Heat-related thermal sensation, comfort and symptoms in a northern population: the National FINRISK 2007 study

Abstract

Background: The occurrence of subjective symptoms related to heat strain in the general population is unknown. The present study aimed to describe the temperatures considered to be comfortable or hot and the prevalence of heat-related complaints and symptoms in the Finnish population. Methods: Four thousand and seven men and women aged 25–74 years, participants of the National FINRISK 2007 study, answered a questionnaire inquiring about the ambient temperatures considered to be hot and the upper limit of comfortable and about heat-related complaints and symptoms. The age trends in threshold temperatures and symptom prevalence were examined in 1-year groups by gender after smoothing with loess regression. The prevalence estimates were also adjusted for age. Results: The temperature considered as hot averaged 26°C and the upper limit for thermal comfort was 22°C. Both temperatures declined with age (from 25 to 74 years) by 1–5°C. Approximately 80% of the subjects reported signs or symptoms of heat strain in warm weather, mostly thirst (68%), drying of mouth (43%), impaired endurance (43%) and sleep disturbances (32%). Cardiac and respiratory symptoms were reported by 6 and 7%, respectively, and their prevalence increased up to the age of 75 years. The exception was thirst, whose prevalence declined with age. Most symptoms and complaints were more prevalent in women than men. Conclusions: A large percentage of this northern European population suffers from heat-related complaints. Information on these is an aid in assessing the burden of summer heat on population health and is a prerequisite for any rational planning of pre-emptive measures.

Introduction

It is well established that high ambient temperatures cause adverse health effects ranging from thermal discomfort to decreased performance and productivity,¹ through increased morbidity to mortality from cardiovascular, lung and other disease.^2,3 Elderly people, women^4,5 and individuals with a pre-existing heart, lung or psychiatric disease^3,6 are at special risk. Since populations are adapted to local conditions, the onset of adverse effects of excessive heat varies depending on the population concerned. For example, heat-related mortality rises more steeply and from a lower temperature in colder climate countries, like Finland, than in warmer southern European countries.^7–9 Hence, a population-based study conducted in a subarctic country, where people are poorly adapted to heat, would add to understanding of health effects of heat in various climates.

While many studies have addressed the severe effects of heat such as mortality, the occurrence of subjective heat-related symptoms that may actually forecast more severe outcomes has remained largely untouched. Such information in the entire population would be useful in identifying periods hot enough to cause significant health effects, assessing their severity¹⁰ and planning and assessing preventive measures. These measures should be based on information on the full spectrum of heat effects,⁵ as the specificity and sensitivity of any single outcome is unknown. Within an individual, the perception of heat depends on environmental conditions, clothing, activity level and various individual factors.¹ Thermal sensations of heat and thermal discomfort indicate potential adverse effects of heating of the body and may occur together with other general or disease-specific symptoms. To our knowledge, there are no reports on perceived temperatures of heat and thermal comfort and the prevalence of heat-related symptoms among the general population.

It is considered likely that climate change will cause environmental temperatures to rise and that heatwaves, i.e. prolonged periods of temperature higher than normal in the area, will occur more often and with a higher intensity.¹¹ Although populations are adapting to warmer temperatures,¹² heat may still be a public health burden by decreasing performance and productivity and, possibly, increased numbers of accidents^1,13 and overloading of health care facilities by people seeking medical attention.¹⁴ These adverse effects will be aggravated by ageing of populations and urbanization.³ It is therefore important to gain understanding on symptom and perception patterns, which may precede such effects.

To provide more information, we asked about heat-related sensations, comfort and symptoms in conjunction with the National FINRISK 2007 study. This is a population-based health survey conducted in Finland at 5-year intervals. The present article reports the self-reported temperatures perceived as hot or comfortable and the prevalence of heat-related symptoms among the Finnish population, together with their sex differences and age trends.

