Hydration Tracking via Saliva Osmolarity in Recruit Firefighters Throughout a 12-Week Fire School

Abstract

Background: The purpose of this study was to assess hydration status via saliva osmolarity throughout a 12-week Fire School in recruit firefighters. Methods: At the beginning (AM) and end (PM) of the workday for 13 weeks (a 12-week Fire School and an additional transition week), saliva osmolarity was measured, recorded, and relayed to each participant in the sample, which included 23 recruit firefighters. The average weekly osmolarity scores were computed for AM and PM. Separate linear mixed-effects models were used to assess the changes in osmolarity at each time of day over the course of the training. Bonferroni-adjusted post hoc tests were used to identify significant differences between weeks while maintaining test-wise error (α = 0.05). Results: AM was significantly lower in week 12 than in weeks 4 and 6, and lower in week 13 than in weeks 2, 4, 6, and 7 (p ≤ 0.035 for all). PM was significantly lower in week 12 than in weeks 3 and 7, and was lower in week 13 than in weeks 1, 3, and 7 (p ≤ 0.019 for all). Conclusion: This study demonstrated that hydration status via saliva osmolarity improved during the last half of Fire School despite those weeks being more physically and thermally challenging.

1. Introduction

Firefighters face significant health risks due to various occupation-related exposures and stressors [1,2,3,4,5]. Identifying and attenuating these health risks is critical to the long-term survival of these central first responders. Although the nature of firefighting poses obvious life-threatening risks from responding to emergencies with varying levels of danger, firefighting also leads to subtle health risks that are often overlooked. For instance, acute and chronic dehydration are common in firefighters and, depending on the severity, may lead to serious cardiac events [6,7].

Dehydration is defined as a ≥2% loss of total body water [8]. This loss of body water stresses the cardiovascular system as compensatory mechanisms are initiated [9]. For instance, dehydration leads to a reduction in stroke volume. To maintain cardiac output, heart rate increases, elevating the workload and stress on the heart [10,11]. Chronic dehydration can thus contribute to serious health risks such as hypertension and cardiovascular disease [12,13].

The occupational demands of firefighting promote body water loss and conflict with the ability to maintain proper hydration. Firefighters lose body water primarily via sweating as they are required to wear restrictive personal protective equipment (PPE) weighing 50–60 pounds, work in extreme temperatures, and perform intense physical labor [6,14,15]. PPE restricts the natural cooling mechanism from sweat, which leads to inefficient sweating and unnecessary loss of body water [16,17]. Reports reveal that over 90% of firefighters are continuously in a state of dehydration, which, as mentioned, may negatively impact acute and long-term cardiovascular health [14,18].

To balance out fluid loss and maintain adequate hydration under minimal conditions (i.e., low physical activity levels, temperate weather, and low altitude), the Institute of Medicine recommends a daily fluid intake of 3.7 L for men and 2.7 L for women [19]. However, due to the elevated sweat losses, firefighters may lose as high as 8.5 L of water in extreme conditions [20,21]. Firefighters who work at higher elevations also lose more water via increased respiratory water loss [22]. Thus, providing recommendations for this population to maintain adequate hydration levels is challenging.

Regular fluid status assessments may help overcome this individualized hydration recommendation barrier when determining hydration needs [10]. Multiple fluid status assessments exist; however, they must be accessible for firefighters to use regularly. For instance, blood-based measurements require a venous blood draw, advanced laboratory techniques, and specialized costly equipment to determine serum osmolarity. While blood osmolarity is a gold-standard hydration assessment, it is impractical for regular use at fire stations [23,24]. On the other hand, urine color and specific gravity, subjective thirst ratings, and changes in body mass are more practical for regular hydration assessments at fire stations [10,25]. Still, they may not accurately describe hydration status when used alone due to measurement limitations [23,26,27].

Saliva osmolarity is similar to serum osmolarity as it reflects hydration status by analyzing the concentration of solutes in saliva [28]. Although it has been shown to measure changes in hydration accurately, it has not been a practical assessment for fire stations due to costly equipment and specialized analysis techniques [29]. However, portable point-of-care saliva osmolarity meters have recently been developed and used in research [14,30]. These allow regular hydration assessments while also being user-friendly for the general population.

