Heat-health vulnerability in temperate climates: lessons and response options from Ireland

Globally, older people are regarded as being particularly vulnerable to heat [40] and the World Health Organization (WHO) project an additional 38,000 older people will die every year globally due to heat exposure [41]. In the global meta-analysis conducted by Bouchama et al. [42], death was more likely during a heatwave for those individuals unable to care for themselves (odds ratio [OR]: 2.97 [95% confidence interval [CI]: 1.8–4.8]), confined to a bed (OR: 6.44 [95% CI: 4.5–9.2]) or do not leave their home (OR: 3.35 [95% CI: 1.6–6.9]); behaviours characteristic of some older people. Mortality and morbidity statistics related to increased temperatures for older people are unknown for Ireland despite 13.4% of the Irish population being over the age of 65, an age group that has been increasing since the 1980s [43]. Roche et al. [44] in their analysis of computer tomography kidney, ureters and bladder scans in an Irish hospital group, did find that older adults (> 65 years) were more susceptible to ureteral colic during warm weather compared to those below 65 years old, potentially due to an increased susceptibility to dehydration. Older adults (> 75 years) were identified as a particularly vulnerable group in Pascal et al.’s [26] study of heatwaves and mortality in Ireland. Analysis from General Practitioner consults and Emergency Department visits in England (Köppen system classification: temperate, Cfb) indicated that during the 2013 heatwave, older people (aged ≥75 years), along with school children, were particularly at risk of heat illness [45]. Notably, ill-health continued after the heatwave ended in the ≥75 years age group [45]. Despite WHO/Europe recommendation for the implementation of heat-health action plans [46], only limited heat response planning has been conducted by relevant bodies in Ireland such as weather warnings from Met Eireann, the Irish meteorological service, and press releases and website information from the Health Service Executive (HSE), Ireland’s community and hospital public health service.

A compounding factor is that a large proportion of older people take medication for chronic conditions. It is estimated that 33% of over 65 s in Ireland report polypharmacy (regularly taking ≥ five medications), with medications for cardiovascular disease and hypertension the most common [47]. Although a topic currently under-researched, medication use can increase vulnerability to suffering heat-related illness. Anticholinergics, antihistamines, phenothiazines, tricyclic antidepressants and antispasmodics can impair sweating; tricyclic antidepressants and ergogenic stimulants can alter heat production; lithium, diuretics, laxatives and levothyroxine disrupt water balance within the body; whilst calcium channel blockers, barbiturates and beta blockers are known to alter skin blood flow and / or blood pressure [48]. Even non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin and ibuprofen, can alter core body temperature and efficient thermoregulation [49]. Indeed, Page et al. [50] in their study on mental illness and temperature-related deaths in England (Köppen system classification: temperate, Cfb) found that taking hypnotic/anxiolytic and antipsychotic medications increased the relative risk of mortality during hot weather by approximately 7% per 1 °C temperature increase, although the association did not seem to hold true for taking antidepressants. A comprehensive review, focusing not only on older people, is required on the implications of medication usage in temperate regions in the context of a warming climate and the current burden of disease.

It is not only older people taking medications that are vulnerable but with the high burden of chronic non-communicable diseases (NCDs) in Ireland, and many other temperate regions, many more could be at risk. According to the WHO, globally, 80% of all premature NCD-related deaths occur from cardiovascular disease, cancers, respiratory disease and diabetes [51]. A global meta-analysis conducted by Bouchama et al. [42] calculated that suffering from cardiovascular disease or pulmonary disease increased the odds of death during a heatwave by 2.48 (95% CI: 1.3–4.8) and 1.61 (95% CI: 1.2–2.1) respectively. In Ireland, Pascal et al. [26] found excess deaths during heatwaves attributable to cardiovascular and respiratory causes. Furthermore, analyses from the UK (Köppen system classification: temperate, Cfb) also found an increased sensitivity for heat-related deaths by cause of cardiovascular or respiratory disease [52]. In a study conducted by Rey et al. [53] the greatest percentage of excess mortality during six heatwaves between 1971 and 2003 in France (Köppen system classification: majority temperate, Cfb) were observed for cardiovascular, respiratory diseases as well as neoplasms, heat-related causes, injuries and poisoning. All neoplasms (i.e. cancers) were included in the analyses (ICD-10, C00-D48). Type-2 diabetic and obese individuals are also at risk of ill-health during high ambient temperatures. In a temperate region (Köppen system classification: temperate, Cfb) English type-2 diabetic patients were found to be at an increased odds of presenting to their medical practitioner on days of high ambient temperature and these odds were elevated for diabetic patients with cardiovascular comorbidities [54].

