The next time a cashier hands you a receipt at Walmart, Whole Foods, or McDonald's, you may want to think twice before touching it. In a neurosis-inducing study commissioned by the Environmental Working Group, 40 percent of cash register receipts printed on heat-activated paper were found to contain the dangerous chemical Bisphenol A (BPA). The alleged risk for cashiers, who handle hundreds of receipts per day, is even higher. (Watch a local report about BPA in receipts.) Here, a quick guide to this unseen danger:

What is BPA?
A synthetic estrogen, Bisphenol A is a carcinogen that is considered toxic to the endocrine and nervous system. It is widely used in both the manufacturing of thermal paper (on which many receipts are printed) and plastics, particularly those used to line food containers.

Which companies' receipts were found to contain BPA?
Sample receipts from CVS, Walmart, KFC, Chevron, McDonald's, Whole Foods, the U.S. Postal Service and a variety of other businesses all contained BPA. Those from Safeway contained the highest levels; 3 percent of the receipts' total weight was BPA. On the plus side: Scientists found that sales tickets from Target, Starbucks and Bank of America ATMs were BPA-free or contained only trace amounts.

Why is BPA on the receipts?
The thermal paper on which receipts (as well as airline tickets, cinema tickets, prescription labels and lottery tickets) are often printed is coated with a dye and BPA, or a similar "developer chemical." When heat triggers a reaction between the dye and the BPA, the black print is revealed.

What health problems can BPA cause?
In animal studies, BPA has been found to cause abnormal reproduction, decreased intellectual capacity and behavior problems — and also contribute to the development of cancer, diabetes, asthma and cardiovascular disorders. Relatively little is known about the effects of the chemical on humans, although detectable amounts of BPA have been found in 93 percent of people over age 6.

How does the BPA get into the body?
Laboratory tests show that the powered chemical can rub off the receipt and onto the skin. It then penetrates so deep into the skin that it can't be washed off, and enters the blood stream.

How can I avoid BPA contamination from receipts?
The Environmental Working Group suggests customers decline receipts whenever possible; store receipts separately in a wallet or purse; and wash hands after handling receipts, especially before eating or food preparation. The EWG also says it's important to "never give a child a receipt to hold or play with," since BPA has been found to potentially affect the brain development of fetuses, infants and children.

Sources: EWG, NIEHS, Time, US News, AOL News