Miners in Australia frequently perform physically demanding work under high ambient temperatures and humidity, often whilst wearing personal protective equipment, leading to heat-related illnesses. This study investigated effects of replacing 100% and 50% sweat losses with 5% carbohydrate liquid or ice-slurry solution on core temperature during simulated mining conditions. Five randomized treadmill trials were performed with: no fluid replacement (NF), 100% (100 ICE) and 50% (50 ICE) sweat loss replaced with ice-slurry (~-1°C) solution and 100% (100 LIQ) and 50% (50 LIQ) sweat loss replaced with liquid (~+4 °C) solution. Time to exhaustion was longer in 100 ICE followed by 100 LIQ, 50 ICE, 50 LIQ and NF. Change in rectal temperature was least in 100 ICE followed by 100 LIQ, 50 ICE, 50 LIQ and NF. Ingestion of ice-slurry resulted in longer time to exhaustion and slower rates of change in rectal temperature. It is recommended that ice-slurry drinks be provided to personnel to lower heat strain during hot working conditions.
Miners perform physically demanding work [
Current interventions used to reduce the development of a heat stress related illness in miners include: setting the upper limit on body temperature during work to 38.5˚C [
A potential alternative intervention that is economically viable, easily implementable, and can concomitantly be used with currently in use heat management protocols, is modification of drinking practices. This method will theoretically enable workers to remain euhydrated and cool by drinking cool fluids and/or ice-slurry beverages. Research has shown that dehydration caused by sweating can impair physical and cognitive performance [
We speculate that ingestion of ice-slurry solutions during exercise may be more effective at cooling than liquid solutions because an ice-slurry elicits a larger heat sink due to the additional heat required to change phase from solid (ice) to liquid water, known as the “enthalpy of fusion”. The potential effectiveness of consuming an ice-slurry solution during exercise was demonstrated by [
Thus, the purpose of this investigation was to compare the effects of ingesting ice-slurry versus liquid beverages on core temperature during simulated mining conditions. It was hypothesised that offsetting sweat losses and remaining euhydrated by drinking an ice-slurry beverage would attenuate rises in Tre and cardiovascular strain to a greater extent than drinking a cold liquid beverage.
Ten healthy non-heat-acclimated Caucasian males (height 1.75 ± 0.05 m; age 29 ± 5 y; body mass 81.8 ± 9.0 kg; 19.2% ± 3.1% body fat;
On their first visit, each subject’s body mass (Model ID1; Mettler Toledo, USA) and height (Seca, USA) were measured. Adiposity was determined through Dual Energy X-ray Absorptiometry (Hologic, Hong Kong). Under ambient room conditions (25.8˚C ± 2.0˚C at 44.1% ± 8.1% RH) (Microtherm; Casella Measurement Ltd., UK), a modified Bruce treadmill protocol was used to determine
Participants visited the laboratory during the spring time on five separate occasions at approximately the same time of day with a minimum of 7 days separating each visit. All participants performed their first condition, considered the control condition (NF), without any drink ingestion. Differences between pre and post exercise nude body mass was used to estimate sweat rate and subsequent drink volumes. The remaining four randomized sessions included 100% replacement of estimated sweat losses with ice-slurry solution (100 ICE), or liquid (100 LIQ), and 50% replacement of estimated sweat losses with ice-slurry solution (50 ICE), or liquid (50 LIQ).
Upon arrival to the laboratory, urine and blood samples were collected and a rectal thermistor (Monatherm Thermistor, 400 Series; Mallinckrodt Medical, USA) was self-inserted by subjects, 10 cm past their external anal sphincter. Nude body mass was measured before instrumentation of skin thermistors and heart rate monitor (RS800 Polar Heart Rate Monitor, Finland). Participants then donned running shoes, cotton pants, t-shirt and mining helmet. Re-usable skin thermistors were fixed to the mid belly of the left gastrocnemius, quadriceps, biceps, and chest; using Ramanathan’s equation to calculate mean skin temperature [
Upon entering the climate chamber, participants were seated in an upright chair for 15 min prior to exercise. Exercise then commenced of walking on a treadmill at a constant workload of ~290 W∙m−2 (3.0 km∙hr−1 at an inclination of 15˚). Ambient conditions were 28.3˚C ± 0.5˚C, 74.2% ± 4.6% RH (25.9˚C ± 0.4˚C WBGT) and wind speed <0.1 m∙s−1. This work intensity and ambient condition does not require an altered rest to work ratio for industrial standards [
During exercise, drinks were administered after 20 min of exercise and every 20 min thereafter. Participants were instructed to ingest fluids as quickly as was comfortably possible. Participants walked until either voluntary exhaustion, achieving a Tre of 39.0˚C, or 120 min of exercise. Immediately after the protocol termination, nude body mass, blood and urine, were reassessed in that order. This experimental protocol for randomization was selected as an extension to the work conducted by [
Confirmation of euhydration was determined by urine osmolarity through freezing point depression (Advanced Instruments Inc, USA) Uosm < 286 mOsm∙kgH2O−1 [
Participants sat upright in a chair for approximately 3 min prior to blood being drawn from the antecubital vein. Plasma osmolality (Posm), through freezing point depression (Advanced Instruments Inc, USA), was determined by collecting 8.5 ml of blood into an SST heprinized tube and centrifuging (Heraeus Multifuge 3 S-R, Australia)
for 15 min at 3000 rev∙min−1 at 4˚C. Changes in plasma volume (∆PV), 8.5 ml of blood were collected into a plain clot tube before immediately separating into 2 aliquots (30 μL each) in non-heparinized capillary tubes. Capillary tubes were spun (MED Instruments, MPW-212, Poland) at 12000 rev∙min−1 for 5 min at room temperature. Haemoglobin concentration was measured (Hemocue, Hb 201, Sweden) using a sample of blood (10 μL each) from the same plain clot tube. ∆PV was calculated based on the method of Dill and Costil [
Tre and
Both ice-slurry (~−1˚C) and liquid drinks (~+4˚C) were composed of orange flavoured cordial (Cottee’s Foods, Australia) with a 5% carbohydrate concentration. Ice-slurries were made using a slushy machine (Essential Slush Co., Australia). Drink volumes administered were determined from individual sweat loss. Participants consumed approximately 1.30 ± 0.31 L for 100 ICE and 100 LIQ conditions while 0.650 ± 0.160 L was administered during 50 ICE and 50 LIQ. The temperature chosen for the liquid solution was based on typical household refrigeration temperature while the warmest temperature was used for the ice-slurry in an attempt to minimize temperature differences between drinks.
