Development of a Cardiorespiratory Risk Surveillance Score Via Comparison of Body Composition Methods, Cardiorespiratory Fitness, and Obesity in Firefighters

Firefighters are exposed to extreme metabolic and cardiopulmonary stressors in the performance of their vocation that are recognized to exceed the actual physical or environmental hazards. The current medical literature regarding firefighting highlights two frequently cited statistics: (1) based on body mass index (BMI), upwards of 73-80% of firefighters are considered overweight or obese, and (2) >45% of firefighter line-of-duty deaths are due to cardiovascular events.

This finding regarding BMI has been widely reported in the U.S. media after being published in a 2014 CDC medical journal. Such a finding is particularly relevant to firefighters given that there are recommended cardiorespiratory performance standards for firefighters, commonly 12 metabolic equivalents (METs) or maximum oxygen consumption during aerobic exercise (VO2max) of 42 mL O2/kg of body weight/min. As a consequence, failure to meet such a standard may limit a firefighter's ability to perform (or be employed) in full-duty tasks based on individual fire station requirements.

Fire suppression activities account for 1-5% of a firefighter's duty time yet result in 32% of firefighter deaths. This is attributable to the extreme physical demands of firefighting activities in which anaerobic threshold (typically 85% of maximal heart rate) may be exceeded and sustained throughout an engagement. Few professions, other than elite athletes, require this level of cardiorespiratory fitness (CRF). As such, CRF is of vital importance to firefighters and yet is rarely, if ever, tested in a systematic fashion over the course of an individual firefighter's career.

The vital importance of CRF is well-established in the medical literature, such that many advocate for its addition as a clinical vital sign. Epidemiological studies have demonstrated an inverse relationship between CRF and coronary heart disease (CHD) and all-cause mortality in healthy individuals. There are incremental health benefits to increased levels of fitness and reduced morbidity and mortality associated with lower levels of fitness. The fitness standard of 12 METs (the equivalent of a VO2max of 42 mL/kg/min) remains the testing metric for firefighters in the NFPA 1582 guidelines; it is applied to all candidates, male or female, and of any age. While a VO2max of 42 may be "good" to "excellent" for a 50-59 year old male, it is only "good" for a 40-49 year old male, and only "fair" for a 30-39 or "poor" for a 20-29 year old male. In women, a VO2max of 42 indicates a "good" level of CRF for a 20-29 year old female and "superior" CRF for a 50-59 year old female. Knowing that CRF declines inevitably with age argues that a firefighter whose VO2max is 42 mL/kg/min at age 20 will decline significantly by age 50 and raises the question of their ability to safely participate in fire suppression activities at that age. CRF measures in firefighters would benefit from age-adjustment and gender stratification. This research would begin that process.

While elevated BMI is associated with cardiovascular disease morbidity and mortality, BMI has been demonstrated to be a non-specific metric of body composition. Specifically, BMI "is limited as an index of obesity (body fatness) because it does not account for the composition of body weight," and the concurrence of BMI and body fat percentage (BF%) is affected by age, gender, ethnicity, and body shape. The misclassification of firefighters as overweight and obese based on BMI has been discussed in a several studies. These studies utilized bioimpedance testing or skin calipers for BF% measurement, methods known to have significant levels of error in terms of false positives and false negatives. Interestingly, a series of these studies cite a single article with a sample size of 39 individuals as validation of bioimpedance in comparison to dual-energy X-ray absorption (DXA), but also report DXA as the gold standard for BF% measurement. This inaccuracy of BF% compared to BMI was addressed thoroughly in one study of U.S. firefighters, demonstrating the potential for false positive and false negative identification of obesity in this population; specifically, they reported rates of 15.4% false positive and 18.2% false negative identification of obesity when identified by BMI as opposed to BF%.