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Increased injuries on night shift
Increased
injuries on night shift
Summary
Methods
Concern over safety has resulted in legislation by, for example, the Commission of the European Union, to limit the number of hours worked at night. There is, however, no direct evidence that injuries are more frequent at night. We analysed all 4645 injury incidents reported for a year on a rotating three-shift system in a large engineering company where the a-priori accident risk appeared to be constant. The relative risk of sustaining an injury was 1·23 (95% Cl 1·14-1·31) higher on the night shift than on the morning shift, which showed the lowest incidence. The type of work (machine versus selfpaced) affected the pattern and nature of injuries; for self-paced work the relative risk of more serious injury on the night shift compared with the morning shift was 1 82
Injury accident records can be a rich source of data.9 We modified the forms at a large engineering company where two production sites worked at different ends of the same assembly line. In addition to data on the shift system and shift being worked, information on whether the work being carried out was self-paced (off-track) or machine-paced (on-track) at the time of an injury was also determined because the opportunity to control the pace of one’s work may affect performance. On-duty accidents resulting in injury incurred by about 4250 shiftworkers were recorded for 12 months. The workers (95-5% were men) had a mean age of 30-7 (SD 87); shiftwork experience was 3-7 (SD 3-9) years, and the absenteeism rate was about 6%. The shift system had a weekly rotation of three 8 h shifts, the changeover times being at 0600, 1400, and 2200. The system provided a constant level of manning across the three shifts of 1417 workers from 0600 on Monday to 0600 on Friday. Every worker, over the 12 months, will have worked roughly the same number of shifts starting at the three times. Every shift team included 1 or more employees to cover absences.
(1·30-2·34).
Introduction There is an increased awareness that circadian rhythms in many of our physiological and psychological processes are at a low ebb during the night.1 The serious industrial accidents at Chernobyl, Three Mile Island, and Bhopal all happened at night. One justification for limiting night work is that safety may be compromised1,3yet there is no direct evidence to substantiate this.4-6 Indirect evidence comes from laboratory studies of non-shiftworkers that show performance and alertness to be reduced at night;1,7 from a few studies that show that shiftworkers are slower or less accurate at night ;2,6,7 and from data on single-vehicle road accidents, which are much more likely at night, often due to drivers falling asleep at the whee1.2,8 Although the indirect evidence does support the suggestion that safety during night shifts may be compromised we clearly need direct evidence. Obtaining this requires large numbers of injuries in a setting where the probability of an accident is, at least on average, constant across the different shifts. Shiftworking tends not to meet these criteria. For example, long runs of a job are often saved for the night shift, and maintenance or supervisory personnel are frequently present during the day only.4,6 Further, the numbers present are often higher during the day due to additional personnel, and the injury report may fail to record the work schedule of those involved.
Body Rhythms and Shiftwork Centre, Department of Psychology, University of Wales Swansea, Swansea SA2 8PP, UK (LSmith PhD, S Folkard DSc); and Occupational Health Department, Dudley Health Authority (C J M Poole MD) Correspondence to: Dr L Smith
Table 1: Shift
injuries and risk ratios relative to morning shift
20% of shiftworkers reported between 2 and 5 injuries over the period of the study. These repeat injuries were evenly spread across shifts and over the year. For example, an individual sustaining 5 injuries reported 1 for the morning shift in early January, 1 for the night shift in mid-April, 1 for the morning shift in late May, another for the afternoon shift in late August, and a final 1 for the night shift in early December. Similarly, those who reported 2 injuries tended to have sustained each injury several weeks or months apart and generally on different shifts. There were 642 injuries recorded for Fridays but there was no night shift that day and the morning and afternoon shifts were shorter than the ones on Monday to Thursday, so analyses were confined to injuries between 0600 on Monday and 0600 on Friday, ensuring equal representation of the three shifts. Supervisory and maintenance personnel worked the same shift system, as did the occupational health nurses who recorded the injury information and were trained to classify the injuries. 1% of injuries were "serious" (eg, requiring sutures, crush injuries, or broken bones) and these were combined with the 9% "moderate" injuries (eg, cuts requiring plasters or steristrips) for the analyses. The "minor" injuries (90°.0) included bruises and foreign bodies in the eye that could be washed out. The timing (relative to start time) and duration of breaks was constant across the three shifts at both sites, but differed between sites. However, the total break duration per shift was the same. The analyses were based only on injuries during work time and the 295 injuries during breaks (an average of 0 104 per break over the year) were omitted. The plant closed down for 2 weeks in the summer-ie, holidays were taken at the same time by all employees. The product changed in minor ways during the data collection period but these changes would have affected shifts similarly because the total production time for each unit was about 20 h. Environmental factors such as lighting, noise, heat, and humidity
1137
Table 2:
Injuries and RRs relative to morning shift for period, site, severity, body location, and beginning versus end of week
similar at both sites and were fairly constant across the three shifts and at all work stations. The on-track workers typically had a cleaner working environment than those off-track. The nature of the work carried out was similar for both sets of workers-namely, continual repetitive movements leading to the progressive assembly of the product. For both on-track and off-track workers, there was ample opportunity to interact but the team structure on the assembly line allowed more interaction. The drinking of alcohol at work was strictly forbidden. Thus it would appear that the a-priori probability of an accident was relatively constant across the three shifts. Hierarchical log-linear analyses were used to examine a total of 4645 accidents. We calculated relative risk ratios (RRs) and 95% CI. Data were missing for the analysis of some accident records, thus the total counts in tables 1-4 may not tally exactly with the total number recorded. The statistical test used in the log-linear analyses was X2, usually with 2 degrees of freedom.