A condition x time repeated measures ANOVA was performed to identify any changes in Tre,
The increases in Usg were observed to be different between pre and post exercise for 50 LIQ (P = 0.005) (
Rate of heat storage (
The rate of change in Tre (∆Tre) are shown in
Uosm | Usg | Posm (n = 9) | % ∆ BV | % ∆ CV | % ∆ PV | ∆ Body Mass (kg) | ||
---|---|---|---|---|---|---|---|---|
No Fluid | Pre | 163 ± 97 | 1.011 ± 0.009 | 296 ± 7 | ||||
Post | 302 ± 178 | 1.010 ± 0.008 | 294 ± 6 | −4.10 ± 2.83 | −6.44 ± 1.78 | −2.81 ± 5.73 | 1.04 ± 0.53 | |
100% Ice Slurry | Pre | 169 ± 75 | 1.006 ± 0.004 | 291 ± 5 | ||||
Post | 239 ± 117 | 1.007 ± 0.004 | 290 ± 5 | −0.11 ± 2.68 (b) | −0.53 ± 4.02 | 0.38 ± 4.68 | 0.11 ± 0.32 (b) | |
50% Ice Slurry | Pre | 120 ± 58 (a) | 1.005 ± 0.005 | 295 ± 5 | ||||
Post | 243 ± 156 | 1.007 ± 0.005 | 294 ± 5 | −1.58 ± 4.14 | −1.35 ± 2.62 (b) | −0.58 ± 6.88 | 0.71 ± 0.39 (c) | |
100% Liquid | Pre | 156 ± 71 | 1.005 ± 0.003 | 293 ± 3 | ||||
Post | 255 ± 194 | 1.007 ± 0.006 | 292 ± 4 | −2.97 ± 3.77 | −3.07 ± 1.75 | −2.30 ± 6.11 | 0.13 ± 0.47 (b) | |
50% Liquid | Pre | 98 ± 64 (a) | 1.005 ± 0.005 (a) | 293 ± 6 | ||||
Post | 385 ± 300 | 1.011 ± 0.010 | 294 ± 6 | −1.89 ± 2.45 | −1.43 ± 3.50 (b) | −1.89 ± 4.62 | 0.64 ± 0.35 (b) |
(a) difference between pre and post values (P < 0.05); (b) different to No Fluid (P < 0.05); (c) different to 100% Ice Slurry (P < 0.05).
tween 100 LIQ and 50LIQ (P = 0.0453). Change in Tre between 100 ICE and 100 LIQ were 0.020 ± 0.008˚C∙min−1 and 0.022 ± 0.007˚C∙min−1; P = 0.3936, respectively with no clear differences between conditions (mean difference 14.6%, 90% CI: −12.6 to 50.1%).
Across all interventions (50 trials in total), 7 trials completed the entire exercise protocol, 27 stopped voluntarily due to exhaustion and 16 reached the maximum allowable Tre. No trials were stopped because of maximum HR. In total 43 trials were terminated due to physical exhaustion. This consisted of all NF and 50 LIQ trialsbeing terminated due to physical exhaustion, 90% of 50 ICE, 80% of 100 LIQ and 60% of 100 ICE trials were terminated on the same criteria. During these trials no participant reached the Tre cut off.
Estimated sweat rates for each drink condition was 22 ± 5 ml∙min−1 (NF), 19 ± 3 ml∙min−1 (100 ICE), 19 ± 4 ml∙min−1 (50 ICE), 20 ± 7 ml∙min−1 (100 LIQ) and 19 ± 7 ml∙min−1 (50 LIQ). Sweat rates were not different (P = 0.104) between conditions.