Regarding other NCDs, in our Irish search of the literature, Roche et al. [44] found an association during the summer months in Ireland (Dublin), for an increased frequency of presentations to Emergency Departments for acute renal stones. Furthermore, a study conducted by Flaherty et al. [55] suggested that specific travel advice should be provided to obese people that may exhibit poor heat tolerance travelling to warm climates. NCDs in Ireland are the greatest cause of death accounting for 575.36 deaths per 100,000, with nine of the top 10 DALYs attributable to NCDs [56]. With such a large burden of disease attributable to NCDs, further determination of the impact of heat on the health of these populations should be urgently conducted in Ireland so as to respond to what could be an emerging, compounding health crisis.

The mentally ill are particularly heat susceptible as the global literature identifies due to medication usage, a limited ability for self-care, poor cognitive awareness of environmental hazards and inadequate adaptive behaviour change such as hydration and clothing choice [28, 42, 48, 57, 58]. This relationship has been identified in a temperate city, Adelaide (Köppen system classification: temperate, Csa), as shown by a 7.3% increase in hospital admissions for psychological disorders during heatwaves [28]. It is estimated that those with a psychiatric illness are at a triple risk of death during a heatwave (OR: 3.61 [95% CI: 1.3–9.8]) and taking psychotropic medication increases the likelihood of death almost two-fold (OR: 1.90 [95% CI: 1.3–2.8]) [42]. Mental ill-health in Ireland carries a prevalence of 18.5% including anxiety and depressive disorders, bipolar disorders and schizophrenia as well as alcohol and drug use disorders [59]. The prevalence of mental ill-health in the Irish population is higher than the EU average of 17.3% and only slightly lower than Finland, which exhibits the highest prevalence in the EU at 18.8% [59]. Given this prevalence of mental ill-health, along with subsequent medication usage to manage disorders, the effects of heat on populations with poor mental health in temperate regions needs further investigation.

Heat stress can affect human health and wellbeing throughout the lifecycle including the unborn child, infants, children and pregnant women [60, 61]. A global systematic review by Zhang et al. [61] found that both high and low temperatures were associated with adverse birth outcomes of pre-term birth, low birth weight and sometimes, stillbirth. The association appeared stronger for high temperatures however the authors called for more research to be undertaken as well as further research in “diversified climate zones” as the review had no geographical boundaries. Epidemiological studies of temperature effects on birth outcomes have been conducted in temperate climates (Köppen system classification: temperate, Cfb) such as London [62] and Aberdeen [63]. Results are mixed, with the London study [62] reporting a non-significant effect for maximum temperature and preterm births, and the Aberdeen study [63] reporting reduced birthweight if the ambient temperature was high during the first trimester of pregnancy. Therefore, we concur that more studies are needed in different climate zones, particularly in temperate regions, to determine the effects of high ambient temperature for a given location on birth outcomes.

As indicated in a study from Melbourne, Australia (Köppen system classification: temperate, Cfb), during warm months, children might desire to spend more time outdoors [64], thus potentially exposing them to environmental heat stress, UV radiation and air pollutants through this behaviour change [65, 66]. Physiologically, there is debate whether children are vulnerable to heat stress through underdeveloped and reduced sweating responses [65, 67‐69], reduced locomotion economy resulting in greater heat production per unit of body weight [68], smaller total blood volume [70] and increased requirement for shunting cardiac output to the skin compared to adults [68]. Although children’s reliance on dry heat dissipation may mitigate their vulnerability [68]. Behaviourally, it is generally accepted that children are vulnerable to heat stress given their decreased awareness of fluid intake requirements [65], a possible extended duration to achieve acclimatization [71], spending time outdoors in play as well as often playing on, or close to the floor where thermal radiation levels can be high depending on solar intensity and surface material.