were
Results There was a significant increase in the frequency of injuries from the morning shift, through the afternoon shift, to the night shift (p < 0.001; table 1). There was no evidence that the morning-to-night injury pattern differed between the first and second halves of the data collection period; nor was there a difference between the two sites. Further analyses indicated that neither the severity of the injury (minor vs moderate/serious) nor whether it was to the hand (the most common) or another part of the body affected the pattern. Injury counts and RRs for these non-significant interactions are given in table 2. Although the variation across shifts appeared to be consistent it did seem to differ according to whether the
Table 3: Injuries for on-track and off-track work and
Table 4:
1138
Injuries and
RR relative to
more
serious
work
being done was machine-paced or self-paced (figure 1). Injuries to on-track workers reflected the basic pattern of a gradual increase from morning, through afternoon, to night shift (p < 0-001). However, the significant effect for those working off-track (p < 0-001) was due to significantly more injuries being recorded on the afternoon and night shifts than during the morning shift and p<0-01, (p<0-01 respectively). There was no difference between afternoon and night shifts significant for off-track workers. A greater proportion of injuries incurred by those working off rather than on-track was classified as moderate/serious (p<0-01), presumably of on the more nature their jobs. This reflecting dangerous increased proportion of more severe off-track injuries varied greatly across the three shifts (p<0-01). For the on-track workers the proportion of injuries classified as moderate/serious was relatively stable across the three shifts, but for the off-track workers this proportion rose substantially on the night shift (table 3, figure 2). There was a significant increase in night shift accidents from the first to the last two days of the week (p < 0-01), but no such significant change was found for the morning or afternoon shifts; nor was there any interaction between shift and first/second half of the week (p=0’07) (table 4, figure 3). Discussion
Safety has often been suggested to be compromised by shiftwork in general and by night work in particular. Although there are good grounds for assuming lower safety at night few systematic investigations of the relation
injuries, by shift
morning shift at beginning of week
u.u————jI Afternoon Morning
I
Night
Shift
Figure 1: Risk of Injury relative to the morning shift
Afternoon
Morning
Night
Shift
Figure 2:
Risk of
more
serious
Injuries relative to morning shift
between night work and injury have been done.6 Our study offers clear evidence that safety is reduced during night work at least relative to the morning shift, but can give no indication that shiftwork per se is less safe than permanent day work. To do that would require otherwise comparable permanent day and shiftworking groups doing similar jobs under much the same conditions, and such a comparison is unlikely to be achievable.
The results point to a significant increase in the frequency of injuries from the morning shift, through the afternoon shift, to the night shift. A different picture emerged when the nature of the jobs was taken into account. Off-track (self-paced) workers showed no significant increase in the number of injuries from the afternoon to the night shift, but in the off-track workers the proportion of injuries classified as moderate/serious rose on the night shift. This finding supports the view that night-time accidents may be more serious,10,11 but this effect may be limited to self-paced work. There was some evidence that the injury risk on the night shift rose towards the end of the working week. This finding seems inconsistent with any beneficial effect of circadian adjustment over successive night shifts and supports the suggestion that more rapidly rotating shift systems may be preferable. 12 However, it is unclear whether the more
complete circadian adjustment likely to occur on a permanent night shift system might result in reduced accident rates.5 Nor was the overall rise in injuries across the three shifts consistent with a simple sleep deprivation interpretation since a separate survey of a random sample of some 300 of the shiftworkers concerned indicated that their mean sleep duration was considerably shorter on the morning shift (5-95 h) than on either the night (7-46 h) or afternoon (895 h) shifts.13 It seems more likely that the increased injury rates at night reflect workers’ circadian rhythms in performance capabilities and alertness, which had failed to adjust sufficiently to the night shift. Under otherwise constant conditions accidents resulting in injury were more frequent in machine-paced workers at night in our study; self-paced work injuries were more serious at night; and night shift injuries may have been more prevalent towards the end of the week. The causes for this reduced work safety at night need to be elucidated but limitations on night work, especially where there may be a risk to the public, seem appropriate.
