That really depends. Plants and animals might be eating it, in which, as long as humans don’t invest it things would be fine. There’s lots of things we use that would be bad when eaten. The danger is if the poison is absorbed through contact.

[He wasn’t sure if ten years was enough to notice anything.]

Lead wasn’t understood as dangerous for centuries, and even in the modern era it took a long time to have it removed from common items. Ten years might not be enough.