Zero. Decompression illness (or the bends), affects scuba divers that do not adequately decompress following prolonged exposure to increased pressure. With a tank you can, of course, remain in deep water for longer period of time. A rapid ascent under these conditions can cause harm. Apnea divers do not stay for a very long time at depth. DCS can only effect freedivers in extreme depths and extreme diving times that are well beyond what we enjoy.