Well, Nathan, I guess that depends on what type of "used tissues" you mean. Are you talking about toilet tissue, or the kind you blow your nose with? Or both?
I'd rather not touch the toilet tissue issue. As for the kind you blow your nose with, like Kleenex, the answer is: don't do either. Use an organic cotton handkerchief insteadso the sap of tens of thousands of dead Canadian old-growth pines won't be on your hands.
To understand why, take a look at a box of Kleenex, sometime. You'll notice there's no label saying anything like "these tissues are made from recycled sources." On the contrary, Kimberly-Clark, the maker of Kleenex, receives nearly one-third of its paper fiber from timber logged in Canada's virgin, 20,000-year-old Boreal Forest, according to the National Resources Defense Council (NRDC).
This vast but ever-shrinking swath of pristine land is home to 30 percent of North America's songbirds, and 40 percent of its waterfowl. Just as important, it stores roughly 186 billion tons of carbon dioxide in its soil and trees, according to a recent Greenpeace report. (I'm not exactly sure what 186 billion tons of carbon dioxide translates to, but it sounds like a hell of a lot.) Then there's the impact of using chlorine bleach to whiten these types of paper products, which pollutes the air and water. So basically, when you catch a cold and use a tissue, the entire planet sneezes.
Some companies, like Seventh Generation, do make chlorine-free tissues from 100-percent recycled paper. The NRDC argues that if every American house switched from one box of standard-fiber facial tissue to all-recycled ones, we could save 163,000 trees. If we did the same with toilet paper, we'd save 423,000 trees. With napkins, 1 million trees. (Dr. Wife, MD, makes sure that we use cloth napkins at home, and avoid this issue altogether.)
If you've got a full-blown cold or the flu, and the thought of carrying a literal snot rag in your pocket all day grosses you out, then go ahead and use that 100-percent recycled facial tissue. But throw it in the trash, and not the toilet. Facial tissue doesn't break down easily in water like toilet paper does, and can clog the plumbing. And speaking of toilet paper: if, on the off chance, that's indeed what your question is referring to, I suppose I can provide an answer. Use a bidet.