The general rationale is vaping rather than smoking tobacco is a lesser of two evils story. For those who smoke tobacco, then e-cigs are less evil as they do less damage body.

http://tasteyourjuice.com/wordpress/is-it-safe/

With respect to general safeguards then one needs to seek out as reliable and safe vape liquid as one can find. Here is a quote from one vendor that offers insight into their products.

http://www.goodlifevapor.com/shipping-returns/

There is a 48 hour processing period for every order. All orders are mixed to order by hand for every single bottle so please understand that we believe in quality and we are working hard to give you the absolute best product we can. You can choose priority shipping but please be aware that this is for shipping, not processing. We mix all orders in the order we receive them, no matter if you choose priority or first class.

All the references to juice I see are clearly just for nicotine delivery, never any reference to using e-cigs for any sort of respiratory relief.

If your husband is not a smoker than I see no reason to vape zero nicotine juice. I agree that there may be risk in this stuff so why take any chance when there is no benefit for cancer side effects.