Although research is sparse and outdated, it has also been suggested that poor regulation of the indoor environment, including high temperatures, can result in lowered test performance in children [72]. Certainly, high temperatures indoors have been shown to reduce adult job speed, concentration and clarity of thought [73, 74]. The impact of air-conditioning regulating the indoor environment to a tolerable thermal comfort level could mitigate temperature-induced performance decrements [75, 76] however, research on this is limited, outdated and requires further investigation, particularly over the loss / gain of an increased energy requirement for air-conditioning and possible emissions.

Outdoor workers, those who are physically active and those who are recreationally active outdoors are also considered heat-vulnerable. Outdoor workers are at particular risk to six out of the seven climate-related occupational hazards identified by Schulte et al.’s [77] framework. That being: increased ambient temperature; air pollution; ultraviolet radiation; extreme weather; vector-borne disease and other biological hazards; industrial transitions and emerging industries (e.g. green jobs). Of note is outdoor workers required to wear personal protective equipment (PPE) who are at an elevated risk of suffering heat-related illness. Under specific conditions, heat strain can even occur in temperatures as low as − 20 °C (if the workload is moderate to high and the worker is wearing semi-permeable or impermeable PPE [78‐80]). Although gender and heat-vulnerability has not been well characterised in the literature, it may be that occupational status may be a relevant factor in many studies suggesting a heat and gender-vulnerability association, usually placing males at greater risk [8]. Indeed, in a study of acute nephrolithiasis during summer in Ireland, men were found to be at greater risk of developing stones in warm weather compared to women [44]. However, in a global study focused on women firefighters, there is evidence to suggest that they suffer from heat-related illness at work with the call for further specific heat-related support for this heat-vulnerable population [81].

A study on non-military working populations in the state of Washington (USA) (Köppen system classification: temperate Csb, Csc, Cfb) found that heat-related compensation claims for workers were associated with high ambient temperatures [82]. Common occupations associated with the claims included: construction workers, fire fighters, labourers and material movers. Other studies from the region have indicated adverse heat-related health outcomes for agricultural workers [83]. Research on climate change-induced warming on occupational health in outdoor workers in Ireland does not yet exist to our knowledge, despite over 10% of Ireland’s workforce currently employed in common outdoor sectors of agriculture, forestry, fishing and construction [84]. Although ISO standards (International Organization for Standardization) exist, Irish-specific policies do not appear to adequately protect the outdoor worker from heat-related adverse health outcomes. For example, to our knowledge, there are no Irish-specific codes of practice for working outdoors in extreme heat. A useful example to adapt to the Irish situation would be the National Institute for Occupational Health and Safety’s (NIOSH) Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments. Another example, although not as comprehensive, is the United Kingdom’s Health and Safety Executive’s guide: Heat Stress in the Workplace - A Brief Guide; as well as various resources from the UK Health and Safety Executive’s website. The development of such policies by the Irish Health and Safety Authority is essential for a proactive approach, rather than reactive response to the occupational health challenge of heat in the workplace. Financially, evidence has shown that the costs of implementing systems proactively to protect against the risks of climate change on health will save costs in the long-term as compared to reacting to incidents [14, 85]. Global statistics are available: an estimated 153 billion hours of labour were lost in 2017 due to a warming planet, 80% of which was from the agricultural sector [86] projected working hours lost by 2030 due to heat are 2% per year [87]. Thus, the opportunity exists for Ireland to adopt evidence-based proactive action on protecting outdoor workers from heat.

