University School of Medicine, Nishi 7, Kita 15, Kita-ku, Sapporo 060, Japan.

Subjects and methods
One hundred and fifty-five male chain-saw workers between the ages of 24 and 53 (mean 45.1, SE 0.6) years were studied. They were surveyed for their chain-saw operating times and their subjectivesymptoms for the six years from 1978 to 1983.
Their skin temperatures, vibratory sense thresholds, recovery time of nail blood color in the nail compression test (I 7), and the pain sense of their fingers were measured annually at a room temperature of 20 to 23°e and at the fifth and tenth minute after a lO-min immersion of the hand in cold water at woe. The data obtai ned from the six-year special health examinations given to the workers were analyzed to evaluate the effect of the new work conditions since 1978.    Table 4 shows that the skin temperature of the chai nsaw worke rs at a ro om tem perat ure of 20 to 23°C did no t reveal an y significant difference by year , bu t that the reco very of sk in temperature afte r the co ld pr o vocation test a mo ng men in th eir 20s was faster th an fo r other age s.

Changes in operating time, skin temperature and vibratory sense threshold of the workers
Details of ch an ges in the reco ver y rate of skin temperature after the lO-min cold provocation test are sho wn in figure 2. T he recov ery rates at th e fifth min ut e a fter the tes t wer e significant ly bett er in 1982 a nd 1983 th an in 1978. Reco very ra tes a t th e tenth minu te aft er the test were also significantly bett er in 1982 and 1983 th an in 1978. The values of the mean ages, operating times, and recovery rates o f skin tempera tu re for the chain-s aw workers in 1983 are shown in ta ble 5.
T he vibrat or y sense thresholds o f the workers at the fifth minute after the cold provocation test were sig-  nificantly better in 1980, 1982, and 1983 than in 1978; similarly the thresholds at the tenth minute after the test in 1979, 1980, 1982, and 1983 recovered significantly (figure 3). Table 6 shows the vibratory sense thresholds of the chain-saw workers in 1983. Analysis also showed significant differences by age under conditions of room temperature and at the fifth and tenth minute after the cold provocation test. The values of the vibratory sense thresholds among the older workers were markedly lower than those of the younger workers. The recovery of vibratory sense after immersion of the hands in cold water showed a tendency to be faster than that of their skin temperature.
In any case, the recovery of skin temperature and the vibratory sense threshold among the chain-saw workers has become progressively better since 1982, three years after improvements in the work conditions.
to dispose of many trees felled by a typhoon in 1953. Since then, the increasing mechanization of forestry processes has meant the increasing use of such tools. But for 20 years the operation time of vibrating tools was not limited, so that forest workers might well have operated for as many as 6 h a day. Between 1966 and 1969, signs and symptoms of illnesses arising from the use of the power saw in forestry operations were reported from Japan, Sweden, and Australia (1, 2, 6, 7. 9). Taylor et al (16) reported that a marked increase in the prevalence of the hand-arm vibration syndrome was apparent over eight years among the chain-saw users in England. Pyykko (13) reported that in eastern Finland 40070 of I 18 lumberjacks operating chain saws were found to have traumatic vasospastic disease and that the mean latency period of the patients was 5 600 workhours and four years after the beginning of chainsaw operation. In our 1978 field study of the northern area of Hokkaido, Japan, the hand-arm vibration

Relationship oj skin temperature and vibratory sense to age and operating time
The correlation coefficients between the findings for age, operating time, and the recovery rates of skin temperature at the fifth and tenth minute after the cold provocation test are shown in table 7. There are significant correlations between age and operating time in each year from 1978 to 1983, between the recovery rates of skin temperature at the fifth and tenth minute after the cold provocation test in 1983, and between the recovery rates in every year from 1978 to 1983. The correlations between the chain-saw operating time and skin temperature recovery rates after the cold provocation test were not significant. The correlations between age and the vibratory sense thresholds before and after the cold provocation test and between the values both before and after the test were positively significant, but the correlations between the chain-saw operating time and vibratory sense threshold were not significant (table 8).

Discussion
It is recognized that the operating time of vibrating tools affects the occurrence of the vibration syndrome. The chain saw was first used in Japanese forestry work  syndrome among 417 chain-saw workers wa s found to be 7.7 0/0. The peak duration of th e operation time of the patients was from 11 to 15 years, and half the patients were over 50 years of age. O f the 417 chain-saw wo rkers, 155 were observed in special health examinations over a period of six yea rs, apart from 32 patients with th e hand-arm vibra tio n synd ro me. Their me an operation time fo r wo rk ing th e chain saws cha nged from 2 990 to 3 944 h o ver th e six-year period after the o ffici a l co untermeasures had been taken in work co nd itio ns. They did not co mpla in of a ny sym pto ms o f the hand-arm vibratio n synd ro me durin g this obser vation period .
M iya shita et al (12) ha ve reported on the relationship be tween the progress o f th e vibra tion syndro me and the tota l chain-saw operating t im e among forestry wor kers. They show ed th at a group wit h 2 000 to 5 000 h of cha in-saw operation had peripheral ne rve a nd circulatory d isturba nces, including Ra ynaud' s ph enomenon, while even the group with less than 2 000 h of expe rience complain ed of ti ng ling, numbness, o r pain and that the group with over 5 000 h of exposure su ffered severe functional , o r o rganic changes owing to vibration. No special counter measur es to prevent the hand-arm vibration synd ro me had been taken, however, until a special health examination was undertaken for an epidemiologic in vesti gation.
Our results suggest that adequate countermeasures to restrict the operating time o f cha in saw s and the age of th e wor kers as shown in table 3 can com pletely prevent the hand-arm vibration synd ro me even if the total operating time is appreciably lengthen ed .