Full-time recruit firefighters are individuals paid to go through Fire School to become full-time firefighters. Recruit firefighters may struggle with dehydration as they learn new hydration strategies to overcome the increased daily body water loss, which they experience from wearing PPE and working in unaccustomed extreme conditions. To offset dehydration-related events, recruits’ hydration status should be monitored daily to ensure they are adequately hydrated before intense training. Also, tracking their hydration status daily may allow recruit firefighters to understand their individual hydration needs when working as firefighters. Thus, our study aimed to assess hydration status via saliva osmolarity at the beginning and end of each work shift throughout a 12-week Fire School in recruit firefighters. An additional 13th week was included in the assessment to view possible hydration alterations after the completion of Fire School.

2. Materials and Methods

2.1. Experimental Design

A longitudinal single-arm design was used to determine if daily hydration status feedback altered hydration status over 12 weeks in firefighter recruits in the southeastern part of the United States. Before starting the 12 weeks, individualized saliva osmolarity optimal hydration zones were identified for each recruit. For the following 12 weeks during Fire School and one week after, saliva osmolarity was recorded at the beginning and end of each 8-h daily work shift. Participants were allowed to see their measurement results each time. This study was approved by the University’s Institutional Review Board (IRBNet ID# 2047457), and all procedures performed followed institutional guidelines.

2.2. Participants

In total, 23 recruit firefighters (21 males, 2 females) from the local fire department participated in this study. The participants were recruited from two classes attending Fire School. The first class included 9 participants who attended Fire School from August to October, and the second class included 14 participants who attended Fire School from April to June of the following year. The data collected from the first class were analyzed and used as pilot data to estimate the required sample size for the study as a whole. A significant effect of Week on salivary osmolarity was observed, suggesting that a sufficiently large sample size was achieved, but that a larger number of participants would make it easier to determine differences in salivary osmolarity between the other time points and thus make the study findings more meaningful and generalizable. A summary-statistics-based power analysis calculator created by Murayama et al. (2022) was used to estimate how many additional participants would be needed for a t-value of 1.96 to be considered significant with traditional parameters (α = 0.05 and power = 0.80) [31]. An estimated total of 19 participants would be needed to identify significant differences at this level of effect size. The size of the second class of recruits was sufficient to meet this requirement while also including additional participants in the event of complete participant dropout from the study. Inclusion criteria included males and females, at least 18 years old and currently employed as a full-time recruit firefighter. The same fire department employed all participants and underwent the same Fire School training, allowing consistency between classes. Participant characteristics are provided in

Table 1. Characteristics for the 23 recruit firefighter participants.

Participant Characteristics	Mean ± Standard Deviation
Age (yrs)	26.54 ± 5.99
Body mass (kg)	87.15 ± 19.10
Height (cm)	179.76 ± 8.40
BMI (kg/m2)	27.14 ± 4.54

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2.3. Protocol

2.3.1. Baseline Measures

To determine and record each participant’s optimal hydration zone (OHZ; personalized target range that reflects a user’s fully hydrated state) via real-time osmolarity testing with the MX3 LAB handheld device (MX3 diagnostics, Austin, Texas, USA; Minneapolis, MN, USA; and Melbourne, Australia), participants followed a one-day hydration protocol during a day spent indoors with little/light exercise as they worked at a desk in a designated classroom. Participants were asked to refrain from alcohol for 24 h prior to the one-day protocol to avoid alcohol-related dehydration effects. At the beginning of the work shift, participants were weighed and asked to consume 5 mL of water per kg body weight per hour from 9:00 AM to 5:00 PM in addition to any water typically consumed during or after work/exercise. Body weight was measured with a floor scale. An investigator provided each participant with the required hourly volume of water intake based on their body weight. Each participant was given the same water bottle with volume amounts listed on the bottle and hourly breaks to refill their bottles. Saliva osmolarity was measured six times at 5:00 PM via the MX3 LAB device, which recorded and saved the OHZ for each participant based on these six readings.

Before the saliva osmolarity measures, participants were asked to refrain from chewing tobacco 2 h and smoking, eating, or drinking 10 min before this assessment. To measure saliva osmolarity, participants were asked to open their mouth to allow access to their tongue. An electrochemical test strip attached to the MX3 Lab device immediately touched the tongue until the device beeped indicating sufficient saliva levels were captured for analysis. The osmolarity result is displayed within 10 s on the meter and recorded internally.

2.3.2. Daily Measures

At the beginning and end of the workday for 13 weeks, saliva osmolarity was measured, recorded, and relayed to each participant. The results provided categories based on their OHZ scores, which included “hydrated,” “mildly dehydrated,” “moderately dehydrated,” and “severely dehydrated.” The salivary osmolarity score reflects the concentration of solutes in the saliva, meaning that a high score is indicative of dehydration and a lower score more indicative of hyperhydration or euhydration. The description of each week during Fire School is provided in

Table 2. Description of the weekly goals and actions for the 12-week Fire School.