No statistical differences in HR were observed between conditions. 100 ICE produced a mean HR value of 163 ± 12 bpm followed by 50 LIQ (167 ± 11 bpm), 100 LIQ (169 ± 9 bpm), NF (169 ± 15 bpm) and 50 ICE (172 ± 6 bpm). Although no statistical differences were measured for HR between conditions; further analysis revealed a “trivial to non-trivial” reduction in HR when comparing ingesting 100 ICE to 50 ICE (mean difference 5.7%, 90% CI: 0.9 to 10.8%), and 100 ICE to 100 LIQ (3.9% mean difference, 90% CI: −1.2 to 9.3%). For 50 ICE vs. 50 LIQ comparison, there was a “trivial to non-trivial” reduction in HR (mean difference 3.1%, 90% CI: −1.1 to 7.1%) when ingesting 50 ICE.
NF | 100 ICE | 50 ICE | 100 LIQ | 50 LIQ | |
---|---|---|---|---|---|
Time 38.0˚C | |||||
Avg | 27.3 | 34.1 | 28.8 | 36.0* | 26.8 |
SD | 9.4 | 14.5 | 7.2 | 7.1 | 8.2 |
Time 38.5˚C | |||||
Avg | 37.0 | 68.5 | 60.1 | 78.3 | 42.0 |
SD | 10.6 | 33.8 | 24.5 | 15.9 | 10.4 |
*difference from 50 LIQ (P < 0.05).
It was hypothesised that replacing 100% of sweat loss with ice slurry ingestion would result in the greatest attenuation of thermal strain during exercise due to the larger cooling capacity associated with the enthalpy of fusion. However, contrary to our hypothesis, both 100 ICE and 100 LIQ provided similar physiological responses in participants as did 50 ICE compared to 50 LIQ. Complete replacement of sweat loss increased Tlim, reduced the rate of ∆Tre and reduced the rate of heat stored when comparing to replacement of half sweat loss during simulated mining conditions.
Although there was an increase in Tlim and an attenuated rise in Tre during exercise for both 100% sweat replacement conditions, this conflicts with the theoretical cooling capacities of the solutions calculated from sweat rates during NF. For each condition, the cooling capacities were: 100 ICE (594.2 ± 104.7 kJ), 50 ICE (320 ± 76.2 kJ), 100 LIQ (179.8 ± 42.8 kJ), 50 LIQ (89.9 ± 21.4 kJ) and NF (0 kJ). The aforementioned cooling capacities were estimated from the specific heat capacities of ice and liquid water in addition to the enthalpy of fusion for the respective volumes. From the mean sweat rates measured during NF and a thermal transition from −1˚C to 38˚C for ICE and 4˚C to 38˚C for LIQ, the presented values were derived. Based on these cooling capacities, it was hypothesised that Tlim and the attenuated rise in Tre would follow a similar trend to that of drink cooling capacities. This was contrary to what was observed. Possible explanations could be due to the thermal inertia within the body. If a large enough heat sink was ingested (i.e., 100 ICE), then this sink could affect the heat stored within the body. Reference [
Our findings extend upon the work of [
Reference [
The deleterious effects of dehydration can potentially be offset by increasing evaporative heat loss [
In the present study, 100ICE had the smallest increase in HR during exercise. This smaller increase in HR, compared to the other conditions, could be attributed to the larger heat sink provided by the ingestion of the cooler ice-slurry [
An alternative explanation to our findings of similar Tlim observed between conditions are the variability in fitness levels between our participants. With standardized work intensity, those individuals with a decreased capacity to do work could have skewed the data. As the purpose of this investigation was to determine the cooling capacity during a simulated mining condition, the participant selection was appropriate. Future investigations incorporating a more homogeneous population could result in observable differences in the various physiological measurements.
Industrial best practices are focused on maintaining a core temperature below that of approximately 38.0˚C; however there are circumstances where Tc can be 38.5˚C [
In conclusion, this investigation has demonstrated the ingestion of 100 ICE or 100 LIQ during exercise at rates equivalent to sweat rate can increase the Tlim, reduce the rate of heat stored, and in turn attenuate the rate of rise in rectal temperature in hot and humid conditions when compared to ingesting 50 ICE and 50 LIQ. Therefore, replacing 100% of lost fluids with either ice slurry or liquid drink will provide greater thermal relief during work in hot and humid conditions, which may be a practical and effective cooling strategy which Occupational Hygienists may wish to consider.
This laboratory study of a heat stress intervention was conducted under simulated mining conditions experienced in Australia during summer. Partial or complete volumes of fluid lost were replaced by ice-slurry or liquid. Fatigue and heat gain outcomes were improved amongst subjects consuming ice slurry. Ice-slurry was more palatable than water.
The authors would like to thank the participants for their time and commitment and also to Edith Cowan University for their financial assistance with this project.
JosephMaté,RodneySiegel,JacquesOosthuizen,Paul B.Laursen,GreigWatson, (2016) Effect of Liquid versus Ice Slurry Ingestion on Core Temperature during Simulated Mining Conditions. Open Journal of Preventive Medicine,06,21-30. doi: 10.4236/ojpm.2016.61002