Methods

The area studied

Finland is a subarctic northern European country with cold winters and cool short summers. The study areas were the province of Oulu, the counties of North Savo and North Karelia, the cities Turku and Loimaa with nine adjoining rural municipalities and the cities of Helsinki and Vantaa (Supplementary 1). In 2007, the population aged 25–74 years in the areas concerned was 1.1 million, which represents 35% of the entire national population of the same age. In the two southern areas, the population is mainly urban and the climate maritime, the mean monthly temperatures in summer (June–August) ranging from +15 to +17°C. The three north-eastern areas are more rural and the climate is more continental, with monthly temperatures in summer between +14 and +16°C.

In summer 2006, i.e. the summer preceding the present survey, the temperatures were higher than normal, exceeding the long-term average (1971–2000) by ∼2°C both in the north and south.

Study population

Participants of the study consisted of a sub-sample of the National FINRISK study conducted in January–March 2007 in the areas mentioned above.¹⁵ A random sample of 2000 people aged 25–74 years, stratified by sex and 10-year age groups, was drawn in each area. The main study protocol included a self-administered questionnaire including questions on socioeconomic background, health behaviour, medical history and symptoms and signs of diseases. Two-thirds of the participants (N = 6671) were invited to participate in a more detailed study on temperature-related symptoms (the remaining one-third underwent a dietary survey). Out of these, 4193 (63%) attended at the clinic and were given a separate questionnaire focusing on heat- and cold-related symptoms, which they were asked to return in a pre-paid envelope. The questionnaire was returned by 4007 subjects, and each record was linked with the FINRISK main study on an individual basis. Thus, out of all those invited to participate, 60% returned the cold–heat questionnaire (48, 55, 63, 67 and 67% in age groups 25–34, 35–44, 45–54, 55–64 and 75+ years, respectively). Details of the study arrangements are reported elsewhere (http://www.julkari.fi/bitstream/handle/10024/78146/2008b34.pdf?sequence=1).

The study protocol was reviewed and approved by coordinating ethics committee of Helsinki University Hospital. All participants gave a written consent.

The Oulu cold and heat questionnaire (OCHQ 2007)

The OCHQ 2007 inquired about sensations of heat, feelings of comfort and symptoms and general complaints in warm and hot weather (Supplementary 2 gives the English translation). The questionnaire was designed by the study team at Kastelli Research Center, Oulu, which comprised specialists in thermophysiology, cardiology, chest medicine, physiatry, epidemiology, occupational medicine and public health science. The questions were designed specifically for the present purpose, using the experience from previous FINRISK studies.^16–19

The respondents were asked to record in writing ‘What ambient temperature do you find hot?’ and ‘What ambient temperature do you find comfortable’ (lower and upper boundary separately). The respondents were also asked to tick ‘yes’ or ‘not’ to ‘Does warm weather produce you any of the following symptoms?’: shortness of breath; prolonged cough or coughing bouts; wheezing of breath; increased excretion of mucus from the lungs; chest pain; cardiac arrhythmias; impaired muscular strength; sleep disturbances; thirst; melancholy or depression; anxiousness; impaired concentration; drying of mouth; headache; nausea or vomiting; unusually strong fatigue; faintness, dizziness or vertigo; impaired endurance.

Data analysis

The ambient temperatures perceived as hot were presented as a histogram together with a Gaussian kernel density estimate. The cumulative distribution was also presented to depict the temperature beyond which people started to perceive the temperature as hot. The age trends in temperatures perceived as hot or comfortable were examined in 1-year age groups, with a smoothing line fitted by loess regression with a smoothing window of 0.8, to show the age-specific mean temperatures. In this analysis, the numbers of subjects in each age group were used as weights. The results were shown as regression-predicted values together with 95% confidence bands. The prevalence of heat-related symptoms was similarly smoothed by 1-year age intervals. The symptom prevalences were also standardized by age by the direct method, using the Finnish population in 2007 as standard. The confidence interval for the standardized prevalence was based on the gamma distribution.²⁰ The calculations were performed using the R software, release 2.15.0 (available at: http://www.R-project.org, 1 April 2012, date last accessed).

