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The vast majority of sack lunches that kids bring to school aren’t properly cooled or refrigerated and could pose a health hazard to young children, a new study in the journal Pediatrics suggests.

In the first study of its kind, researchers in Texas visited nine preschool child-care centers on three separate occasions and measured the temperature of hundreds of sandwiches, yogurts, and other perishable lunch foods brought from home, using a heat-sensing gun.

What they found shocked them: 97% percent of meats, 99% of dairy, and 99% of vegetables were stored at unsafe temperatures. Of the 1,361 perishable foods that were tested, only 22 were at temperatures considered safe.

“This study should be an eye-opener for the public,” says Fawaz Almansour, the lead author and a researcher in the department of nutritional sciences at the University of Texas at Austin.

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Harmful bacteria that can cause food poisoning, such as Salmonella, E. coli, and Staphylococcus aureus, grow and multiply at temperatures between 40 and 140 degrees Fahrenheit, a temperature range commonly known as the “danger zone.”

The average temperature of the perishable items in the study was 63.7 degrees. Only half of all lunches included ice packs, but even those containing multiple ice packs were found to be in the danger zone.

Foods stored at unsafe temperatures for more than two hours are considered dangerous and should be discarded. The researchers tested the lunches of the 3- to 5-year-old preschoolers about 90 minutes before lunchtime, so it’s very likely that the foods stayed overly warm for at least that amount of time.

Limiting exposure to harmful bacteria is particularly important for young children. According to the study, kids under the age of 4 experience food-related bacterial infections at up to 4.5 times the rate of adults ages 20 to 49 (depending on the bacteria).

“It is more of a scary situation for children than for adults,” Almansour says. “Kids’ immune systems have not adapted to these diseases.”

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Dr. Denice Cora-Bramble, senior vice president of the Goldberg Center for Community Pediatric Health at the Children’s National Medical Center, in Washington, D.C., says the study results surprised her.

Talking to parents about healthy eating is a common part of her practice, but until now food safety has rarely entered the discussion.

“Now that I have seen this, it is something that I will definitely include when giving parents advice,” says Cora-Bramble, who was not involved with the study. “We need to talk to parents and raise their awareness.”

To reduce the risk of food poisoning, the study authors and Cora-Bramble both recommend the following steps:

Make sure your child’s day care center or school stores lunches in a refrigerator

Don’t refrigerate lunches in insulated bags, which can prevent the food from staying cool

Avoiding making lunches with mayonnaise, which contains eggs and spoils quickly

Freeze juices and waters; this will keep the juice fresh and will also keep other foods chilled

Most of the parents and child-care centers in the study failed to follow commonsense precautions such as these. Only half of all lunches included ice packs, and only three of the nine centers provided refrigerators for the kids’ lunches. In the centers that did have a fridge, the teachers often didn’t use them.

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Almansour says it’s not clear how many instances of food-borne illness can be attributed to sack lunches, since tracing food poisoning back to a specific food or meal isn’t always possible. But his team’s eye-opening findings should encourage parents to rethink how they pack their children’s lunches, he says.

“We don’t always know what got us sick,” he explains. “You can’t easily point a finger at a particular food. But sack lunches could easily be the culprit.”