Eating out often is not good for your health in all the obvious ways — there's usually too much fat, too much salt and the portions tend to be huge. But a less obvious danger lurks in restaurant food containers, including take-out and fast food packaging. A harmful class of chemicals called PFAS found in food packaging has been linked to ill health, reports a new study. People who cook at home more have fewer of these chemicals in their bodies.

PFAS, or perfluoroalkyl substances, are manmade chemicals that have properties that enable them to repel both water and oil. They are used in a variety of nonstick, stain-resistant and waterproof products. In addition to being used in food packaging, they are also found in carpeting, outdoor apparel and cookware, as well as livestock and food crops that come into contact with contaminated soil and water.

People who ate out more often, whether at restaurants, fast food joints or via take-out, had higher levels of PFAS in their blood.

To study the association between PFAS and human health, researchers used dietary information from over 10,000 people who took part in the National Health and Nutrition Examination Survey (NHANES). The participants also provided blood samples that were analyzed for the presence of PFAS chemicals.

People who ate at home more often had significantly lower levels of PFAS in their bodies. Ninety percent of the food they ate was prepared from food purchased at a grocery store.

On the other hand, people who ate out more often, whether at restaurants, fast food joints or via take-out, had higher levels of PFAS in their blood, suggesting that food eaten away from home in fast food joints, pizza parlors and other restaurants was more likely to be contaminated with PFAS. The likely cause was food that came into contact with food packaging containing PFAS.

The study, by researchers at Silent Spring Institute, raises questions about the use of microwave popcorn. People who reported eating microwave popcorn often had higher PFAS levels, likely the result of chemicals leaching out of the popcorn bag. This study detected four PFAS chemicals that have already been associated in other studies with microwave popcorn bags.

“Our results suggest migration of PFAS chemicals from food packaging into food can be an important source of exposure to these chemicals,” explained Laurel Schaider, a researcher at Silent Spring Institute, an organization dedicated to uncovering the environmental causes of breast cancer.

Cancer, thyroid disease, immune suppression, low birthweight and decreased fertility are some of the negative health effects that have been linked to PFAS. The reason scientists are concerned about PFAS is because they are so pervasive in our daily lives. Many believe there need to be restrictions on all PFAS.

The study only looked at results for one type of PFAS because it was the most frequently detected, but, recently, food manufacturers have begun replacing this type of PFAS with newer types, which also appear to be harmful.

PFAS are not the only chemicals of concern in food packaging. Hormone-disrupting compounds like BPA and phthalates are also present.

You can avoid ingesting these chemicals by limiting how often you eat at restaurants, order take-out, eat fast food or use microwave popcorn.

Sure, all these foods are convenient, but at what price?

The study is published in Environmental Health Perspectives.