Results

Characteristics of subjects

The median age of the subjects was 52 years, and the percentages of those belonging to age groups 25–34, 35–44, 45–54, 55–64 and 65–74 years were 16, 18, 21, 22 and 23%, respectively. The respondents were evenly distributed between the areas (18, 20, 21, 20 and 21% in Helsinki, Turku, North Karelia, North Savo and Oulu, respectively). Fifteen percent of the subjects had academic education and 22% only basic education. In all, 45% were engaged in office work or services (planning, management or administration), 11% in industry (factory work, mining or construction work), 3% in agriculture, forestry or stock raising and 41% were students, housewives, pensioners or unemployed.

Ambient temperature perceived as hot or comfortable

The mean temperature perceived as hot was 26.4°C (men 26.7°C, women 26.1°C) (figure 1). There was a wide range of perceived hot temperatures, with 5% of the subjects reporting the threshold for hot temperatures as low as 19.0°C or lower and 5% 33.0°C or higher. The hot-perceived temperature showed almost no age trend in men, but in women, it declined from 27.0°C at the age of 25 years to 24.8°C at the age of 75 years (figure 2).

Figure 1

Distribution of the self-reported ambient temperature regarded as hot. Left panel: histogram (bars) and smoothed kernel density estimates (line). Right panel: cumulative percentages of subjects feeling hot by increasing temperature (thin line: empirical, thick line: smoothed)

Figure 2

The ambient temperature perceived as hot, and the upper limit of the comfortable temperature, by age. Reported temperatures averaged by 1-year age groups and smoothed by loess regression with a smoothing window of 0.8. Continuous line represents smoothed values and dotted line their 95% confidence band. Circles indicate mean temperatures in each 1-year age interval

The upper limit of the comfortable temperature averaged 22.2°C (men 22.4°C, women 22.0°C) with 90% of the respondents reporting temperatures between 12.9 and 28.7°C. In women, this temperature declined throughout the age scale, from 23.5°C (at age 25 years) to 18.6°C (at 75 years) while men over the same age range showed a similar but milder decline (23.5–21.0°C) (figure 2).

Prevalence of heat-related symptoms

Most respondents (81%) reported at least some signs or symptoms of heat strain. The commonest symptoms were thirst (68%), drying of mouth (43%), impaired endurance (43%) and sleep disturbances (32%) with other complaints ranging from 1 to 29% (table 1). Cardiorespiratory symptoms were reported by 11% of respondents (6% cardiac, 7% respiratory), the most frequent individual symptoms being cardiac arrhythmia (6%) and dyspnoea (5%).

