Golf season is here, bringing joy and exercise to many. But new research has linked the links to a dangerous health risk, particularly for those living nearby.

People residing within 3 miles of a course faced a greater risk of developing Parkinson’s disease, the new study shows. Most of those people — 90% — shared a groundwater-sourced water service with the golf course. Others in the study who lived farther away but still shared water service with a golf course also had a higher risk of developing Parkinson’s, a brain degeneration disorder that causes severe movement problems.

The reason? It could be exposure to pesticides that leach into groundwater and contaminate the drinking supply, the researchers say. “In certain parts of the country, where summers are hot and humid, golf courses use pesticides to maintain the course aesthetics and keep bugs under control,” said lead author Brittany Krzyzanowski, PhD, a geographer and spatial epidemiologist at Barrow Neurological Institute in Phoenix.

“Some of these pesticides have been linked to negative health outcomes including Parkinson’s disease,” Krzyzanowski said. “So it’s important to be mindful and take steps to mitigate exposure.” The analysis included health and residence records for 419 people living in Minnesota and Wisconsin who were diagnosed with Parkinson’s disease between 1991 and 2015. They ranged in age from 65 to 80, and the average age was 73. For the comparison group, each person was matched to 20 additional people without Parkinson’s of the same age and gender.

Here’s what the results showed: The odds of developing Parkinson’s disease increased 126% for people living within 1 mile of a golf course, compared to living more than 6 miles away. The heightened risk persisted within 3 miles of golf courses, but decreased beyond that. The risk nearly doubled for people with shared water service with a golf course, compared to those without a shared service. The greatest risk was for people living in areas with what’s called “karst topography,” which is “a region with limestone bedrock that slowly dissolves over time, creating underground voids that allow water from the surface to move more rapidly through it,” Krzyzanowski said. “This means that pesticides applied to grass or crops can more readily move into the groundwater supply after a rain.”

Like any analysis, this one had limitations. It only looked at data for three years prior to diagnosis. Because Parkinson’s can take up to 40 years to develop, the researchers want to do a follow-up study with 40 years of address data to see if there’s any difference in the results. That would let them analyze whether risk changes based on how long someone lives near a course. It can’t account for changes in pesticide practices over time. “Our study assumes exposure to pesticides on golf courses occurred many decades ago,” Krzyzanowski said. “It’s possible that the pesticide practices from years ago do not reflect the pesticide practices on these golf courses today.”