Extreme heat can trigger excessive sweating that overwhelms the body's natural cooling system, potentially leading to heat cramps, heat stroke and even death in severe, untreated cases.
An international team of researchers has identified a more subtle consequence of extreme heat -- it may accelerate the aging process.
Environmental epidemiologists analyzed 15 years of health data from nearly 25,000 adults in Taiwan and determined that two years of cumulative heat wave exposure can add eight to 12 days to a person's biological age, a number that reflects the true health of their cells, tissues and organs.
The research, published this week in the journal Nature Climate Change, compared the effect to the impact of a tobacco or alcohol habit.
"While the number itself may look small, over time and across populations, this effect can have meaningful public-health implications," said Cui Guo, an assistant professor at the University of Hong Kong who led the study.
Last year was the planet's warmest year on record, according to several scientific organizations.
The US experiences about six heat waves on average each year, up from two in the 1960s, according to the Environmental Protection Agency. Heat waves also tend to be more intense and last longer than 60 years ago.
The EPA notes that the definition of extreme heat depends on geographic location. It counts extremely hot days as those where temperature reaches or exceeds 95 degrees Fahrenheit.
Extreme heat kills about 2,000 Americans each year, though this may be an undercount because of inconsistent reporting methods and inadequate resources for comprehensive death investigations.
Heat can exacerbate pre-existing health problems like heart disease, asthma, kidney disease, neurological disorders and mental health conditions.
"Manual workers, rural residents and participants from communities with fewer air conditioners were more susceptible to the health impacts" of heat waves, the researchers wrote.
Their findings follow a recent University of Southern California study that warned that extreme heat may speed up biological aging in older adults.
A higher biological age than chronological age -- the number of years you've been alive -- increases the risk of chronic diseases such as dementia, stroke, cancer, heart disease, Type 2 diabetes and kidney disease.
"Participants living in areas where heat days, as defined as Extreme Caution or higher levels (≥90°F), occur half the year, such as Phoenix, Arizona, experienced up to 14 months of additional biological aging compared to those living in areas with fewer than 10 heat days per year," Jennifer Ailshire, senior author of the USC study, said in February when the research was published.
"Even after controlling for several factors, we found this association," she added. "Just because you live in an area with more heat days, you're aging faster biologically."
While the dog days of summer are behind us in the US, hot and humid weather can persist into early September.