Table 1

Age-adjusted prevalence (%) of various symptoms and complaints in warm weather

Symptom/complaint	Both sexes		Men	Women	Difference women-men
	%	CI	%	%	%	CI
Thirst	67.5	64.8 to 70.2	61.4	72.5	11.1	7.4 to 14.9
Drying of mouth	43.0	40.9 to 45.1	40.0	45.4	5.5	2.6 to 8.5
Impaired endurance	42.7	40.6 to 44.8	36.3	48.1	11.8	8.9 to 14.9
Sleep disturbances	32.4	30.6 to 34.3	26.1	37.7	11.5	8.9 to 14.3
Flushing of skin	29.3	27.6 to 31.1	17.6	39.2	21.6	18.9 to 24.4
Headache	19.0	17.7 to 20.5	10.6	26.1	15.5	13.4 to 17.9
Impaired concentration	18.6	17.2 to 20.1	13.2	23.1	9.8	7.8 to 12.0
Unusually strong fatigue	16.1	14.9 to 17.5	10.8	20.7	10.0	8.1 to 12.0
Faintness, dizziness, vertigo	13.9	12.8 to 15.2	7.7	19.2	11.5	9.7 to 13.5
Impaired muscular strength	11.2	10.2 to 12.3	8.5	13.6	5.1	3.6 to 6.8
Nausea or vomiting	4.4	3.8 to 5.2	1.7	6.7	5.0	3.9 to 6.2
Anxiousness	3.8	3.2 to 4.5	2.9	4.6	1.7	0.8 to 2.7
Feelings resembling chills	3.6	3.0 to 4.2	2.8	4.2	1.3	0.5 to 2.3
Melancholy or depression	2.4	1.9 to 2.9	2.2	2.5	0.3	−0.4 to 1.1
Sore throat	0.9	0.6 to 1.3	0.6	1.2	0.7	0.2 to 1.3
Musculoskeletal symptoms	7.2	6.5 to 8.1	6.1	8.3	2.2	1.1 to 3.6
Neck pain	2.4	2.0 to 2.9	2.5	2.3	−0.2	−0.8 to 0.6
Back pain	3.0	2.5 to 3.6	2.7	3.3	0.6	−0.1 to 1.5
Pain in knees/ankles etc	4.5	3.9 to 5.2	3.3	5.6	2.3	1.4 to 3.4
Muscle pain	3.5	3.0 to 4.1	2.9	4.0	1.1	0.3 to 2.1
Respiratory symptoms	7.4	6.6 to 8.3	5.5	9.1	3.6	2.4 to 5.0
Dyspnoea	4.8	4.1 to 5.5	2.7	6.6	3.9	2.8 to 5.1
Prolonged cough	1.4	1.1 to 1.9	1.1	1.7	0.6	0.1 to 1.3
Wheezing	1.7	1.3 to 2.2	1.6	1.8	0.2	−0.4 to 0.8
Excretion of mucus	2.7	2.2 to 3.3	2.7	2.7	0.0	−0.6 to 0.8
Cardiac symptoms	6.4	5.7 to 7.2	3.6	8.8	5.2	4.0 to 6.6
Chest pain	1.7	1.4 to 2.2	1.5	2.0	0.5	0.0 to 1.2
Arrhythmia	5.5	4.8 to 6.3	2.9	7.9	5.0	3.9 to 6.3
All cardiorespiratory symptoms	11.2	10.2 to 12.3	7.7	14.3	6.6	5.0 to 8.3
Any complaints or symptoms	80.5	77.7 to 83.5	73.3	86.5	13.2	9.2 to 17.4

CI = 95% confidence interval.

Sex differences

Almost all symptoms and perceptions were more common among women than men, notably flushing of skin (female–male excess 22 percentage points); headache (excess 16%) and thirst (11% excess; see table 1). None of the symptoms were significantly more common among men, and a few symptoms showed no sex difference at all, e.g. wheezing of breath, back pain and melancholy or depression.

Age trends

Many heat-related symptoms showed a monotonic increase with age (figure 3). Few subjects (1% of men, 5% of women) reported respiratory symptoms at the age of 25 years, but with increasing age, these symptoms became more common (16% of men, 19% of women) at age 75 years. Similarly, the prevalence of heat-related cardiac symptoms increased by age, from nil at the youngest age to 12 and 21% at the highest age in men and women, respectively.

Figure 3

Prevalence of heat-related symptoms and complaints experienced in summer by age. Smoothed by loess regression, with smoothing window of 0.8. Solid line: smoothed prevalence, dotted line: 95% confidence band. Circles indicate empirical prevalence in each 1-year age interval

Figure 3 also shows significant age trends in some heat-related general complaints. Thus excessive fatigue, impaired muscle strength and impaired endurance increased with advancing age. Thirst was exceptional since its prevalence declined with age. Among all subjects, the linear decrease in feeling of thirst was 24% per a decade of age (95% confidence interval 12–35%). A greater decrease was observed among women (27% per decade; 95% CI 14 to 40%) compared with men (16% per decade; 95% CI −2 to 34%). Weak age trends were seen in most other symptoms.