Week	Description of Weekly Goals and Actions
1	This week was an introduction to the class. Physical training (PT) was very strenuous to assess the physical and mental status of the recruits. PT was not performed in turnout gear (PPE) this week because the recruits had not learned about their gear yet. PT sessions were longer because the students spent the rest of each day in the classroom.
2	The recruits learned about their PPE gear and SCBA this week. They learned and conducted many repetitions of getting dressed in their gear in two minutes or less. The recruits completed the fire department’s physical agility course in full gear for the first time and conducted a longer version of the course in full gear for PT each morning.
3	PT continued to include the physical agility course in full gear. This week, the recruits learned about forcible entry and ladders. These skills were more technical, but the recruits practiced them in full gear to build confidence in their gear and acclimate them to the stress it puts on the body.
4	The recruits continued learning about ladders and performed many repetitions to build proficiency. PT continued to build cardiovascular endurance, and the recruits continued to perform the agility course in gear.
5	This week, the recruits received a lot of classroom instruction on ropes, knots, and ventilation. These skills are not very strenuous, but this allowed for longer PT sessions in the morning.
6	This was the hardest week for the recruits to this point in the class. This is search and rescue week. Monday was a classroom day, but the remainder of the week was spent in full gear performing search and rescue techniques. The recruits performed many repetitions in full gear and with their vision blacked out. The week ended with the recruits performing repetitions in the SCBA confidence maze and then in the afternoon, performing search and rescue drills under live smoke conditions. PT was still conducted to start each day.
7	During this week, the recruits learned about fire hoses and how to operate them. PT was still conducted daily, and the physical agility course was conducted in full gear. There was a lot of classroom time, and the skills were low-impact. At the end of the week, the recruits performed muster drills involving fire hoses to improve their proficiency and confidence in the fire hose.
8	This week, the recruits began prepping for their live burns. For the first four days of the week, the recruits practiced moving the fire hose through the burn building in full gear to simulate their live fire rotations. On Friday, the recruits performed their live fire burns. The recruits had three evolutions they had to complete. PT was still completed each day except on Friday.
9	This week, the recruits began learning how to operate on the fire grounds as part of an engine company. They combined all their fire ground skills to function as part of an engine company. The recruits also conducted a simulated 24-h shift during this week. PT continued to train endurance stamina.
10	This week continued week 9.
11	This was the hardest week of the class. During this week, the recruits completed strenuous PT sessions, and the entire PT session was in full gear. The recruits learned firefighter survival, self-rescue skills, and skills to rescue injured or trapped firefighters. These skills were stressful and extremely strenuous. The week ended with the recruits performing a firefighter rescue scenario and then a long hose crawl designed to test the recruits’ self-rescue skills and mental toughness.
12	This was a test week for the recruits. The recruits practiced and studied for their final written and practical exams. PT was kept light this week to help the students recover.
13	This week was a transition week into Hazmat School.

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2.4. Statistical Analysis

All analyses were carried out using SPSS, version 29 (IBM, Armonk, NY, USA) with a predetermined alpha level of 0.05. Data for each analysis at each time point were screened for outliers (>3.29 standard deviation units from the group mean) and for the assumption of normality using the Shapiro–Wilk test. The average weekly osmolarity scores were computed for the morning (AM) and the afternoon (PM). The difference in osmolarity scores between the morning and afternoon (AM-PM) were also computed and averaged for each week of the study.

Separate linear mixed-effects models were used for analysis of AM, PM, and AM-PM to account for missing data scores which resulted from logistical conflicts for some of the participants [32]. Individual participants were specified as a correlated random effect [33]. Averages for each week were designated as the repeated-measures fixed factor Week, with 13 time points (weeks 1–13). A compound symmetry covariance structure was selected for each linear mixed-effects model. Tests of fixed effects were generated alongside Bonferroni-adjusted post hoc tests, which were used when appropriate to identify significant differences between weeks.

3. Results

Descriptive results (observed mean and standard deviation) for AM, PM, and AM-PM are presented in

Table 3. Descriptive data for the observed scores for average osmolarity in the morning (AM), average osmolarity in the afternoon (PM), and the average daily difference in osmolarity from the morning to the afternoon (AM-PM) for each week of the study. Values are presented as mean (standard deviation).

