Actually, both D1 and Marquis have "contact chambers" for their ozone, as well as producing zero off-gas. There are, however tubs of lesser quality than the sundance, marquis, hot springs, and d1 which actually inject ozone into the tubs main water body producing that sweet smell of ozone. It won't really harm a human in the doses it disperses at, however, it won't be an effective sanitation method either.