Searches for hyperbaric oxygen have risen over the last five years, and devices — that range from large, hard-shell chambers to soft “body bags” — are being sold online for thousands of dollars, allowing consumers to try “HBOT” at home. Athletes such as LeBron James, Novak Djokovic and Michael Phelps have said it helps their recovery, and so-called biohackers like Bryan Johnson and Gary Brecka have touted it as well.