Week	AM	PM	AM-PM
1	66.9 (18.1)	68.9 (17.2)	−2.0 (19.8)
2	70.9 (18.9)	64.0 (14.0)	6.9 (13.5)
3	69.1 (15.3)	70.7 (18.5)	−1.6 (14.9)
4	72.6 (20.2)	71.9 (13.9)	−3.8 (16.3)
5	63.6 (7.6)	69.1 (11.8)	−5.6 (10.0)
6	71.9 (14.5)	64.7 (12.2)	5.3 (14.4)
7	70.8 (17.2)	72.3 (11.8)	−1.6 (16.3)
8	66.6 (14.5)	64.5 (14.7)	2.9 (15.0)
9	61.2 (12.5)	53.5 (8.6)	7.7 (11.8)
10	63.3 (15.2)	55.2 (8.4)	−1.0 (10.4)
11	60.3 (19.5)	53.4 (9.9)	−6.2 (10.6)
12	53.3 (13.4)	55.6 (10.5)	−2.3 (16.2)
13	52.7 (11.0)	54.0 (11.3)	−1.3 (11.4)

. Estimated fixed effects mean scores with standard error values for AM, PM, and AM-PM, which account for varying sample sizes at some time points, are presented visually in Figure 1. Results presented in the text for significant differences between weeks include the average differences between indicated weeks (Δ), the 95% confidence interval of this average difference (CI₉₅), and the Bonferroni-adjusted p-value of this observed difference.

There was a significant effect of Week on AM (F_{12, 204.6} = 3.5, p < 0.001), which was lower in week 12 than in week 4 [Δ = −19.19, CI₉₅ = (−36.78, −1.60), p = 0.016] and week 6 [Δ = −18.53, CI₉₅ = (−36.12, −0.93), p = 0.026], and which was lower in week 13 than in week 2 [Δ = −18.10, CI₉₅ = (−35.70, −0.51), p = 0.035], week 4 [Δ = −19.77, CI₉₅ = (−37.36, −2.17), p = 0.010], week 6 [Δ = −19.11, CI₉₅ = (−36.70, −1.51), p = 0.017], and week 7 [Δ = −17.95, CI₉₅ = (−35.55, −0.36), p = 0.039].

There was a significant effect of Week on PM (F_{12, 153.2} = 5.2, p < 0.001), which was lower in week 12 than in week 3 [Δ = −18.23, CI₉₅ = (−35.16, −1.31), p = 0.019] and week 7 [Δ = −19.88, CI₉₅ = (−36.81, −2.96), p = 0.005], and which was lower in week 13 than in week 1 [Δ = −18.04, CI₉₅ = (−34.97, −1.12), p = 0.022], week 3 (Δ = −19.79, CI₉₅ = (−36.72, −2.87), p = 0.006], and week 7 [Δ = −21.44, CI₉₅ = (−38.36, −4.51), p = 0.001). There was no effect of Week on AM-PM (F_{12, 151.75} = 1.3, p = 0.215).

4. Discussion

Firefighters are susceptible to dehydration and potential heat injury due to the physical demands placed upon them, the extreme temperatures of working conditions, and the PPE, which lacks breathability for evaporative cooling. Quickly assessing and monitoring hydration status is an important step to limit the incidence of dehydration and resulting negative effects on firefighter health and performance. This study aimed to assess hydration status via saliva osmolarity at the beginning and end of each work shift throughout a 12-week Fire School in recruit firefighters. An additional 13th week was included to assess possible impacts on hydration status from week 12 of the Fire School. The main finding was that there was an overall pattern of reduced salivary osmolarity in firefighter recruits throughout the course of the Fire School, which was lowest in weeks 12 (the final week of Fire School training) and 13. These results indicate that hydration status improved over time as the academy training progressed and remained as such after intense physical training had ceased in week 13. The practical relevance and potential causes of this change in salivary osmolarity will be discussed, in addition to potential avenues of future research.

Saliva osmolarity values obtained from the meter used for this study were categorized into four hydration status classifications: “hydrated” (<66 mOsm), “mildly dehydrated” (66–100 mOsm), “moderately dehydrated” (101–150 mOsm), and “severely dehydrated” (151+ mOsm) [34]. On average, the recruit firefighters were classified as mildly dehydrated during weeks 1–8 except for week 5. After week 7, osmolarity values began to decrease, with average scores classed into the hydrated range during weeks 9–13. Interestingly, hydration status improvements occurred despite the progressive increases in physical challenges and thermal stressors, such as working in PPE and fighting simulated fires, throughout the 13-week period. The improvements may have been due to physiological adaptations and/or behavioral modifications.

