Contrary to popular perception, temperature alone is not the best indicator of heat stress, and the heat index - a measure that combines temperature and humidity - is more useful, scientists said.
Humidity should be taken into account because it limits the body's ability to cool via sweating, said Tom Matthews, a climatologist at Liverpool John Moores University in England who contributed to the UK research paper.
A 2014 study conducted by ISET-Pakistan looked at two of the largest hospitals in two Pakistani cities, and found heat stroke was occurring not in the hottest month but when the heat index was highest.
Heat stroke occurs when the body overheats and can be life-threatening.
Extreme heat can also lead to heat exhaustion and severe dehydration, and can aggravate cardiac conditions, kidney disorders and psychiatric illness, said Poonam Khetrapal Singh, Southeast Asia director at the World Health Organization.
Most warnings about heat stress focus on peak temperatures during the day, but rising night-time temperatures are adding to the indirect effects, said NCAR scientist Caspar Ammann.
A report by Australia's Climate Council estimated that in 2015, nearly 3,500 people died in India and Pakistan from heat waves - defined as three exceptionally hot days in a row.
Recent fatal heat waves were a result of a 0.8 degree Celsius rise in temperatures from pre-industrial levels, the UK study said. Heat waves caused by a further increase of 0.7 degrees - if the world sticks to its 1.5 degree Celsius limit - could be even more severe, it warned.