Extreme heat is the most common cause of weather-related deaths in the US.
The research team, including Jennifer F. Bobb of the Department of Biostatistics at Harvard School of Public Health (HSPH) in Boston, MA, notes that extreme heat is the most common cause of weather-related deaths in the US.
Earlier this year, a report from the Centers for Disease Control and Prevention (CDC) found that between 2006 and 2010, around 31% of weather-related deaths were caused by exposure to excessive natural heat, heat stroke, sunstroke or a combination of all three.
The number of fatalities as a result of extreme heat is expected to rise, according to the researchers, as climate change means heatwaves are likely to become more intense, more frequent and last longer.
It is well established that elderly populations are at highest risk of illness from heat exposure; the CDC note that older people do not adjust as well to sudden changes in temperature as younger individuals, and they are more likely to have a medical condition that adjusts the body's normal response to heat.
However, Bobb and colleagues note that current understanding of the health effects of extreme heat among the older population is based on studies that have only assessed a small number of possible outcomes, such as heat stroke, dehydration and respiratory and cardiovascular diseases.
As such, the team set out to determine how periods of extreme heat may be linked to hospitalizations due to serious illness among seniors.
To reach their findings, the researchers used Medicare inpatient claims data to analyze 23.7 million individuals aged 65 or older from 1,943 counties across the US. All patients had been admitted to the hospital between 1999 and 2010, and 214 diseases were identified that accounted for these hospitalizations.
The researchers also gathered information from more than 4,000 temperature monitors from at least five summers in the US.
Seniors at greater risk of five diseases during heatwaves
The team identified five diseases that were associated with increased risk of hospitalization among older adults during periods of extreme heat - defined as 2 or more consecutive days that exceeded the 99th percentile of daily temperatures in each county.
Heat stroke was the most common cause of hospitalization during such periods; older adults were 2.5 times more likely to be admitted to the hospital for heat stroke during periods of extreme heat than non-heatwave periods.
Elderly individuals were also 18% more likely to be hospitalized for fluid and electrolyte disorders during extreme heat periods and were at 14% higher risk of renal failure, 10% higher risk of urinary tract infections and 6% greater risk of septicemia - a potentially life-threatening blood infection.
The risk of hospitalization as a result of these five illnesses was greater when extreme heat periods were longer and more intense. Hospitalization risk was highest on the heat wave days, but the researchers note that this risk remained "elevated and statistically significant" throughout the following 1-5 days.
Commenting on the relevance of these findings, Bobb says:
"Knowledge of which diseases are most likely to occur during heatwaves could help health systems to be better prepared to prevent and treat excess heat-related hospitalizations now and as climate change progresses."
This study follows a report published by the UK's Royal Society last month, in which the organization warned of the climate change risks for human populations.
In the report, the researchers say climate change could increase the number of heatwave exposure events that people over the age of 65 experience by three times by the year 2100.
As such, the report authors call for greater governmental action worldwide to enable human populations to better deal with extreme weather conditions. "If we continue on our current trajectory, the problem is likely to get much worse as our climate and population change," says Prof. Georgina Mace, chair of the working group for the report. "By acting now, we can reduce the serious risks to our children and grandchildren."