Walk into any grocery store in the United States and you’ll always find eggs in the refrigerator section. But if you’ve ever ventured to a country in Europe or the United Kingdom, you’ve likely been surprised to find egg cartons on the shelf in a regular aisle sans refrigeration. So why is there such a big difference in how eggs are sold? Selling eggs both ways is safe—it all comes down to how the eggs are cleaned and processed to prevent foodborne illness.

How Salmonella Contamination Occurs

The USDA regulates how eggs are processed to reduce the possibility of foodborne illness from spreading. Salmonella is a common bacteria that proliferates easily. Chickens can have salmonella and pass it on through their eggs. And because hens lay eggs and excrete waste through the same place, cross contamination can easily occur.

“Chicken fecal matter contains a high concentration of salmonella, and can get on the surface of an egg, which contaminates the shell,” says Bryan Quoc Le, Ph.D, food scientist and author of “150 Food Science Questions Answered.” “The pores or cracks of the egg shell can also allow salmonella to seep into the interior of the egg.”

How Eggs Are Processed in the US

Commercially produced eggs are required to be cleaned and sanitized for food safety before they reach the consumer. “In the United States, eggs are washed with warm water and a detergent, which helps strip the salmonella biofilms that can form on the surface of an egg,” says Le. “The eggs are then rinsed with a sanitizer to further reduce the number of bacteria on the surface of the egg.”

But when eggs are washed and sanitized, the shell loses a protective barrier called the cuticle, making the eggs more vulnerable to bacteria. Tiffany Swan, food scientist and chef, explains that washing eggs “removes the natural protective coating from the shell, making the shell more porous and thus putting the egg at greater risk of contamination.”

Because washed eggs no longer have their natural protection, refrigeration becomes required to prevent bacteria from growing. “Eggs that have been washed should be kept refrigerated, which prevents salmonella from growing,” says Le. “Cracked or chipped eggs should be thrown away.”

Testing Eggs for Freshness

The Local Eggception

If you buy eggs from a local egg farmer that sometimes have traces of a feather or are never cold, the farmer may not be required to wash eggs or refrigerate them thanks to state laws. Swan advises that you wash local eggs with warm water and soap before using them. “Don’t use cold water and don’t submerge them, as this can push bacteria through the shell into the egg,” she says.

Le says local eggs can be left on the counter as long as they aren’t chipped or cracked, but as soon as the egg is washed at home it should be used immediately or refrigerated to minimize the risk of salmonella.

Once an egg, no matter its origin, has been refrigerated, keeping it refrigerated until you’re ready to use it is important. If you leave a washed, refrigerated egg out at room temperature, throw it out if more than two hours have passed, explains Swan. “The risk of the bacteria proliferating at room temperature is pretty high, if it is present,” she says. “We have to assume all eggs are contaminated because there is no way to know if they are safe or not.”

What About the EU and UK?

As for Europe and the United Kingdom, many countries have different perspectives on the risk of salmonella. Each European Union country has their own rules for commercially produced eggs to comply with food-safety measures and reduce salmonella outbreaks.

According to Swan, “salmonella isn’t as common in Europe1 [as in the US2, and] hens may be vaccinated against salmonella.” Plus, eggs aren’t cleaned with water so the protective coating stays intact, lowering the chances of salmonella entering the egg. And it’s not only the US that washes eggs. Japan and Sweden do as well.