This blog post from Paul Ebeling discusses the link between lawn chemicals and cancer in dogs. I recommend you read it if you use lawn chemicals on your lawns or if you take your dog any place where such chemicals are in use. Many states require that commercial applicators post notices that such chemicals have been used; however, in many cases if a property owner applies these chemicals themselves, they may not be required to post a warning. Just be cause a lawn has not been posted does not mean it is free of chemicals or is safe.