Updating the “Risk Index”: A systematic review and meta-analysis of occupational injuries and work schedule characteristics

This systematic review & meta-analysis evaluated the evidence surrounding the risk of occupational injuries relative to work scheduling.

[Note: This study is an update on previous studies the authors published many years ago. Although there’s more recent studies not included in this data-set, I like how Simon Folkard presents the data in his line of studies.]

29 high-quality studies were included in the analysis.

The data is presented as risk ratios (relative risks). Say the risk of an injury over the course of a 9 to 5, Mon to Fri shift schedule is presented as 1, and the risk of changing to an afternoon shift is now 1.5, then that group is 1.5 times the risk to experience an accident (50% higher than day shift). A change from a RR of 1 to 2 means 2 times the risk of an injury (100% increase, or a doubling of injury risk). Values lower than 1 indicate a lesser chance of injury.

Results

The RR of an injury/accident wasn’t elevated between morning shifts or afternoon/evening shifts. However, risk of accident increased by 33% on nightshift relative to morning.

When evaluating age, only the workers’ age (≤ 20 yrs vs. > 20 yrs) was found to be statistically significant, where adolescent workers ≤ 20 yrs showed a lower RR on afternoon compared to morning shifts; but not night shift.

Compared to the first shift in a series of consecutive shift, RR “rose exponentially” for morning shifts and night shifts as the succession of shifts passed (e.g. RR of 1.05 on the second shift and 1.36 by the fourth shift; graph not shown). However, the “trends observed for morning shifts and night shifts were however non-significant except for a significantly increased risk on the fourth night shift by 36%. Removing the highest and lowest estimates did not change the findings” (p10).

Hours on duty (graph c) and shift length (graph not shown) were next evaluated. Found was that compared to the 8th hour at work (generally set as the baseline risk of 1), RR wasn’t significantly elevated between hours 1 to 9, but “appeared slightly increased between the 2nd and 5th hour on duty” (p10).

Beyond the ninth hour at work, RR rose exponentially and almost tripled after the 12th hour at work (RR of 2.73 at the 12 hour mark).

Quoting the paper, these trends “were mirrored when looking at shift length (e.g., using meta-analysis to synthesize the risk estimates for hours 1 to 8 into an 8-h shift length, the estimates for hours 1 to 10 into a 10-h shift length, etc.): relative risk for shifts longer than 12 h increased by 34% (RR = 1.34 [1.04–1.51]) (graph not shown)” (p10). 

Two aspects of rests breaks were evaluated: total duration of rest breaks and interval between any two rest breaks (called “time on task or minutes since last break”).

RR for injury/accident significantly decreased for any rest break duration (graph not shown). RR increased significantly for every additional half hour spent working (without a break) relative to the first 30 minute working period (e.g. 90 – 119 mins working since the first 30 mins had a RR of 1.62).

Next, they explored the interaction of rest break duration & intervals. They found that relatively short rest breaks (< 1 hr) had a significant effect on injury risk such that “the more frequent the breaks, the lower the relative risk” (p12). However, while break frequency had a pronounced effect, the break duration has less effect such that there was practically no effect on break durations when added up to 2 hrs (i.e. short & frequent rest breaks are likely the most protective rest break characteristics when accounting for productivity).

The key findings are then summarised:

·        Injury risk increased by 36% on night shifts compared to morning shifts.

·        Injury risk on afternoon/evening shifts increased by 12% relative to morning, but this wasn’t statistically significant.

·        Number of hours on duty significantly increased injury risk > 9th hour exponentially and “almost tripled it after the 12th hour at work resulting in a 34% increase for shifts longer than 12 hours” (p12).

·        For number of consecutive shifts, RR wasn’t significantly elevated for consecutive morning shifts but injury risk was elevated by the 4th consecutive night shift (36% increase).

·        Rest breaks—both frequency & duration—were protective of injuries, but more pronounced with frequency such that injury risk increased with every additional half hour spent working relative to the first 30 min of work (but it’s said there was a paucity of data here, so caution is needed).

Link in comments.

Authors: Dorothee Fischer, David A. Lombardi, Simon Folkard, Joanna Willetts & David C. Christiani, 2017, Chronobiology International