The cumulative physiological effects of consecutive days of simulated occupational heat stress: study protocol for a randomised controlled trial with healthy adults undertaken in Australia

Background: Fuelled by rising global temperatures, occupational heat stress has become a critical threat to the health and safety of workers. This threat may be particularly great for workers who are required to perform prolonged work in the heat over multiple consecutive days; however, the cumulative physiological impact of these work conditions remains unclear. As such, the primary objective of this study is to determine the effects of three consecutive simulated physically demanding workdays in a hot and humid environment on core temperature (primary outcome), with secondary objectives examining the impact on heart rate, hydration status, physical task performance, and cognitive function. Methods: The study will involve an open-label exploratory randomised within-subject cross-over experimental trial, lasting 74 h. All trials will be conducted at Deakin University, Australia, in a climate-controlled environmental chamber. Participants will complete three consecutive days of simulated physically demanding work. Participants will be healthy, physically active male and female non-smokers aged 18–40 years, with a body mass index (BMI) < 35 kg·m−2, who meet Australian Army physical fitness entry standards. Individuals with metabolic disorders, acute illness, or conditions that may impair their ability to safely complete the protocol will be excluded. A sample of 14 participants will be recruited. Participants will be recruited by the project coordinator through university channels, local fitness facilities, and online platforms. Each day will involve cycles of 30-min work and 30-min seated rest for 8 h, with each work period comprising 15-min treadmill walking and 15-min manual handling (both ~4.3 metabolic equivalents of task (METs)). Participants will be randomly allocated in a 1:1 ratio, stratified by sex, to either an experimental hot humid (HOT: 35 °C, 63% relative humidity (RH), 31 °C wet-bulb globe temperature (WBGT)) or control condition (CON: 18 °C, 53% RH, 14 °C WBGT). After a > 6-week wash-out period, they will return to complete the other condition in the environmental chamber (i.e. HOT or CON). Safety monitoring includes continuous core temperature and heart rate measurement with predefined stopping criteria and post-trial well-being assessments. Discussion: Growing evidence indicates that working in the heat on consecutive days, a common situation for many workers, may lead to a cumulative increase in the physiological strain on the body. This will commensurately increase the risk of heat-related injuries while also increasing susceptibility to a range of health issues. Our findings will inform whether current heat management practices need to be adjusted to consider the impact of repeated heat exposures. Australian New Zealand Clinical Trials Registry, https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=386410; First planned enrolment 15/04/2024. Trial registration: ACTRN12623001069640. Registered on 06/10/2023.