Firefighting is a highly stressful occupation, with firefighters exposed to a multitude of physical and environmental stresses during their normal duties [1]. Work can be highly variable, ranging from short duration structural fires, to long shifts (10–15 h) over multiple days (3–5 days) when fighting wildland fires [2]. In addition, this work can involve both low intensity exercise of long duration and periods of high intensity exercise of unpredictable duration [3–5]. Perhaps of most distinction, firefighting tasks are often performed in environmental extremes, such as high heat and dense smoke, while wearing impermeable, heavy and restrictive Personal Protective Equipment (PPE) [2, 6–8].

These factors create a uniquely arduous occupation, leading to significant heat and cardiovascular strain [1, 9–11]. Dehydration is one likely consequence [6]. This is due to the significant challenge to the thermoregulatory and body water balance systems caused by wearing PPE, the physical demands of firefighting and the high temperatures in which firefighters operate [1, 6, 12]. Firefighter PPE, which is typically heavy, thick and encompasses the head, induces profuse sweating [10]. The physical demands of firefighting combined with the substantially reduced water permeability, evaporative heat capacity and increased energy demands of wearing the PPE mean firefighter cooling capacity is reduced, causing the observed high sweat rates—in turn resulting in dehydration [10, 13–15].

Dehydration has been identified as having the potential to impact firefighter health, safety and performance [6, 16], and may exacerbate the effects of heat exposure by impairing the firefighter’s thermoregulatory response [17]. If firefighters are not receiving adequate fluid they may experience the adverse effects of dehydration [6, 18]. Dehydration is known to impair both cognitive and cardiovascular function [10] and reduces tolerance time in uncompensable conditions when compared to euhydration [19]. It should also be noted that excessive fluid intake during sustained physical activity can be equally dangerous, if it is sufficient to cause exercise-associated hyponatremia, or dangerously-low sodium concentrations in key body fluids [20]. As such, any hydration guidelines need to balance these concerns [21].

Accurately documenting the effects of firefighting on hydration status across multiple scenarios will enable the development or refinement of health and safety guidelines for firefighters [18]. Many fire agencies currently prescribe fluid intake through guidelines, with large variability (500–3000 mL/h), to combat occupational stresses [18]. Raines et al. (2015) state that these guidelines are largely based on data from sport and other occupations, due to the limited evidence in firefighting. Carlton and Orr [22], in a critical review on the effects on fluid loss on physical performance, state that the impacts of dehydration need to be studied in the specific environmental context.

On this basis, the purpose of this review was to critically appraise the current literature investigating the impact of fighting actual, or ‘live’, fires on firefighter hydration.

A PRISMA flow diagram [23] showing the literature search, screening and selection results at each stage of the process is shown in Fig. 1.

A total of 148 potentially relevant articles were identified in the primary literature search (Fig. 1). The secondary search by hand of the reference lists of included articles yielded 18 further articles, while consulting subject matter experts did not produce any additional articles. A total of 10 articles examining the impact of fighting live fires on firefighter hydration were retained for critical appraisal and review [1, 2, 6–9, 12, 15, 18, 28].

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 [33]. Both Angerer et al. [9] and Eglin et al. [28] 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. [6], 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. [1] 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 [15], 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. [6] and Eglin et al. [28] 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. [6]. 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 [12]. In the study by Ruby et al. [15], 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. [6] 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 [34].

Although urinary measures are associated with well-established indexes of dehydration [6], 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. [12] 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 [37]. Authors suggested that a shift towards the use of plasma osmolality [2, 12] and salivary osmolality [6] 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 [38]. 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 [39]. Considering this, Raines et al. [18] 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 [40]. Salivary osmolality in response to a 3 h structural fire suppression exercise was investigated by Horn et al. [6]. 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 [6]. Horn et al. [6] 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. [7] states that haematocrit is an important blood marker of hydration. Both Holsworth et al. [7] and Smith et al. [8] 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. [8], 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. [15] or 8–11 l according to the results of Cuddy et al. [1], although Cuddy et al. [1] 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. [18], 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.

In conclusion, there is fair to good evidence to indicate that firefighting results in dehydration when fluid intake is inadequate. This situation is exacerbated during intense activity and extreme environments. During wildfire operations, ad libitum drinking is sufficient to maintain hydration, however during structural fire suppression tasks, ad libitum drinking may not be sufficient to maintain hydration and dehydration can occur rapidly, possibly due to the intensity of the tasks and lack of time or ready access to fluids. As such, while ad libitum drinking should be encouraged during all firefighting tasks, special consideration may need to be given to ways to enhance access to fluids and capacity for ad libitum fluid intake while fighting structural fires, and this concern also warrants further investigation.