Introduction
Extreme heat poses a risk to the health of perinates and maternal women.1 Extreme heat is increasing in frequency, duration and intensity, because of human-induced climate change.2 3 It has been found that nowhere is safe from the occurrence of heatwaves.4 This poses a health risk to a growing proportion of the population because extreme heat has many impacts on health outcomes and has been associated with increased hospital admissions and mortality rates.5 6 A recent study reported that 65 000 people died across Europe in heatwaves during Summer 2022.7 In addition, it has been found that babies born in 2020 will be exposed to 30 heatwaves over their lifetime with a 1.5°C warming, over six times more than someone born in 1960.8 Extreme heat also has indirect impacts on health, for example, posing challenges to infrastructure resilience and food systems.8 9
Humans are homotherms and must maintain an internal temperature of around 37°C despite the surrounding environmental conditions.7 9 Groups vulnerable to the health effects of extreme heat and heatwaves are those with a limited capacity to thermoregulate.10 Newborns, children, pregnant women, elderly and those with a range of existing medical conditions for example, diabetes, cardiovascular diseases and respiratory conditions, such as asthma, are in this category.7 10 Those who work outdoors and with heavy physical labour are more vulnerable because of their prolonged exposure to extreme heat.7 10
Pregnant women and neonates are two important groups that are at risk to extreme heat. Pregnant women are particularly vulnerable due to reductions in their ability to thermoregulate and are known to experience adverse health outcomes because of extreme heat exposure.1 Neonates also experience adverse health outcomes, which in some cases is thought to affect development throughout childhood.11 Through the Sustainable Development Goals (SDG) and other global programmes, targets have been set for countries to attain, which could be jeopardised by exposure to extreme heat.12
A growing body of literature demonstrates that temperature alone is not an accurate indication of the impact of the thermal environment on the body’s thermoregulatory response for heat stress.13 14 For example, a previously published heat primer for public health describes heat stress as the buildup of body heat resulting from exertion and/or the external environment, with the external environment, including air temperature and other factors, such as wind speed, humidity and incidence of radiation.15 In addition, physical activity and clothing also influence human body heat balance and heat strain. There is also a primer discussing the importance of humidity, a factor in heat stress16 and human thermoregulation and heat stress is reviewed by another.17
Over time, heat stress and thermal comfort indicators have been developed with the aim of better capturing the risk levels associated with the thermoregulatory response of the body to different aspects of the thermal environment.18 19 Multiple reviews and studies consider the effectiveness and ease of use of these metrics.19 20 Consistently studies find that heat and thermal comfort indices that consider temperature, humidity, solar radiation and wind speed are a better indication of the risk than simpler metrics.21 However, the literature states that no perfect indicator is present.18 19
There is no consensus on how to include extreme heat in studies, making it a challenge to build up a reliable evidence base for how extreme heat affects health across the globe. For maternal and perinatal health, most studies only use mean and/or maximum air temperature when considering the impact of heat.1 22–30 Reviews showed that the health outcomes associated with exposure to extreme heat were preterm birth, low birth weight, congenital anomalies and stillbirth.1 22–27 Very few studies considered by these reviews use comparable methodologies and none of these reviews focused solely on heat stress exposure, often combining the metrics with air temperature.1
Therefore, given targets to improve quality of care and protect the lives of mothers and neonates and because heat indices better capture body responses and risk levels associated with the thermal environment, it is important to outline the evidence to date for heat stress and maternal and neonatal health. The purpose of this scoping review is to answer three questions: (1) what is the biometeorological background of heat indices? (2) How does heat stress influence maternal and perinatal health outcomes? And (3) how do heat indices fit within the maternal heat balance?