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The roofing material that the rainwater is collecting from is probably a bigger health concern than the plastic barrel. If you have an asphalt shingle roof you will likely be getting some of the tar substances in your garden. If you have cedar shingles your plants may not be the happiest (although if moss is growing on the shingles then maybe all the natural toxins are already gone). If you have a steel roof, you are probably getting safe water in your rain barrel.

I use a rain barrel, but only use it to water my flowers. I just don't trust the asphalt shingles.