Whilst Pascal et al. [26] identified and analysed mortality attributable to heatwaves for vulnerable groups in Ireland, such as older people and those living in rural or urban areas, a further analysis on vulnerable groups was not undertaken although the authors mention the importance of focusing on outdoor workers including farmers. We would echo this call for research prioritization of farmers and would also add other vulnerable outdoor workers of specific relevance to Ireland such as those required to wear PPE and construction workers. Particularly as the construction sector is experiencing steady growth in Ireland following recovery from the recession. It is essential that health and safety policy specific to Ireland, and other temperate regions, include basic guidelines for employers to adhere to workplace heat limits and surveillance, hydration strategies, mandatory rest breaks during hot weather and special considerations for those wearing PPE.

We also advocate that a needs assessment be undertaken to determine training and capacity needs and research priorities for outdoor workers leading to improved health and safety. Adam-Poupart et al. [88] provide a comprehensive overview of the main impacts of a changing climate on occupational health and safety in Quebec, Canada (Köppen system classification: continental Dfa, Dfb, Dsc and polar ET). Heat was identified as a key hazard with the following potential consequences on occupational health and safety: heat-related illnesses (including cardiovascular and renal issues, dehydration); increased absorption of chemicals; reduced cognitive performance; dermatological issues; increased accidents related to lowered vigilance and manual dexterity; perturbed emotions and mortality [88]. Priority focus areas for research included the interactions of chemical toxicity and heat as well as the physiological strain of wearing PPE during high temperatures [88]. It was also recommended that heat-health-focused training tools be developed and disseminated [88]. Investigation of heat-related risks, research priorities and recommendations for improved occupational health and safety, are also needed for Ireland.

Further factors compounding vulnerability to heat stress, even in temperate climates includes those of a poor socio-economic status [89] and those living in urban heat islands (UHI) [90]. In an analysis of the European heatwave in 2003, Rey et al. [89] identified that in Paris (Köppen system classification: temperate, Cfb), those who were deprived were more vulnerable to death during the heatwave. Vulnerability of the poor to heat may be through reduced adaptive capacity such as the inability to afford air conditioning, poor general health, lifestyle risk factors and living in densely populated cities. Although race and ethnicity and heat-vulnerability has not been well characterised in the literature, it may be that community-level characteristics may be a relevant factor in many studies suggesting a race or ethnicity-vulnerability association [8]. Although the most recent Survey on Income and Living Conditions (2017) in Ireland revealed that overall, poverty and deprivation is decreasing as the mean annual household disposable income rose by 4.7% from 2016; the “at risk of poverty rate” was at 15.7% of the total population [91]. This indicates the percentage of the population “whose equivalised income was less than 60% of the national median equivalised income”.

Heaviside et al. [90] assert that the projected impact of heat on populations is likely underestimated given that temperature monitoring stations are usually on the outskirts of heavily built up areas. With high spatial resolution modelling over two different scenarios (one taking into account the UHI), Heaviside et al. [90] estimated that the UHI contributed to 50% of the mortality during the August 2003 heatwave in the West Midlands of the UK (Köppen system classification: temperate, Cfb). Sheridan and Allen (2018) argue that even within cities, vulnerabilities differ spatially [92]. Internal migration statistics for Ireland indicate that with a 3.8% population increase between 2011 and 2016, the rural / urban divide grew during those years, with more people living in urban areas (62.7%) compared to rural areas (37.3%) [91]. Pascal et al. [26] caution that rural communities should also benefit from appropriate heat-health prevention plans, particularly in countries such as Ireland which has a significant rural population and where access to healthcare may be more remote. The long-term trend for Ireland is one of increased urbanisation, which from a heat-health perspective, translates to a steady increase in the proportion of the population at risk of heat-related morbidity and mortality.

The direct and indirect impacts of climate change on the natural environment are expected to be significant in the long term (2050 and beyond), potentially further exacerbating existing pressures on ecosystems and contributing to the further decline of some species as well as impacting water, food production and land and soil erosion [93]. In particular, the natural environment is vulnerable to cascading effects that can potentially result in non-linear ecosystem responses, resulting in loss of ecological function and with major implications for human health and wellbeing. As function is reduced or lost, the buffering the natural environment provides against hazards such as top soil erosion or pests, is sharply reduced.

