Some health conditions are more likely to kill you when the weather is extremely hot or cold, scientists have said.
According to a study, people are more likely to die from heart failure on very hot or very cold days and climate change is increasing the risk of dying from cardiovascular disease.
For every 1,000 cardiovascular deaths, extreme hot and cold days accounted for 2.2 and 9.1 more, respectively - rising to 2.6 and 12.8 for those with heart failure.
The findings, published in the journal Circulation, are expected to have implications for global warming after temperatures in the UK hit a record 40C last summer.
Lead author Dr Barrak Alahmad, of Harvard University in Boston, said: "The decline in cardiovascular death rates since the 1960s is a huge public health success story.
"Cardiologists identified and addressed individual risk factors such as tobacco, physical inactivity, Type 2 diabetes, high blood pressure and others.
"The current challenge now is the environment and what climate change might hold for us."
Cardiovascular disease already claims around 18 million lives a year - a figure that will soar as the planet heats up.
Scientists found there were more deaths on days when temperatures were at their highest or lowest.
The study was based on a global analysis of 32 million cases over four decades and people with heart failure were found to be the most vulnerable.
Other conditions included hardening of the arteries that can trigger heart attacks, stroke and irregular heartbeats known as arrhythmias.
More research is required to develop strategies to mitigate the impact, say the US team.
The largest analysis of its kind was based on health records from 567 cities in 27 countries on five continents between 1979 and 2019.
The global data came from the Multi-Country Multi-City (MCC) Collaborative Research Network.
A consortium of epidemiologists, biostatisticians and climate scientists are studying the effects of global warming and related environmental stressors on mortality.
Climate change is associated with substantial swings in extreme hot and cold temperatures, so the researchers examined both.
They looked at cardiovascular deaths on the hottest and coldest 2.5 percent of days for each city.
These were compared with numbers on the days that had optimal temperatures associated with the fewest.
Co-author Dr Haitham Khraishah, of Maryland University in Baltimore, said: "One in every 100 cardiovascular deaths may be attributed to extreme temperature days.
"Temperature effects were more pronounced when looking at heart failure deaths.
"While we do not know the reason, this may be explained by the progressive nature of heart failure as a disease, rendering patients susceptible to temperature effects.
"This is an important finding since one out of four people with heart failure are readmitted to hospital within 30 days of discharge.
"Only 20% of patients with heart failure survive 10 years after diagnosis."
Targeted warning systems and advice for vulnerable people may be needed to prevent cardiovascular deaths during temperature extremes.
Dr Alahmad said: "We need to be on top of emerging environmental exposures.
"I call upon the professional cardiology organisations to commission guidelines and scientific statements on the intersection of extreme temperatures and cardiovascular health.
"In such statements, we may provide more direction to health care professionals, as well as identify clinical data gaps and future priorities for research."
Estimates could be conservative due to a lack of data from South Asia, the Middle East and Africa.
Professor Robert Harrington, a heart specialist at Stanford University in California who was not involved in the project, said: "This study contributes important information to the ongoing societal discussions regarding the relationship between climate and human health.
"More work is needed to better define these relationships in a world facing climate changes across the globe in the years ahead, especially as to how those environmental changes might impact the world's leading cause of death and disability, heart disease."
The analysis was funded by the Kuwait Foundation for the Advancement of Science (KFAS).
