Exertional heat injuries surge at dew-point temperature ≥25.1°C

Combined data from the Singapore Armed Forces (SAF) and the Meteorological Service Singapore (MSS) show that a dew-point temperature ≥25.1°C on the same day (D) often leads to an increased risk of exertional heat injury (EHI) on the following day (D+1) during military training, reports a study.

“This finding can be practically applied to supplement the current EHI risk mitigation system,” the researchers said.

The study used data from the SAF’s heat injury registry and the MSS’ meteorological data between 2012 and 2018 to establish a consolidated dataset of EHI incidents and same-day weather parameters rank-ordered in deciles. The incidence rate ratios (IRRs) of the EHI were determined via Poisson regression modelling, referencing the first decile of weather parameters.

The researchers performed two frames of analysis: the relationship between the weather parameters and the adjusted IRR for D was described for the first and that between the weather parameters and the adjusted IRR on D+1 for the second.

The IRR on D+1 for wet-bulb temperature approximated unity for the first nine deciles but soared to 3.09 (95 percent confidence interval [CI], 1.45‒6.57; p=0.003) at the 10th decile. Likewise, for dew-point temperature, the IRR on D+1 approached unity for the first nine deciles but climbed to 3.48 (95 percent CI, 1.61‒7.53; p=0.002) at the 10th decile. [Singapore Med J 2021;doi:10.11622/smedj.2021183]

On days with dew-point temperature ³25.1°C (transition between the ninth and 10th decile), the adjusted IRR on D was 2.39 (95 percent CI, 1.58–3.62; p<0.001) relative to days with dew-point temperature <25.1°C, and the adjusted IRR on D+1 was 2.26 (95 percent CI, 1.49‒3.42; p<0.001).

“Comparing the two parameters, the raw data showed that many EHI incidents occurred at the boundary of the ninth and 10th decile for mean wet-bulb temperature in contrast to dew-point temperature, which had a more distinct spread,” the researchers said.

“Dew-point temperature could, thus, serve as a better supplementary parameter for practical application than wet-bulb temperature would,” they added.

Dew-point temperature refers to the temperature to which air must be cooled to reach saturation of water vapour, with the assumption that air pressure and moisture content are constant. Water vapour is released into the atmosphere in liquid form when air is cooled to below the dew-point temperature. [www.weather.gov/arx/why_dewpoint_vs_humidity]

Moreover, dew-point temperature indicates the absolute amount of moisture in the air, while relative humidity indicates the amount of moisture in the air relative to the amount of moisture that the air can hold at that temperature.

“A literature review of PubMed, Cochrane, Embase, and ProQuest databases … did not return any studies that demonstrated a significant correlation between dew-point temperature and the risk of heat injuries,” the researchers said.

“More primary research can be conducted to understand the relationship between physiological thermoregulatory mechanisms, especially evaporative heat loss, and thresholds in relation to the dew-point temperature,” they added.