Ireland’s National Adaptation Framework (2018) states that both the agricultural and the marine and fisheries sectors will be deeply affected by climate change [7]. Changes in air and soil temperatures and changes in rainfall patterns and extreme events are predicted to cause water stress for crops and heat stress for animals. In addition, plant diseases which are currently rare may occur more frequently and the mobility of machinery on fields may be affected due to increased levels of winter rainfall. Increased sea surface temperatures will affect the biogeographical ranges of species distribution including major commercial fish stocks. Warmer waters support lower levels of dissolved oxygen and provide favourable conditions for the growth of algal blooms resulting in increased health risks [7]. In addition to direct impact such as heat stress on food systems, there is an increasingly growing global literature on the impacts of climate change on food insecurity (e.g. [94]), food-related illnesses (e.g. [8]) and the loss of nutritional value of food (e.g. [95]). This combination of impacts will have wide ranging implications at the global scale as well as the national one.

Linked closely with the vulnerability of outdoor workers, the Irish food system, with a large focus on beef and dairy production, is particularly vulnerable to rising temperatures. Just as humans have a physiological thermal tolerance limits, so too do crops and livestock. In the temperate climate of the UK, Part et al. [96] found that broiler chickens were vulnerable to heat stress-induced mortality during warm and humid weather. Palacio et al. [97] found financial benefits (decreased water consumption) when pasture dairy cows were provided with shaded areas during the summer in a temperate climate (Ontario, Canada). The call for providing livestock shelter in warm weather of temperate climates was also advocated by Moons et al. [98] for improving welfare and productivity. Initial reporting into the impacts of climate change on dairy production in Ireland have identified extreme weather events, fodder production challenges and the emergency of new diseases and pests as major threats but did not identify heat specifically as a challenge to the industry [99]. This suggests that the industry is either not yet aware of the issue or that sufficient evidence has not yet been provided for sector decision-makers.

Global estimates of the impact of heat stress on crop yields (wheat, maize, rice and soybean) found that a particularly sensitive region globally were continental regions between 40° and 60° North [100]. Although Ireland’s location is at 52° N, it is not continental and may not be at the highest risk. However, warming temperatures, often in conjunction with increased wetness, provide more suitable habitats for growth of biological contaminants [101] and it is projected that globally increasing temperatures might introduce new pests and pathogens [102]. All of these changes would result in a greater need for pest and contaminant control, which is often in the form of chemicals (pesticides, herbicides, fungicides etc.). This will increase the chemical load on ecological systems and increases the risk of hazardous chemical exposure and poisonings in humans. Indeed, Patterson et al. [103] state that another concern is how climate change may affect the potency or effectiveness of pesticides used for pest control and project a possible further increase in chemical use because of this. It is not only the direct health impacts of potentially greater exposure to chemicals that is a concern for farmer-health, but also the mental distress possibly experienced in light of the financial stress arising from these, and other climate-related complications.

The health and social care sector is a large employer in Ireland. While it forms a significant component of the economy, it is also recognised as a source of carbon emissions. Therefore, the sector must play a dual role: to lead by example in acting to mitigate climate change as well as reduce the overall vulnerability of the population to the impacts of climate change. However, the sector itself is also vulnerable to rising temperatures from the perspective of healthcare quality and essential public health infrastructure and services such as power [8]. Most notably, this would be through an increased capacity need especially during extreme events. The projected increase in days of hot weather or heatwaves will likely increase the levels of morbidity and mortality exhibited in vulnerable populations both during and also following an event (lag structures) [15, 104]. This anticipated increase in service demand, dictates the need for a thorough analysis of hospital admissions during, and considering an admission lag after, the hot event, is necessary for detailed planning of particular care units or specializations that might require extra capacity such as geriatric, psychiatric and neonatal. Preparedness in supplies, capacity and other resources will involve further research of a detailed analysis of the anticipated health outcomes of rising temperatures to include both direct (such as dehydration) and indirect (such as food-borne diseases) health outcomes.