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Howdy,
I do this all the time as a way to remember important things, so I was very eager to find you an answer.
According to WiseGeek, "occasionally writing on skin with water-based inks is considered non-hazardous."
The site goes on to explain that the main concern with writing on skin is the xylene found in permanent ink. "Some permanent inks now use an alcohol-based substance instead of xylene." This is less hazardous, but "it still may irritate the nose, throat, mouth and sensitive skin."
In general, writing on skin with ballpoint pen isn't considered dangerous, however "the occasional person might be irritated by the dyes in inks. Normally for such ink to be considered poisonous, one would have to ingest three or four penfuls of ink."
Mike
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