When temperatures between day and night range by as much as 10 degrees Celsius, it doubles the risk of death, especially among those suffering from respiratory and circulatory diseases, an international study showed Tuesday.

According to a joint study on 30 cities in four countries -- Korea, China, Japan and Taiwan, the death risk rose by 0.58 percent on average for every degree Celsius increase in the daily temperature range.

The study, led by public health professor Kim Ho at Seoul National University, was conducted based on weather and death toll data between 1979 and 2010. It was published in the latest issue of Science of the Total Environment.

Considering the average temperature range of 10 degrees Celsius during the transition between seasons, the study suggested that the overall death risk could jump by 5.8 percent. The average daily temperature range of the overall year is 5 degrees Celsius, the researchers said.

Such large temperature gaps can particularly impose health dangers on those with respiratory problems and circulatory system diseases. The death risk of those patients surged by 9.7 percent and 8.1 percent, respectively, when temperature ranges reached 10 degrees Celsius.

The city of Incheon recorded the highest death risk of 2.84 percent per 1 degree Celsius difference for people with respiratory diseases, followed by Tangshan and Tianjin in China, with 2.25 percent and 1.8 percent, respectively.

The study also showed that the health impact was more serious for people aged 65 or older.

“The larger the daily temperature difference is, the more it affects the heartbeat, blood pressure, immune system and respiratory organs, indicating that the drastic temperature change can affect the internal immune mechanism,” Kim said.

In order to reduce health risks, keeping the immunity level up is essential, such as by keeping the body warm during outdoor activities and by controlling personal hygiene, experts said. Drinking enough water and taking vitamins is also helpful.

By Lee Hyun-jeong (rene@heraldcorp.com)