References 1
2
3 4 5
6
7
8 9
10
0.yo Afternoon Morning Night Shift Figure 3: Risk of injury relative to morning shifts at beginning of week
11 12 13
Commission of the European Communities. Directive on working time. Off J Eur Commun 1993; 93/104/EEC. Mitler MM, Carskadon MA, Czeisler CA, Dement WC, Dinges DF, Graeber RC. Catastrophes, sleep and public policy. Sleep 1988; 11: 100-09. Folkard S. Circadian performance rhythms: some practical and theoretical implications. Phil Trans R Soc Lond 1990; B327: 543-53. Harrington JM. Shift work and health: a critical review of the literature. London: HM Stationery Office, 1978. Waterhouse JM, Folkard S, Minors DS. Shiftwork, health and safety: an overview of the scientific literature 1978-1990. London: HM Stationery Office, 1992. Carter FA, Corlett EN. Shiftwork and accidents. Dublin: European Foundation for the Improvement of Living and Working Conditions, 1982. Folkard S, Monk TH. Shiftwork and performance. Hum Factors 1979; 21: 483-92. Lavie P. The 24-hour sleep propensity function (SPF). In Monk TH, ed. Sleep, sleepiness and performance. Chichester: Wiley, 1991: 65-93. Drury CG. The use of archival data. In: Wilson JR, Corlett EN, eds. Evaluation of human work: a practical ergonomics methodology. London: Taylor Francis, 1990: Ong CN, Phoon WO, Iskandar N, Chia KS. Shiftwork and work injuries in an iron and steel mill. Appl Ergon 1987; 18: 51-56. Anon. Statistics and news. Bull Eur Stud Time 1993; 6: 46-50. Knauth P. The design of shift systems. Ergonomics 1993; 36: 15-28. Barton J, Folkard S, Smith L, Poole CJM. Effects on health of a change from a delaying to an advancing shift system. Occu Envir Med
(in press).
1139
injuries on night shift
Summary
Methods
Concern over safety has resulted in legislation by, for example, the Commission of the European Union, to limit the number of hours worked at night. There is, however, no direct evidence that injuries are more frequent at night. We analysed all 4645 injury incidents reported for a year on a rotating three-shift system in a large engineering company where the a-priori accident risk appeared to be constant. The relative risk of sustaining an injury was 1·23 (95% Cl 1·14-1·31) higher on the night shift than on the morning shift, which showed the lowest incidence. The type of work (machine versus selfpaced) affected the pattern and nature of injuries; for self-paced work the relative risk of more serious injury on the night shift compared with the morning shift was 1 82
Injury accident records can be a rich source of data.9 We modified the forms at a large engineering company where two production sites worked at different ends of the same assembly line. In addition to data on the shift system and shift being worked, information on whether the work being carried out was self-paced (off-track) or machine-paced (on-track) at the time of an injury was also determined because the opportunity to control the pace of one’s work may affect performance. On-duty accidents resulting in injury incurred by about 4250 shiftworkers were recorded for 12 months. The workers (95-5% were men) had a mean age of 30-7 (SD 87); shiftwork experience was 3-7 (SD 3-9) years, and the absenteeism rate was about 6%. The shift system had a weekly rotation of three 8 h shifts, the changeover times being at 0600, 1400, and 2200. The system provided a constant level of manning across the three shifts of 1417 workers from 0600 on Monday to 0600 on Friday. Every worker, over the 12 months, will have worked roughly the same number of shifts starting at the three times. Every shift team included 1 or more employees to cover absences.
(1·30-2·34).
Introduction There is an increased awareness that circadian rhythms in many of our physiological and psychological processes are at a low ebb during the night.1 The serious industrial accidents at Chernobyl, Three Mile Island, and Bhopal all happened at night. One justification for limiting night work is that safety may be compromised1,3yet there is no direct evidence to substantiate this.4-6 Indirect evidence comes from laboratory studies of non-shiftworkers that show performance and alertness to be reduced at night;1,7 from a few studies that show that shiftworkers are slower or less accurate at night ;2,6,7 and from data on single-vehicle road accidents, which are much more likely at night, often due to drivers falling asleep at the whee1.2,8 Although the indirect evidence does support the suggestion that safety during night shifts may be compromised we clearly need direct evidence. Obtaining this requires large numbers of injuries in a setting where the probability of an accident is, at least on average, constant across the different shifts. Shiftworking tends not to meet these criteria. For example, long runs of a job are often saved for the night shift, and maintenance or supervisory personnel are frequently present during the day only.4,6 Further, the numbers present are often higher during the day due to additional personnel, and the injury report may fail to record the work schedule of those involved.
