A large Korean study tracking over 5 million adults for up to 10 years found that people with asthma or allergic rhinitis (hay fever) were roughly 16–18% more likely to develop Parkinson's disease (PD) than people without those conditions. The more severe the allergy — measured by how often someone visited a doctor for it — and the more allergic conditions a person had, the higher the risk climbed. Importantly, eczema (atopic dermatitis) did not show the same pattern, suggesting the link is specific to airways-related allergies rather than allergies in general.
The researchers also compared blood samples from 234 hospital PD patients with 468 healthy controls and found that PD patients had higher levels of eosinophils — a type of white blood cell involved in allergic reactions and inflammation. This points toward a possible shared biological pathway: chronic allergic inflammation, driven in part by eosinophils, may contribute to the brain inflammation that is already known to play a role in Parkinson's.
What this means in practice: This study does not mean that having asthma or hay fever will cause Parkinson's — the absolute risk increase is modest, and most people with these conditions will never develop PD. It also does not suggest changing how you manage your allergies right now. However, it does reinforce the idea that controlling systemic inflammation may matter for long-term brain health, and it opens the door to investigating eosinophil counts as a potential early-warning biomarker. Patients with both allergic disease and a family history of PD may wish to discuss this emerging research with their neurologist.