Procter & Gamble (the maker of Downy brand products) does not recommend using Downy Unstopables (or any other product) in a manner inconsistent with the directions on its label.
Whether using Downy Unstopables in a tart or wax warmer is a dangerous practice that can cause injury or even death.
The advent of Pinterest and its format friendly to the sharing of household tips has introduced a wealth of homekeeping innovations to the Internet, and popular among them has been the notion that the use of fabric fragrance beads (primarily Downy Unstopables, often incorrectly spelled as "Downy Unstoppables") in a tart or wax warmer presents a money-saving option preferable to costlier versions offered by companies such as Pink Zebra or Scentsy.
One of the earliest iterations of this tip appeared on a coupon blog on 27 February 2013, in a post titled "7 New Uses for Downy Unstopables":
My favorite idea by far is to use Unstopables in place of higher-priced wax blocks in a Scentsy holder or wax burner. Discard when the scent weakens; replace as necessary. Scents from two or more containers can even be mixed!
For fans of laundry smells, the tip was certainly attractive: if the scent of plain clean laundry could be likened to cocaine, then the condensed fragrance beads were more like crack. Moreover, the use of a standard scent warmer could quickly become an expensive proposition due to the high cost of direct sales-distributed Scentsy products (and its competitors).
Soon after the penny-pinching Downy Unstopables tip began to circulate, folks (a large number of whom were independent sales consultants for home fragrance products) started buzzing about the grave risk posed by putting laundry scent beads in a wax warmer. One of the top comments on the post excerpted above responded:
Do NOT use as a melt!!! They are dangerous to heat!! See the MSDS sheet
The Downy Unstopables MSDS sheet is available on Procter & Gamble's website [PDF], but no portion of that document specifically addresses whether heating the product is "dangerous." The most relevant portion (under "Inhalation") states that "[p]rolonged or repeated exposures to perfume oils may cause respiratory tract irritation." However, perfume oils are a key component of all home fragrance products; and there is no indication that the specific ones used in Downy Unstopables are riskier than any others with respect to inhalation or general fragrance sensitivity. A number of rumors claim that individuals who attempted to use Downy Unstopables in place of approved products in a Scentsy warmer were sickened; but we were unable to turn up any proof such an incident had occurred outside of anecdotes posted in the comments sections of housekeeping blogs (overwhelmingly provided by independent sales reps for Scentsy and Pink Zebra).
We contacted Procter & Gamble's customer service line to ask about the practice and whether it was safe to use Downy Unstopables in a scent warmer. The representative with whom we spoke stated that the company was aware of the warning rumor and asserted that the company did not recommend using Downy Unstopables or any other product in a manner inconsistent with the instructions on its label.
So while the use of Downy Unstopables in a wax warmer is both popular and recommended by Pinterest users, Procter & Gamble does not recommend using the product in that fashion. However, we were unable to find any indication anyone had fallen ill (or required a "funeral") due to their substitution of Downy Unstopables for another product in a scent warmer.