Smokers may be getting a lot more than they bargain for when lighting up — like inhaling traces of pig’s blood. Pig hemoglobin is sometimes used in cigarette filters, according to recent Dutch research reported by The Daily Telegraph — a disclosure that is bound to upset many devout Muslims and Jews.

Islamic and Jewish religious texts expressly forbid consumption of pork in any form, and vegetarians may find this hidden ingredient unpalatable as well.

"It just puts into hard relief the problem that the tobacco industry is not required to declare the ingredients of cigarettes ... they say 'that's our business' and a trade secret,” University of Sydney public health professor Simon Chapman told The Daily Telegraph. Chapman is a sociologist with a Ph.D. on the study of the signs, symbols, and significance of cigarette advertising as well as a book author.

Though many tobacco companies now voluntarily provide ingredient lists on their website, the swine-derived blood protein is reportedly hidden within "processing aids ... that are not significantly present in, and do not functionally affect, the finished product,” says Chapman.

But cigarette formulas won’t remain so secretive for much longer. In June, tobacco companies must furnish complete ingredient lists to the FDA, including any studies the companies have done on those ingredients.

The FDA will then determine which ingredients could make cigarettes more harmful or addictive and could eventually ban certain ingredients. However, quick action is unlikely.

"Tobacco products today are really the only human-consumed product that we don't know what's in them," Lawrence R. Deyton, the director of the Food and Drug Administration's new Center for Tobacco Products and a physician, told The Associated Press.