可能因为这个 not to mention ... 是个插入语，所以有逗号并不表示是非限定性从句，可以用 that。原文摘录如下供你参考：
Parents who smoke often open a window or turn on a fan to clear the air for their children, but experts now have identified a related threat to children’s health that isn’t as easy to get rid of: third-hand smoke.
That’s the term being used to describe the invisible yet toxic brew of gases and particles clinging to smokers’ hair and clothing, not to mention cushions and carpeting, that lingers long after second-hand smoke has cleared from a room. The residue includes heavy metals, carcinogens and even radioactive materials that young children can get on their hands and ingest, especially if they’re crawling or playing on the floor.
（NY Times 2009年1月）

那么问题来了，that 是指什么？