Dementia prevalence rate has been rising steeply to affect one out of four Koreans in their 80s with the nation’s overall elderly dementia rate increasing around 20-fold between 2002 and 2013, a study shows.

A report by the Korea Institute for Health and Social Affairs published Friday noted dementia rates increased 28 times for those in their 70s, and 24 times for those in their 80s. The report pointed to the lack of physical activity as the biggest cause of dementia.

The study found that in 2002, the prevalence of dementia in those older than 60 hovered around 0.52 percent (2,881 people out of 555,566). Follow-up studies found the dementia prevalence rate in the same group rose to 10.7 percent in 2013, indicating a 20.6-fold increase in 11 years.

According to the survey, the risk factors affecting dementia onset included lack of activity at 28.74 percent, obesity (15.87 percent), smoking (11.29 percent), and education level (13.11 percent).

The National Assembly Budget Office estimates the cost of dementia will rise sharply from around 11.7 trillion won ($10.4 billion) in 2013 to 23.1 trillion ($20.6 billion) in 2030 and 34.2 trillion ($30.5 billion) in 2040.

The report was based on records of patients treated with dementia in the National Health Insurance Service’s cohort study that followed 558,147 men and women over 60 years of age from 2002 to 2013.