OSHKOSH, Wis. (AP) — The Winnebago County Health Department has sent letters to some residents who offer short-term apartment rentals through a popular website. It warns them of fines if they continue to list their property on the Airbnb site without a permit. While the letters focus on Airbnb users, the issue could have implications for the hundreds of property owners who rent during the Experimental Aircraft Association's AirVenture. Health Department Director Doug Gieryn says the letters focused on properties that involve the owners leaving the premises, which makes them transient properties. A bed and breakfast, on the other hand, has owners on the premises. Gieryn says he's requested more guidance from state officials to see if Wisconsin law would impact those who rent their homes during AirVenture.