Above the average daily temperature of 29.9°C, each degree increase was associated with approximately 190 meters less in total distance travelled. When the average exceeded 32.9°C, the maximum distance reached from home decreased by approximately 540 meters for each additional degree. The heat, it seems, can also tighten the geography of our days. This is what a new study published in Innovation in Aging by a group of researchers from Stanford University and the University of Alabama at Birmingham.
The researchers followed 82 people between the ages of 61 and 89, with an average age of 74.5, through an app installed on the iPhone. For two weeks the phone recorded the movements, allowing us to measure what scholars call life-space mobility: the geographical footprint within which a person lives, moves, works, meets others and carries out their daily activities. Then those data were related to the average temperature of the day.
When the temperature rises, everyday space shrinks
The first signal appears already around 28.8 °C on average daily. Above that temperature, for each additional degree the geographical area that encompassed the participants’ movements was reduced on average by 2.97 square kilometers. We are talking about a surface, therefore, and not about three kilometers less traveled or a radius shortened by the same size.
At 29.9°C another change occurred: the total distance traveled began to decrease by 0.19 kilometers per degree. Above 32.9°C the maximum distance reached from home also narrowed, by 0.54 kilometers for each additional degree. The association emerged during the summer and remained statistically significant after accounting for, among other things, humidity and some sociodemographic characteristics. In autumn and winter, researchers observed curvilinear trends, but did not find statistically significant associations in the adjusted models.
These are thresholds that should be read carefully: the study uses the average temperature of the entire day, so 30 °C here does not mean the normal afternoon maximum of a summer day. A daily average of that flow indicates decidedly warm conditions even for many consecutive hours.
For the elderly, reducing travel when the thermometer rises may be, at least in part, a prudent response. The National Institute on Aging reminds us that with age the body tends to adapt less effectively to sudden changes in temperature; Chronic diseases and some medications can also make thermoregulation more difficult or increase the risk of dehydration. Going out less during a heat wave can therefore protect us from the heat. The problem comes when that inside the house starts to last a long time.
Can heat increase insulation? Meters do not measure loneliness
A distinction is needed here. The study did not measure loneliness, nor did it ask participants how often they met with friends and relatives. We cannot use those 190 meters as a unit of measurement of social isolation.
It recorded something more material: with very high summer temperatures, in the small sample studied older people occupied a smaller geographical space. And there life-space mobility It is of interest to those who study aging precisely because it includes many dimensions of daily autonomy. The same authors recall that previous research has linked more limited mobility to various indicators of health and well-being, from depressive symptoms to cognitive decline. These links do not prove that heat causes loneliness, but they explain why the restriction of travel deserves attention.
The matter becomes particularly concrete during long heat waves. Staying cool may mean giving up a walk, an errand or a usual activity. For a person with an already fragile social network, a succession of too hot days can therefore also reduce the opportunities for meeting. It is a possibility consistent with mobility data, yet to be directly measured.
The answer cannot simply be “go out more”
The authors also see the results as a possible tool for rethinking urban planning in a warming climate. If above certain temperatures the elderly tend to move less, accessible and cool places are needed where they can continue to carry out daily activities and maintain relationships, especially in the neighborhoods most exposed to the heat.
Health indications already go in this direction. The Centers for Disease Control and Prevention advises those over 65 to spend the hottest days in air-conditioned environments, drink without waiting to feel thirsty and maintain contact with friends or neighbors. A library, a senior center or an air-conditioned public space thus becomes something more than a place to wait for the heat to pass.
Before transforming the thresholds found at Stanford into a new summer law, however, the champion calls for a lot of caution. There were 82 people observed and only 17 participated during the summer. They were also predominantly white, well educated, relatively wealthy, living in urban areas, and in fairly good health. To enter the study you also needed to own at least an iPhone 8 and regularly carry it with you. The researchers were unable to take into account the means of transport used or the possible use of mobility aids.
These are US data and are preliminary, so they do not say how all elderly people react to the heat, much less allow those thresholds to be automatically transferred to the Italian population. But they photograph a very simple mechanism that we lose sight of while we discuss record temperatures: extreme heat can also change how far we can, or choose, to go.
In the small group observed during the summer, once a certain temperature was exceeded, the daily territory literally began to contract. The park, friends and errands remained where they were. What was moving was the boundary within which it became reasonable to reach them.
