Firefighting is a highly stressful occupation involving unique physical challenges, PPE and environments that increase the potential for dehydration to occur. The results of the studies included in this review indicate that dehydration is a reality in many firefighting contexts, but that this can be adequately addressed in most instances by ad libitum fluid intake. Dehydration leaves the firefighter at risk of harm to their health, safety and performance [6, 16]. The evidence provided by these studies assists in determining fluid replacement requirements for firefighters to combat occupational fluid loss. In general, the results of this review have shown fair to good quality evidence that firefighting will result in dehydration if adequate fluid is not provided or available to firefighters, over both short and long duration fire operations, but that if adequate fluid is available and ad libitum fluid intake is allowed and feasible, dehydration can be prevented, minimised or reduced in both structural and wildfire firefighting scenarios.
When considering the methodological quality of the included studies, there were some consistent weaknesses identified when viewed through the lens of the Downs and Black protocol, suggesting caution should be applied in interpretation of data. The lack of randomisation and blinding in the study designs, relatively small sample sizes (n = 11–49), and variability in interventions and outcome measures must all be considered when interpreting the findings. As must failures to state details of the firefighting interventions, such as ambient temperatures and durations of exposure. When comparing the studies involving similar interventions, preliminary conclusions can, nevertheless, be drawn, while taking into consideration the outcome measures used and their validity.
Results from those studies which utilised nude body weight [1, 6, 9, 28] demonstrated varying results. Previous research has shown that acute changes in body mass reflect changes in body water [31, 32]. A loss of greater than 1–2 % of body mass indicates insufficient fluid intake . Both Angerer et al.  and Eglin et al.  demonstrated loss of body weight from 30 mins of structural firefighting of only 0.47 and 0.79 %/h, indicating hydration was maintained, with no drinking and ad libitum drinking, respectively. Horn et al. , on the other hand, found a mean body weight loss of 2.2 % over 3 h of structural firefighting with encouraged ad libitum drinking, indicating dehydration. Cuddy et al.  was the only author to study body weight changes in wildland fires and found no significant difference in body weight across 3 days of wild firefighting with ad libitum drinking. Studying body mass over multiple days introduces many sources of error, as weight can fluctuate for a variety of reasons including energy balance and glycogen stores , however it remains likely that hydration was well maintained in this group.
Urine-based hydration measures including USG, colour and osmolality were used throughout the selected studies, producing varying results. Horn et al.  and Eglin et al.  both found no significant change in urine measures following structural firefighting with ad libitum drinking. These results are in direct opposition to the reported significant loss of body mass in the study by Horn et al. . Urine measure results followed similar trends in indicating hydration maintenance during wildland fire operations. Raines et al. [2, 12] found significant improvements in urine measures, whereby pre-intervention levels of hydration were maintained [2, 12] or a change from a dehydrated to an euhydrated state with higher water intake in the prescribed drinking group . In the study by Ruby et al. , urine results showed no significant changes over 5 days of wildfire fighting with ad libitum drinking. Considering these findings, it should be noted that Horn et al.  were the only authors to state the timing of urine samples (within 30 min of intervention), with the potential range of timings being a factor that might affect the reliability of these measures, as USG and urine osmolality may lag behind during periods of rapid body fluid turnover due to the protective role of the kidneys .
Although urinary measures are associated with well-established indexes of dehydration , all authors expressed concerns regarding the validity of urinary measures and their poor correlation with other hydration measures, including body weight, total body water, and plasma and salivary osmolality [6, 15, 18]. Raines et al.  highlighted articles [34–36] indicating that there is little to no evidence that USG is sensitive to changes in hydration status, unlike plasma osmolality. As urine osmolality can be used interchangeably with USG, this also casts doubt on its clinical utility . Authors suggested that a shift towards the use of plasma osmolality [2, 12] and salivary osmolality  as more valid measures of hydration was needed for clinical trials to improve methodological quality.
Plasma osmolality has been promoted as the current gold standard marker of hydration status . A change of 5 to 13 mOsm/L signals an 80 % (likely) and 99 % (near certain) likelihood that a meaningful change in hydration status has occurred . Considering this, Raines et al.  found no significant change (p = 0.73) in plasma osmolality following firefighting with ad libitum drinking, while in their earlier studies, Raines et al. [2, 12] found significant reductions (p < 0.0001) in plasma osmolality and a shift from dehydration at the start of a firefighter’s shift to euhydration by the end. These results differed from urine measure results but followed similar trends, indicating maintenance or improvement in hydration when ad libitum or prescribed drinking was implemented during wildland fire suppression.
