Hello –
Yes, it is OK to be concerned about pesticide spray drift from your neighbors. Just do a quick Google search and you’ll see that the web is filled up with these complaints. It’s noted that the EPA — the company in charge of protecting you from pesticide poisoning — gets thousands of calls per year from poisonous sprays drifting off target. How well do you know your neighbor? Can you talk to him about your concerns and ask that he take special care when applying pesticides? Does he apply them by aircraft? Pesticide applicators must follow certain practices, and you are protected. If you suspect chemical drift, call your state department of agriculture and have them run some tests. If your neighbor is negligent, he could be subject to fines by the state.
Good luck!