Body Rhythms and Shiftwork Centre, Department of Psychology, University of Wales Swansea, Swansea SA2 8PP, UK (LSmith PhD, S Folkard DSc); and Occupational Health Department, Dudley Health Authority (C J M Poole MD) Correspondence to: Dr L Smith
Table 1: Shift
injuries and risk ratios relative to morning shift
20% of shiftworkers reported between 2 and 5 injuries over the period of the study. These repeat injuries were evenly spread across shifts and over the year. For example, an individual sustaining 5 injuries reported 1 for the morning shift in early January, 1 for the night shift in mid-April, 1 for the morning shift in late May, another for the afternoon shift in late August, and a final 1 for the night shift in early December. Similarly, those who reported 2 injuries tended to have sustained each injury several weeks or months apart and generally on different shifts. There were 642 injuries recorded for Fridays but there was no night shift that day and the morning and afternoon shifts were shorter than the ones on Monday to Thursday, so analyses were confined to injuries between 0600 on Monday and 0600 on Friday, ensuring equal representation of the three shifts. Supervisory and maintenance personnel worked the same shift system, as did the occupational health nurses who recorded the injury information and were trained to classify the injuries. 1% of injuries were "serious" (eg, requiring sutures, crush injuries, or broken bones) and these were combined with the 9% "moderate" injuries (eg, cuts requiring plasters or steristrips) for the analyses. The "minor" injuries (90°.0) included bruises and foreign bodies in the eye that could be washed out. The timing (relative to start time) and duration of breaks was constant across the three shifts at both sites, but differed between sites. However, the total break duration per shift was the same. The analyses were based only on injuries during work time and the 295 injuries during breaks (an average of 0 104 per break over the year) were omitted. The plant closed down for 2 weeks in the summer-ie, holidays were taken at the same time by all employees. The product changed in minor ways during the data collection period but these changes would have affected shifts similarly because the total production time for each unit was about 20 h. Environmental factors such as lighting, noise, heat, and humidity
1137
Table 2:
Injuries and RRs relative to morning shift for period, site, severity, body location, and beginning versus end of week
similar at both sites and were fairly constant across the three shifts and at all work stations. The on-track workers typically had a cleaner working environment than those off-track. The nature of the work carried out was similar for both sets of workers-namely, continual repetitive movements leading to the progressive assembly of the product. For both on-track and off-track workers, there was ample opportunity to interact but the team structure on the assembly line allowed more interaction. The drinking of alcohol at work was strictly forbidden. Thus it would appear that the a-priori probability of an accident was relatively constant across the three shifts. Hierarchical log-linear analyses were used to examine a total of 4645 accidents. We calculated relative risk ratios (RRs) and 95% CI. Data were missing for the analysis of some accident records, thus the total counts in tables 1-4 may not tally exactly with the total number recorded. The statistical test used in the log-linear analyses was X2, usually with 2 degrees of freedom.
were
Results There was a significant increase in the frequency of injuries from the morning shift, through the afternoon shift, to the night shift (p < 0.001; table 1). There was no evidence that the morning-to-night injury pattern differed between the first and second halves of the data collection period; nor was there a difference between the two sites. Further analyses indicated that neither the severity of the injury (minor vs moderate/serious) nor whether it was to the hand (the most common) or another part of the body affected the pattern. Injury counts and RRs for these non-significant interactions are given in table 2. Although the variation across shifts appeared to be consistent it did seem to differ according to whether the
Table 3: Injuries for on-track and off-track work and
Table 4:
1138
Injuries and
RR relative to
more
serious
work
being done was machine-paced or self-paced (figure 1). Injuries to on-track workers reflected the basic pattern of a gradual increase from morning, through afternoon, to night shift (p < 0-001). However, the significant effect for those working off-track (p < 0-001) was due to significantly more injuries being recorded on the afternoon and night shifts than during the morning shift and p<0-01, (p<0-01 respectively). There was no difference between afternoon and night shifts significant for off-track workers. A greater proportion of injuries incurred by those working off rather than on-track was classified as moderate/serious (p<0-01), presumably of on the more nature their jobs. This reflecting dangerous increased proportion of more severe off-track injuries varied greatly across the three shifts (p<0-01). For the on-track workers the proportion of injuries classified as moderate/serious was relatively stable across the three shifts, but for the off-track workers this proportion rose substantially on the night shift (table 3, figure 2). There was a significant increase in night shift accidents from the first to the last two days of the week (p < 0-01), but no such significant change was found for the morning or afternoon shifts; nor was there any interaction between shift and first/second half of the week (p=0’07) (table 4, figure 3). Discussion
Safety has often been suggested to be compromised by shiftwork in general and by night work in particular. Although there are good grounds for assuming lower safety at night few systematic investigations of the relation
injuries, by shift
morning shift at beginning of week
u.u————jI Afternoon Morning
I
Night
Shift
Figure 1: Risk of Injury relative to the morning shift
Afternoon
Morning
Night
Shift
Figure 2:
Risk of
more
serious
Injuries relative to morning shift
between night work and injury have been done.6 Our study offers clear evidence that safety is reduced during night work at least relative to the morning shift, but can give no indication that shiftwork per se is less safe than permanent day work. To do that would require otherwise comparable permanent day and shiftworking groups doing similar jobs under much the same conditions, and such a comparison is unlikely to be achievable.