The remaining measures used to determine hydration levels following structural fires consistently indicated dehydration following firefighting. Salivary osmolality has been shown to be a marker of acute hydration but questions still remain over its utility as a field measure due to concerns with the practicality of sample collection . Salivary osmolality in response to a 3 h structural fire suppression exercise was investigated by Horn et al. . Results were interpreted based on estimates of change in hydration and compared against body weight changes, as there were no salivary osmolality criterion measures of dehydration status . Horn et al.  concluded that levels of dehydration increased post-fire exposure when drinking ad libitum was allowed and that with future research and the development of a suitable portable tool, salivary osmolality could be a reliable and valid field measure, overcoming the impracticality of nude body mass and urinary measures.
Holsworth et al.  states that haematocrit is an important blood marker of hydration. Both Holsworth et al.  and Smith et al.  found significant changes in haematocrit indicating dehydration, following structural firefighting without fluid intake. However, concerns regarding the differentiation between dehydration and heat stress were noted by Smith et al. , as changes in blood chemistry are typically transient and depend on the extent of hypohydration and cellular damage. The authors concluded that pre-hydration and rehydration need to be a priority in hot and arduous conditions.
Water turnover provides a valuable guide to drinking requirements needed to maintain hydration during wildland fires. A day of wildland fighting requires a minimum of 6–8 l to be ingested according to the results reported by Ruby et al.  or 8–11 l according to the results of Cuddy et al. , although Cuddy et al.  note that these values can vary considerably based on ambient temperature and drinking habits of each individual. Of note, both authors concluded that firefighters failed to consume sufficient fluids to maintain hydration. These fluid requirements are considerably greater than those reported by Raines et al. , who found intakes of 420 ± 132 ml/h in hot environments and 264 mL/h in mild to warm conditions, over 10 and 12 h shifts, were required.
Despite variations in outcome measures, results of the included studies of wildfire suppression all followed similar trends, indicating that ad libitum drinking was sufficient to maintain or improve hydration status across single or multiple day wildland fire suppression operations in cool to hot conditions [1, 2, 12, 15, 18]. Evidence in structural fires is less consistent, and utilised less valid measures of hydration. Results tended to indicate that regardless of whether ad libitum drinking is allowed or not, structural firefighting of greater than 1 h and possibly as short as 30 min resulted in some level of dehydration [6–9, 28]. This is likely due to the more intense physical nature, carriage of potentially heavier loads in PPE, higher environmental temperatures and limited opportunity to consume fluids during structural fire suppression, and extra effort may be required to ensure ready access to palatable fluid and opportunity to drink.
Strengths and limitations of the critical review
Key strengths of this review are its systematic and critical approach and inclusion of 10 studies involving a variety of relevant firefighting scenarios, hydration measures and fluid consumption models. These factors have allowed for useful comparisons and consideration of possible reasons for observed heterogeneity in reported results, and these will usefully inform both future research and interim policies and procedures. The review was limited, however, by the relatively small number of studies meeting the review criteria and the methodological quality of these studies. Firm conclusions are difficult to draw, given these limitations and both the variable nature of the reported firefighting operations and the variation in reporting of tasks performed, time of exposure and ambient temperatures. Furthermore, the variance and questionable validity of some outcome measures limits the development of dedicated recommendations.
The significance of quantifying dehydration that occurs during fighting of actual fires is recognised by fire agencies, with the release over previous years of guidelines targeted at minimising the risk of dehydration [41–44]. A better understanding of the incidence of dehydration assists in the development of such policies and procedures to ensure firefighter health and safety. Horn et al. (2012) state that guidelines from athletic populations provide a reasonable benchmark on which to base guidelines, but may have limited utility with firefighters due to PPE worn and higher ambient temperatures. It is clear that firefighting will result in dehydration if adequate fluids are not consumed. Further research into the fluid requirements to maintain hydration, particular with use of valid and reliable outcome measures such as plasma osmolality will increase the homogeneity of the evidence and provide a clearer understanding. Raines et al. [2, 12, 18] have provided good evidence quantifying the hydration requirements of fighting wildland fires in mild to hot conditions. However, due to the intense nature of structural firefighting and the potential for rapid dehydration, more research to determine fluid replacement requirements is particularly warranted. Future studies should report ambient temperatures, duration of exposure and justifications of outcome measures used. With further research, evidence based guidelines can be developed to ensure the health and safety of firefighters across multiple scenarios and environmental conditions. In the interim, general advice to firefighters and their managers to ensure ready access to fluids and to consume fluids ad libitum, guided primarily by thirst, would seem most appropriate and supported by the available evidence from this review and from previous research . Care should be taken to avoid promoting over-hydration, as noted in the introduction to this review .