New research is helping confirm smoking as a risk factor for the devastating brain illness amyotrophic lateral sclerosis (ALS).

ALS affects roughly 31,000 Americans each year, with about 5,000 new cases diagnosed annually, according to the U.S. Centers for Disease Control and Prevention. It's a progressive, fatal illness that causes nerves cells controlling muscles to slowly die, leading to weakness and paralysis.

The causes of ALS are unknown, and even risk factors for the disease remain unclear.

In their new research, a team from South Korea looked at the data from 32 different studies for possible links between smoking and ALS.

They found that people who had smoked had a 12% higher odds of developing ALS compared to those who hadn't, with risks rising even higher if the person was a current smoker.

“One of our most pivotal findings was the dose-response analysis, revealing an inverted U-shaped curve,” said study lead author Dr. Yun Hak Kim, of the department of biomedical informatics at Pusan National University in Busan, Korea.

According to Kim, a U-shaped curve means that the “risk associated with smoking isn't a linear progression."

"Instead, it peaks and then starts to decrease or plateau, suggesting that the risk of ALS is influenced by the intensity of smoking," he said in a university news release.

Gender seemed to play a role as well.

The link between smoking and ALS appeared stronger for women smokers, who had a 20% higher odds for the disease than women who hadn't smoked. After adjusting for other potential risk factors, that added risk rose to 25%, the study found.

The findings were published in the January issue of the journal Tobacco Induced Diseases.

There are lots of great reasons to quit smoking, and now "encouraging individuals to discontinue smoking is crucial, given its tangible impact on reducing the likelihood of ALS onset," the researchers said.