Discussion

The present study is the first one to report the prevalence of heat-related sensation, complaints and symptoms in a representative sample of adult population undergoing a health survey. Despite the cold climate in Finland, a considerable fraction of people perceive general or disease-related symptoms during summer. Most symptoms were more common in women than men, and they usually increased with age, with the exception of thirst, which declined with age. The findings have obvious public health significance since heat-related symptoms involve impaired bodily or mental function that could lead to increased accidents or a worsening of pre-existing disease that results in an acute event and possibly death. Any national heatwave plan requires such quantitative information to design and target interventions.

The main strength of our study is the large representative sample of the adult Finnish population. The flexible smoothing with loess regression allowing for curvilinear trends enabled us to discern the prevalence of heat-related symptoms at the highest ages where they most likely occur.⁶ Further strengths include that we asked about pulmonary and cardiac symptoms, which can be linked to the diseases that are most often associated with heat-related mortality. We also asked about more general heat-related complaints, which some authors believe are good indicators of heat stress.¹⁰ We also report perceptions and symptoms experienced during a normal summer in the northern climate even though the summer preceding the survey was slightly warmer than average.

The main limitation of the survey is the self-reported nature of the onset temperatures and diagnosed diseases. It is possible that some important heat-related symptoms were not included in the questionnaire, but the choice of symptoms was based on expert opinion from a group of scientists experienced in this type of research.^16–19 Another limitation is that increased symptoms do not always translate into worsening disease since patients can have dyspnoea or episodes of chest pain for many years before developing a cardiorespiratory disease. We acknowledge the possibility of selection bias due to symptomatic people having greater propensity to answer, which may have led to overestimates of prevalence figures in older age groups. However, the rising age trends in symptoms are most likely explained by increased morbidity at higher ages, which was not taken into account in the present analysis.

The average ambient temperature considered hot was 26°C. This is almost identical to the threshold for hot weather defined by the Finnish Meteorological Institute (25°C). This temperature declined with age but only in women. The upper limit of thermal comfort was considered to be 22°C, but there was a strong declining trend by age, especially in women. This could indicate an impaired heat tolerance related to reduced sweating ability,²¹ or reflect the higher prevalence of diseases affected by heat-stress, or use of medications affecting thermoregulation. In addition, subcutaneous fat thickness is greater in women, and it increases with age,²² which would impair heat loss, and thus older women would feel heat stress at a lower temperature. The point at which the environment is considered too hot is complex to define, as it reflects the complex interplay between sensory and psychological responses and a variety of environmental, individual and motivational factors.²³ Hence, the questions used in this study may not describe the true temperature, but rather represent how people recollect themselves feeling during the past summer. Nevertheless, it provides a real world estimation that could not be obtained by experiments in an environmental chamber.

We have shown here that while 11% of this northern population on the average suffer from cardiorespiratory symptoms due to hot weather, more people do so among the highest ages or are female. A number of studies have reported heat-related chest pain or cardiac arrhythmia in connection to heat stroke,²⁴ hot baths²⁵ and sauna.²⁶ Dyspnoea has been reported among patients admitted to hospital for heat illness.²⁷ We were unable to find any studies linking wheezing, mucus production or cough with environmental heat exposure. The low level of atmospheric pollution in the area is unlikely to explain the symptoms. Admissions for psychiatric conditions reportedly increase during major heatwaves,²⁸ as do admissions for alcohol and drug abuse and aggressive behaviours.²⁹ The investigations cited above are either small, based on hospital admissions, or focused on severe heatwaves or otherwise extreme conditions. No previous study has reported the prevalence of heat-related cardiac, respiratory or psychiatric symptoms in the entire population exposed to normal summer heat in a subarctic climate.

