"Limit your physical activity until you become accustomed to the heat." (World Health Organization, Preventing Harmful Effects of Heat Waves, 2006)

A minority of public heat warning web sites advise older people to acclimatize gradually to heat in order to build up their physiologic defences.¹ However, the degree to which they can acclimatize to heat the way young, fit people do is relatively unclear. Pre-seasonal advice on acclimatization may also be useful for people who are newly at-risk, people who have recently moved to warmer climates, or for parts of the world where extreme heat is becoming more frequent.

The Possibilities and Limitations of Acclimatization for Protection Against Extreme Heat

Our survey of current hot weather messaging found little mention of acclimatization, or instructions on how to acclimatize as a means to protect against elevated temperatures.¹

Protective benefits of acclimatization

Early observational studies evaluated acclimatization as a protective factor against heat-related mortality.^2-4 It has been noted that extreme heat episodes, later in the summer, typically extract a lesser toll of heat-related deaths than heat waves of equal levels of intensity in late spring and early summer.^{3, 5} While this relative drop in impacts later in summer may be partly attributable to an early summer reduction in the pool of vulnerable people after the first heat event, evidence suggests that survivors of the first heat wave become physiologically acclimatized and hence deal more effectively with subsequent episodes of extreme heat.⁵

Adaptive behaviours and physiological changes that enable people to better cope with heat can be described as short-term or long-term. Long-term adaptation to heat (sometimes termed “acclimation”) is largely dependent on behavioural adjustments (clothing, activity level, etc.) as well as cultural changes (housing design, use of air-conditioning, siestas, etc.). Short-term acclimatization depends on physiologic changes, typically over a period of days to weeks, that represent reduced strain for a given heat load.

The changes associated with acclimatization include more effective sweating mechanism (lower volumes and salt content) as well as cardiovascular changes.

Temperature indices, used currently by meteorologists to convey the combined comfort impact of heat and humidity (Humidex in Canada/Heat Index in the US), use current day values only, without taking previous weather into account.⁵ This may not give an accurate picture of risk due to exposure to heat (and humidity) over time, or of the potential of acclimatization in mitigating this risk.

What is the evidence that gradual exposure to heat may confer benefits to older or vulnerable populations?

Physiological studies on typical subjects:
Extensive experimental studies were carried out, mostly in the 1960s/1970s on fit, young, male subjects largely in support of increasing fitness and productivity in industrial settings,^{6, 7} in sports training, and for military manoeuvres in tropical climates. Subjects exposed to vigorous exercise in heated thermal chambers were found to have improved heat stress responses (heart rate, core temperature, stroke volume) after a period of several days.^8-11

Measurements of certain stress parameters following this type of controlled, artificial acclimatization generally show improvement starting around day three, with increases up to days 10-14.^6-11

For workers in hot environments, the ACGIH threshold limit values (TLVs) set different guidelines for the acclimatized and the unacclimatized.¹²

Physiological studies on atypical subjects:
We were unable to find experimental studies of acclimatization using subjects older than 60, but two studies based on older men still in the workforce were found.^{13, 14}  Lind, in 1970 published the results of the influence of age and heat exposure on two groups of British coal miners.¹³ The average age of the younger group was 27 (range 23 – 31), while that of the older group was 47 (range 39 – 53). Thermal chamber experiments, where subjects simulated an eight-hour-day’s work, were carried out over a two week period, but temperatures were varied randomly between neutral, warm, and hot so acclimatization effects were not significant. However, differences between the age groups was marked in hot conditions; older men showed a pattern of consistently higher core temperature, less sweat during work and more during rest, and indicators of increased cardiac strain. Lind concluded that rates of acclimatization in the older group may be delayed and diminished.¹³

Robinson conducted a small study where he retested men between the ages of 44 and 60 (mean age 52), who had completed thermal chamber acclimatization studies 21 years prior, to see if they could obtain the same level of acclimatization to heat.¹⁴ All but one of four subjects tolerated work in the heat as well or better than 21 years earlier. Essentially the same pattern of change in heart rate, rectal temperature, and average skin temperature was obtained after 5 – 13 days of exposure, as was seen in the earlier study.

