Well, here is an explanation from the page http://www.api4animals.org/doc.asp?ID=79
"What's really in pet food?". There is argument about these preservatives, some say they may cause cancer and be otherwise bad for our pets' health if eaten constantly throughout their lives, others say they're perfectly safe. It appears no reliable studies have been done so far. I personally think that if there's some doubt, I'll avoid buying cat food with these preservatives if I possibly can. Oh yeah, and usually pet food manufacturers don't have to mention they've used these particular preservatives, in my country they can just say "EU approved antioxidants". Anyhow, below is a snip from the article, read more if you're interested on the url I put above.
"Synthetic preservatives include butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), propyl gallate, propylene glycol (also used as a less-toxic version of automotive antifreeze), and ethoxyquin. For these antioxidants, there is little information documenting their toxicity, safety, interactions, or chronic use in pet foods that may be eaten every day for the life of the animal.
Potentially cancer-causing agents such as BHA, BHT, and ethoxyquin are permitted at relatively low levels. The use of these chemicals in pet foods has not been thoroughly studied, and long term build-up of these agents may ultimately be harmful. Due to questionable data in the original study on its safety, ethoxyquin's manufacturer, Monsanto, was required to perform a new, more rigorous study. This was completed in 1996. Even though Monsanto found no significant toxicity associated with its own product, in July 1997, the FDA's Center for Veterinary Medicine requested that manufacturers voluntarily reduce the maximum level for ethoxyquin by half, to 75 parts per million. While some pet food critics and veterinarians believe that ethoxyquin is a major cause of disease, skin problems, and infertility in dogs, others claim it is the safest, strongest, most stable preservative available for pet food. Ethoxyquin is approved for use in human food for preserving spices, such as cayenne and chili powder, at a level of 100 ppm -- but it would be very difficult to consume as much chili powder every day as a dog would eat dry food. Ethoxyquin has never been tested for safety in cats."