Should you count shift work in estimations?

Question: When we estimate how much business to expect from a client, a major factor is how many on-the-job injuries will be typical for that employer. If we have no experience with them and they don’t have sufficient records for an estimate, we estimate using standard injury rates in that industry. But I’m curious about the effects of shift work. Do workers on the second and third shifts suffer more injuries?

Answer: Yes, shift work is an important factor when estimating how much business to expect from a client. The most pertinent research is a 1994 study from the University of Wales in Great Britain showing there definitely is a higher risk of injury on the second and third shifts.1

Lawrence Smith, MD, and others at the university studied the injury rates at a manufacturing site where employees worked around the clock. The shifts were not exactly the same as the most common shifts in the United States, but they were similar to the American first, second, and third shifts. The first shift at the factory started at 6 a.m. and ended at 2 p.m. The second was 2 p.m. to 10 p.m., and the third was 10 p.m. to 6 a.m.

The researchers analyzed 4,645 injuries over a one-year period. They found there was a significant increase in the frequency of injuries from the morning shift to the afternoon shift, peaking in the night shift. Of all the injuries in the study period, 29.6% occurred on the morning shift, 34% on the afternoon shift, and 36.4% on the night shift. Using the morning shift as a baseline, the afternoon shift had a 15% greater risk of injury, and the night shift had a 23% greater risk.

The study also showed that the effect was even greater when looking at only serious injuries, defined as those requiring stitches or involving broken bones and crush injuries. There were 14% more serious injuries on the afternoon shift than the morning shift, and 43% more on the night shift than the morning shift.

Two variables in the work were noteworthy. Night shift injuries were more likely to occur to workers whose work pace was controlled by a machine, such as an assembly line, than to workers who controlled their own pace. Night shift injuries also occurred more at the end of the work week, possibly due to the cumulative effects of lost sleep.

The researchers concluded that employers should limit night shift work when possible, particularly when the work is inherently dangerous or when accidents could threaten the public.

(For more on how to estimate the amount of business you may get from a client, see related story in Occupational Health Management, March 1997, p. 44.)


1. Smith L, Folkard S, Poole CJM. Increased injuries on night shift. Lancet 1994; 344:1,137-1,139. n