Physiologic age-related changes:
Circulatory system
With the elderly, there is a decreased ability to pump and redistribute blood to the skin.  There is a decline in cardiac reserve and age-related reduction in vascularity which subsequently decreases peripheral blood flow. These changes would reduce the efficiency with which heat can be removed.¹⁵ Furthermore, conditions such as atherosclerosis, heart failure, and hypertension reduce even further the body’s ability to respond to extreme heat.¹⁶

Sweat glands
Sweat gland function gradually declines in the age range of the 70s and 80s.¹⁷ Aging causes a reduction in the number of sweat glands and the sweat gland response.¹⁶ Lifetime ultraviolet exposure and other environmental factors contribute with chronological age to reduce sweat gland responsiveness.¹⁸

Cardiovascular disease—how does it affect heat response?
Aging is associated with reduced cardiac output as well as less redistribution of blood flow from the intestinal and renal circulations.¹⁹ In a study comparing the cardiovascular responses of young and old men during direct passive heating to the limits of thermal tolerance, Minson et al. found that reduced cardiac output was primarily the result of a lower stroke volume, since the older men were able to increase their heart rate to a similar extent as the young men.²⁰ However, the older men had to attain a greater proportion of their heart rate reserve. People suffering from cardiac disease are unable to augment their cardiac output adequately to produce the necessary level of vasodilation to dissipate heat by evaporation, conduction, and convection.²⁰

Behavioural age-related and chronic disease-related changes
Described risk factors for death or morbidity during severe heat episodes characterize the “at risk” population as being socially isolated and elderly (esp. over 75 years old),²¹ living in urban areas, often housebound or unable to care for themselves independently, and disproportionately living in institutions.^22-25 From experimental studies on people in their 40s and 50s, it appears that healthy older people may be able to attain levels of physiologic acclimatization close to younger counterparts, but lack of activity and lack of exposure to a range of temperatures may deprive them of the opportunity to effect this protective adaptation.¹⁴ However, people with cardiac and renal impairment may not be able to achieve these results.

Can air conditioner use impede “natural” warm weather acclimatization?

No scientific papers have addressed the issue of failure to achieve acclimatization by air conditioning use among the older population who are most at risk for heat-related mortality.

Ellis cites  reductions in indoor temperature limits (from 25.6˚C  to 23.3˚C ) on U.S. Navy ships as evidence that modern sailors acclimatized to air-conditioned decks have lost their long-term acclimatization to heat.²  In his 1972 study of mortality from heat illness in the US, Ellis criticizes the design requirements of modern buildings which rely entirely on air conditioning and lack windows that open. He states that these structures are at the mercy of power supplies.² He illustrates this danger by citing the case when 21 of 89 residents of a nursing home in Florida became hyperpyrexic and five died when the air-conditioning system was shut down for repairs.²⁶ Ellis reports significant mortality excess from the summer of 1970 in New York resulting from electrical power cuts during a heat wave.²

What advice regarding acclimatization should we give to vulnerable populations?

• Acclimatization to heat appears to be protective in both experimental and epidemiologic studies. Several public heat-warning websites target this advice to people exercising or playing sports in the heat; however, ways to effect heat adaptation in vulnerable people remains unclear. Experiments on men as old as 60 suggest that training in thermal chambers can promote physiologic changes which indicate an improvement in cardiovascular and sweating mechanisms to lower heat strain.
• Among frequent recommendation, given for heat protection, is to stay in a cool or air-conditioned environment. This advice seems well supported by evidence to protect vulnerable people from episodes of extreme or unusual heat (see subsequent   section on mechanical air cooling). However, there may be benefits derived from some degree of heat exposure when temperatures are not as extreme. It is possible that the avoidance of outdoor temperatures and strenuous exercise deprive seniors of the opportunity to achieve some degree of acclimatization. Experiments on men as old as 60 suggest that training in thermal chambers can promote physiologic changes which indicate an improvement in cardiovascular and endocrine mechanisms to lower heat strain.¹⁴
• Research into the benefits and limits of heat acclimatization in vulnerable populations is extremely limited. We still do not know to what extent these changes can be attained by people in their 70s and 80s, people who do not engage in strenuous exercise, or those with chronic medical conditions. More research needs to be done to answer these questions.

References

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March 2010