During Fire School, physiological adaptations may develop due to heat acclimatization from regular exposure to hot environments. These adaptations include enhanced sweat sensitivity and output (i.e., sweating is initiated at a lower body temperature), increased skin blood flow, expanded plasma volume, and improved sodium conservation [35,36,37]. Plasma volume has been shown to increase within the first week of heat acclimatization, potentially raising total body water by 2–3 L (L) [38,39]. Maintaining sodium levels is important for preserving extracellular fluid osmolarity and volume as total body water increases [39]. This sodium balance is achieved through improved reabsorption in the eccrine sweat gland with heat acclimatization [40]. Allan and Wilson et al. demonstrated that heat acclimation enhances sodium reabsorption in these glands, reducing sweat sodium concentrations from approximately 60 mEq/L to as low as 10 mEq/L [41].

In addition to physiological adaptations, behavioral modifications such as increasing fluid consumption may have improved hydration status over time. Heat acclimatization can enhance thirst sensitivity, which may have prompted the recruit firefighters to drink more fluids throughout the day to meet the body’s hydration needs [42]. It has been suggested that this improved thirst response may result from heat-induced changes in plasma volume and osmolarity, which influence thirst sensation and drinking behavior when fluids are readily available [43].

Before this study began, each recruit firefighter received their personalized optimal hydration zone value, as described in the Methods Section 2.3.1. Throughout this study, their hydration levels were tested each morning and evening, which provided feedback on their hydration status and how it aligned with their individualized optimal hydration zone. This biofeedback likely helped them gauge how much fluid to consume to maintain optimal hydration levels while adapting to the new physical and environmental challenges they were experiencing.

A systematic review demonstrated that dietary and physical activity feedback interventions via health applications (apps) significantly improved behavioral and health outcomes, especially when the apps designs included goal setting, self-monitoring, and performance feedback [44]. The feedback from the hydration monitoring in our study provided a personalized hydration goal by finding their optimal hydration zone and daily monitoring. Thus, this individualized approach to hydration feedback may serve as a valuable tool to improve hydration behaviors and promote adherence to adequate fluid intake.

Fire departments should implement hydration assessment protocols to mitigate dehydration-related issues among recruits and firefighters. These protocols should include evaluating hydration status at the beginning and end of each shift to ensure firefighters are adequately hydrated for physically demanding occupational tasks. If a firefighter is found to be dehydrated, they should be encouraged to take the necessary time to rehydrate properly. Maintaining adequate hydration for long periods may also reduce cardiovascular strain and thus, the risk for developing cardiovascular disease [15]. Other disorders associated with long-term hypohydration include hypertension, coronary artery disease, and type 2 diabetes [12,13]. Therefore, adequate hydration overtime may be beneficial to overall health, especially in this population that struggles with dehydration due to the nature of their occupation.

One limitation of this study was that no control group was included to observe what changes in salivary osmolarity, if any, would occur during the Fire School training without daily feedback. Thus, it is difficult to parse out exactly how much of the changes in hydration status can be attributed to the physiological adaptations to heat acclimatization and how much can be attributed to receiving daily personalized feedback on hydration status. This issue presents a unique methodological challenge, as salivary osmolarity measurements cannot be collected without the knowledge of the recruits and their personal interaction with the test administrator. This makes the inclusion of a true control group difficult, if not impossible, as behavior may still be influenced to a certain degree by testing requirements even if the measurement result is not relayed to the participant. Another limitation was the absence of an accurate daily fluid intake record. Although participants logged their fluid intake, the records were too inconsistent to be included in this study due to missing data when participants were too busy to document refills. Maintaining precise fluid intake logs would help determine whether improvements in hydration status resulted from changes in hydration behaviors or physiological adaptations. In addition, subjective dietary, medication, sleep, and stress logs may be beneficial for determining their role in hydration status.

Overall, this study demonstrated that hydration status via saliva osmolarity improved during the last half of Fire School despite those weeks being more physically challenging with increased exposure to heat. To identify the reasons for this improvement, future explorations should examine physiological changes associated with heat adaptations, such as plasma volume expansion and altered sweating patterns, as well as behavioral factors like daily fluid intake throughout the program. Understanding the mechanisms behind improved hydration patterns, as demonstrated in our study, will help fire departments and researchers know whether regular hydration tracking and feedback are required to encourage drinking behavior adjustments or if the improvement occurs naturally as a result of heat acclimatization, rendering frequent monitoring unnecessary. In addition, future explorations should examine the effects of maintaining adequate hydration for long-term periods on overall health as long-term hypohydration has been shown to increase risk for health conditions like cardiovascular disease.