Between April 1 and July 4, a total of 520 people died across the country, said Bianca Cox, a scientific collaborator at the Scientific Institute of Public Health, which provides research on public health issues.

It is not clear yet why they died, but researchers speculate it could be linked to the heat and lower concentrations of ozone levels, Cox said.

"There are no other real factors at the moment," she said. "But of course, we can't be sure that all 520 deaths are due to heat. There will be some that are due to other causes."

The deaths were mainly among those 80 and older, she said, and many already had underlying conditions such as heart and respiratory problems. The elderly and those with health problems can be particularly susceptible to hot weather.

There were 300 deaths in the last week of June and the first week of July alone, a week in which Belgium had unusually high temperatures of 25 degrees Celsius or higher for seven days in a row, Cox said. For three days that week, the country also had "critical" levels of ozone, she said.

For six days so far in July, temperatures at Brussels International Airport have topped 30 degrees Celsius, with a maximum of 33 degrees Celsius. The average high for this time of year is only 22 degrees Celsius.

Neither April nor May was terribly warm as a whole, with the highest temperature during either month about 26 degrees Celsius.