Eggs are quite possibly the world's perfect protein source. The six grams of protein in each egg has the highest biological value—a measure of how well it supports your body's protein needs—of any food, including beef. The yolks contain vitamin B12, deficiencies of which can cause attention, mood, and thinking problems.

Depending on where you're getting your eggs, though, you could be getting a lot more of stuff you don’t want. First you'll get some arsenic, added to feed to promote growth in hens but linked to various forms of cancer in people, and an extra dose of antibiotics, also used to promote growth but linked to antibiotic resistance and even obesity in people. Then add a heaping helping of salmonella. A 2010 study published in the journal Veterinary Record found that the eggs from hens confined to cages, as they often are in factory farms, had 7.77-times greater odds of harboring salmonella bacteria than eggs from non-caged hens.

You wouldn't know that based on what's starting to appear on egg cartons. Labels like "natural" and "cage-free" make eggs seem like they came from down on the farm, from chickens living happy lives and eating bugs. But that's not always the case. If all you want is healthy protein, it's time to start scrutinizing egg cartons. Following are nine of the most common egg-carton claims and what they mean for your health.

Here's how to make sense of the different egg labels: "Cage-Free" "Free-Range or "Free-Roaming" "Organic" "Natural" "Pastured" "Omega-3 Enriched" "Animal Welfare Approved" "Certified Humane" "United Egg Producers Certified"