This is just a wild-a$$ guess but it's hard to imagine it being so toxic it couldn't be used safely outdoors...but just as hard to imagine it being better than cotton. Cotton is so cheap as to be almost free and can be treated in many ways.
Still, you've piqued my curiousity. I wonder what it is?
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman