According to a report by the organisation Climate Central, since the 1970s, dangerous heat-and-humidity days have increased from an average of 10 per year to 23 per year.

Dangerous heat and humidity occur when high temperatures and humidity combine to prevent sweat from evaporating from the skin, leading to rapid overheating and dehydration.

According to Climate Central, which analyses and disseminates information on climate science, climate change caused by human activity is now the main driver of dangerous heat and humidity, contributing to almost two-thirds of all such days.

The organisation estimates that extreme heat has caused more than 250,000 deaths globally since 2000, making it one of the deadliest climate hazards.

Research indicates that the areas most affected are those in a band on and above the equator. There are no predictions in the document that dangerous values will be reached in Portugal, but the report's authors warn that heat and humidity can still affect health, especially among vulnerable populations.

According to the document, humidity exacerbates the risks, and seemingly mild days can be "much more dangerous than they appear."

Climate Central warns that climate change is causing more dangerously hot and humid days in 69% (665) of the 961 global cities analysed.

The report also explains that when high humidity reduces the body's ability to cool down through perspiration, it "causes heat to accumulate internally, increasing the risk of dehydration, cardiovascular and respiratory problems, heat exhaustion, heatstroke, and other serious health impacts."

The Climate Central analysis was based on the so-called "wet-bulb temperature," a measure that combines heat and humidity to assess the physiological stress on the human body.