24-hour pattern of work-related injury risk of French firemen: nocturnal peak time

The first aim of the study was to assess clock-time patterning of work-related injuries (WRIs) of firemen (FM) of Saône et Loire-71 (France) during the 4-yr span of 1 January 2004 to 31 December 2007. FM of this service are legally required to log every WRI and seek its evaluation by the medical service, whether the WRI was the result of worksite duties or exercise/sport activities at the station. WRI was defined specifically as a (nonexercise, nonsport, and nonemotional/stress) work-associated trauma, verified both by log book and medical records. For the corresponding years, the 24-h pattern of emergency calls (Calls) plus road traffic (Traffic) on the main roads of the service area was also assessed. Relative risk (R) of WRI was calculated as the quantity of WRIs/h divided by the quantity of Call responses/h × 1000, which takes into account the number of at-risk FM/unit time, since each dispatched emergency vehicle is staffed with 4 FM. Comparably trained regular (RFM) and volunteer (VFM) FM experienced a total of 187 WRIs. The 24-h WRI curve patterns of RFM and VFM were correlated (r = 0.4, p < .05), with no histogram difference (p > .05). Analysis of variance (ANOVA) validated comparable clock-time patterns in WRIs of RFM and VFM each year and each season (all p < .0001). Thus, time series of the RFM and VFM were pooled, revealing a statistical significant 24-h variation in WRIs (ANOVA, p > .0006; Cosinor analysis, p < .0001), with peak at 16:00 h and trough at 04:00 h. The 24-h pattern in Traffic, which mirrors that of human activity, with peak ∼18:00 h and trough ∼03:00 h, was also verified (ANOVA, p < .0001; Cosinor, p < .0001). Calls (n = 112,059) resulting in FM responses also exhibited statistically significant 24-h variation, with peak at ∼20:00 h and trough at ∼06:00 h. The 24-h pattern of R showed a nocturnal peak at 02:00 h (R = 2.87 ± 0.46; mean ± SEM) and diurnal trough 14:00 h (R = 1.30 ± 0.05) (t test, p < .02); clock-time-related changes in R were further validated by ANOVA (p = .0001) and Cosinor (p < .0001), with acrophase (peak time, Ø) of 02:43 h ± 68 min (SD). The second aim of the study was to evaluate the relationship between the 24-h patterns of WRIs and lag-time (LT) response (used as a measure of work performance) of FM of the same service to urgent medical calls for out-of-hospital cardiac arrests. Highest R of WRI at 02:00 h corresponded closely to longest LT (raw data at ∼02:00 h and Cosinor derived Ø of 02.54 h ± 71 min [SD]), thereby supporting the hypothesis of a common mechanism underlying the two 24-h profiles. A third aim was to determine the relevance of a new concept in work safety, "chronoprevention," for future FM training programs.