"While working at Polaroid Corp. for more than a decade, John C. Warner learned about the chemistry behind some carbonless copy papers (now used for most credit card receipts) and the thermal imaging papers that are spit out by most modern cash registers. Both relied on bisphenol-A.
Manufacturers would coat a powdery layer of this BPA onto one side of a piece of paper together with an invisible ink, he says. “Later, when you applied pressure or heat, they would merge together and you’d get color.”
At the time, back in the ‘90s, he thought little about the technology other than it was clever. But when BPA exploded into the news, about a decade ago, Warner began to develop some doubts."
Janet Raloff reports for Science News October 7, 2009.