The results point to a significant increase in the frequency of injuries from the morning shift, through the afternoon shift, to the night shift. A different picture emerged when the nature of the jobs was taken into account. Off-track (self-paced) workers showed no significant increase in the number of injuries from the afternoon to the night shift, but in the off-track workers the proportion of injuries classified as moderate/serious rose on the night shift. This finding supports the view that night-time accidents may be more serious,10,11 but this effect may be limited to self-paced work. There was some evidence that the injury risk on the night shift rose towards the end of the working week. This finding seems inconsistent with any beneficial effect of circadian adjustment over successive night shifts and supports the suggestion that more rapidly rotating shift systems may be preferable. 12 However, it is unclear whether the more
complete circadian adjustment likely to occur on a permanent night shift system might result in reduced accident rates.5 Nor was the overall rise in injuries across the three shifts consistent with a simple sleep deprivation interpretation since a separate survey of a random sample of some 300 of the shiftworkers concerned indicated that their mean sleep duration was considerably shorter on the morning shift (5-95 h) than on either the night (7-46 h) or afternoon (895 h) shifts.13 It seems more likely that the increased injury rates at night reflect workers’ circadian rhythms in performance capabilities and alertness, which had failed to adjust sufficiently to the night shift. Under otherwise constant conditions accidents resulting in injury were more frequent in machine-paced workers at night in our study; self-paced work injuries were more serious at night; and night shift injuries may have been more prevalent towards the end of the week. The causes for this reduced work safety at night need to be elucidated but limitations on night work, especially where there may be a risk to the public, seem appropriate.
References 1
2
3 4 5
6
7
8 9
10
0.yo Afternoon Morning Night Shift Figure 3: Risk of injury relative to morning shifts at beginning of week
11 12 13
Commission of the European Communities. Directive on working time. Off J Eur Commun 1993; 93/104/EEC. Mitler MM, Carskadon MA, Czeisler CA, Dement WC, Dinges DF, Graeber RC. Catastrophes, sleep and public policy. Sleep 1988; 11: 100-09. Folkard S. Circadian performance rhythms: some practical and theoretical implications. Phil Trans R Soc Lond 1990; B327: 543-53. Harrington JM. Shift work and health: a critical review of the literature. London: HM Stationery Office, 1978. Waterhouse JM, Folkard S, Minors DS. Shiftwork, health and safety: an overview of the scientific literature 1978-1990. London: HM Stationery Office, 1992. Carter FA, Corlett EN. Shiftwork and accidents. Dublin: European Foundation for the Improvement of Living and Working Conditions, 1982. Folkard S, Monk TH. Shiftwork and performance. Hum Factors 1979; 21: 483-92. Lavie P. The 24-hour sleep propensity function (SPF). In Monk TH, ed. Sleep, sleepiness and performance. Chichester: Wiley, 1991: 65-93. Drury CG. The use of archival data. In: Wilson JR, Corlett EN, eds. Evaluation of human work: a practical ergonomics methodology. London: Taylor Francis, 1990: Ong CN, Phoon WO, Iskandar N, Chia KS. Shiftwork and work injuries in an iron and steel mill. Appl Ergon 1987; 18: 51-56. Anon. Statistics and news. Bull Eur Stud Time 1993; 6: 46-50. Knauth P. The design of shift systems. Ergonomics 1993; 36: 15-28. Barton J, Folkard S, Smith L, Poole CJM. Effects on health of a change from a delaying to an advancing shift system. Occu Envir Med
(in press).
1139