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In the second task, participants were asked to memorize a small set of letters, and then indicate whether individually presented “probe” letters were in this mental inventory. This task is widely used in the psychological literature to measure declines in cognitive ability, and provided additional indices of response orientation and reaction time, as well as a measure of the time needed to scan a mental inventory.
Error rates in the window-memory task show the effects of increasing call volumes the most clearly. Figure 1 shows an increasingly severe impact of call volumes after 10 calls per shift is exceeded. Indeed, for firefighters who had been on 17 calls (a situation frequently encountered and exceeded in this population), the error rate is approximately double what it is for low call volumes. In other words, these cognitively fatigued firefighters forgot twice the number of occupied rooms as their well-rested colleagues. It is important to note however, that these effects were only acute in those who were unable to obtain in-service rest and recovery periods (colloquially referred to as “safety naps”). Scheduling such a rest and recovery period should be an important tool in combating the effects of cognitive fatigue when other measures cannot be taken.
In the second task, firefighters scanned a mental inventory (a list of letters) as quickly as possible to determine whether the list included a particularly item. This memory scanning method is widely used in psychological investigations of cognitive impairment, for example in elderly populations and in the assessment of drug side effects. Figure 2 shows that at higher call volumes there was an accumulating cost in terms of reaction time. For example, when we compare 10 calls versus three, there was an increase of a quarter-second in overall response speed, but the increase for 17 calls versus. 10 (which is the same relative amount of increase) carried with it an additional two-fifths of a second of an increase in the time it took to detect the target. This delay in reaction time could have profound effects for on-the-job performance. Although seemingly small, a one-fifth-of-a-second delay in response time can be the difference between a serious accident on the road and applying the brakes in time to avoid running into another motorist who has rapidly slowed down.
In addition to the tasks measuring the accumulating effects of cognitive fatigue, the researchers also looked at practical consequences of the slowdown in decision making and increase in errors. One important consequence was a dramatic spike in the number of industrial and accident reports that were filled out. Figure 3 clearly shows that these rates were relatively low – fewer than 4% of shifts required such reports – until call volumes exceeded 12 per shift, at which point the number quadrupled to 16.3%. Cognitive fatigue is a likely cause of the lapses and delays in judgment that led to such an increase in accidents.
Conclusions and Future Directions
These two studies clearly suggest that high call volumes have consequences that go beyond physical and emotional fatigue, affecting memory and the speed and accuracy of decision making. Such cognitive fatigue is relatively unstudied in first-responder populations, but the consequences associated with it are of critical importance and should inform public policy. Most notably, the current results suggest that call volumes for any apparatus should not exceed 10 calls in a 24-hour period. While there will always be days that involve workloads in excess of this, cities should endeavor to allocate resources so as to minimize these occurrences.