A high percentage of the Finnish population suffers from general heat-related symptoms such as impaired endurance, strong fatigue or dizziness. These symptoms may be directly or indirectly related to physiological effects of heat on human thermoregulation and are in line with studies on extreme heat stress that, before collapse or death, manifests itself as fatigue, dizziness, nausea, vomiting and headache.¹ We are not aware of comparable studies conducted in general populations elsewhere. Symptoms of heat illness have been reported among special groups such as farmers,³⁰ miners,³¹ athletes³² and the military.³³ Hot conditions are known to cause sleep disturbances, e.g. worsening of REM sleep, sleep efficiency and increased awakenings.³⁴

A rising trend by age is seen in heat mortality^4,8 and morbidity.^2,3 Pre-existing diseases play a significant role in the susceptibility to heat-related adverse health effects among the elderly people.⁶ Age-related deterioration of thermoregulation could explain this. The mechanisms include diminishing ability to perceive temperature change with advancing age,³⁵ declining sweat gland activity, often in conjunction of anticholinergic medication,^1,3,21 a reduced sweat rate, diminished vasodilation and lower maximal skin blood flow, reduced cardiac output and a smaller redistribution of blood flow from renal and splanchnic circulation to the skin for conductive heat loss.³⁶ In addition, poor physical fitness, with or without disabling health conditions,¹ contributes to the adverse health effects of heat. Based on similar mechanisms, the age patterns in heat-related symptoms can be expected to follow those in mortality and morbidity but have not been previously described.

One finding highly relevant to public health advice, but consistent with previous knowledge, was the declining age trend in the prevalence of thirst. Elderly individuals have a decreased sense of thirst,³⁷ it has been proposed as a factor in heatwave mortality³⁸ and is partly attributable to pharmacological effects of anticholinergics, beta-blockers, diuretics, sympathomimetics and antipsychotics, which elderly people are frequently prescribed.

In line of our findings, other studies have noted a greater vulnerability of women than men to heat. Women usually have greater heat mortality,^4,5 although exceptions exist,³⁵ and the effect of heat on the incidence of myocardial infarction is greater in women.² Women also report several bodily symptoms such as fatigue, headache and dizziness more often than men.³⁹ Our study is the first one to report the gender difference in heat-related symptoms in the entire population.

In conclusion, this is the first study to report heat-related symptoms among the general population living in a cool subarctic climate. Even though summer heat in this area is rarely extreme, a considerable fraction of people suffer from various heat-related general, cardiorespiratory or psychiatric symptoms. We identified women and the oldest as the most vulnerable groups. The prevalence figures provided here are useful in assessing the public health burden of summers in the subarctic climate. The information can be used for proper heat risk management for people living and working in environments where heat can be a problem for human performance and health. Public health advice should encourage the elderly to drink more water during hot weather even if they do not feel thirsty. People should be warned against excessive alcohol consumption and too much physical exercise during warm periods and ask their doctors whether their diuretic, anticholinergic and other medications³ should be adjusted. A follow-up study to assess the predictive value of heat-related symptoms in terms of morbidity and mortality is underway.

Funding

Institutional Funds.

Conflicts of interest: None declared.

Key points

• It is well established that high ambient temperatures cause adverse health effects ranging from thermal discomfort to increased mortality from cardiovascular, lung and other diseases. Subjective heat-related symptoms may forecast more severe outcomes but their prevalence in the general population is not known.

• Our study shows that during a normal summer, ∼80% of the adult population in Finland perceive heat-related general, cardiorespiratory or psychiatric symptoms even though summer heat in this area is rarely extreme. Most symptoms increase by age, except thirst, whose prevalence declines with age, and the symptoms are more prevalent in women than men.

• A large percentage of this northern European population suffers from heat-related complaints. Information on these is an aid in assessing the burden of summer heat on population health and is a prerequisite for any rational planning of pre-emptive measures. We identified women and the oldest as the most vulnerable groups.