Despite mothers' best intentions, packed lunches that save money could end up costing more in the end at the doctor's office, as new research shows that many school lunches may reach unsafe temperatures by the time a child eats.

According to a new study published in the September issue of Pediatrics, packed lunches can be inexpensive alternative to school-prepared lunches, but they can also make kids sick if not kept at a safe temperature.

The study, titled Temperature of Foods Sent by Parents of Preschool-aged Children, stated that even lunches that include ice packs can reach unsafe temperatures.

Researchers measured the sack lunches 705 preschoolers at nine Texas child care centers with noncontact temperature guns about an hour and a half before the food was served.

Researchers found that 39 percent of the 705 lunches had no ice packs, 45.1 percent had at least one ice pack, and 88.2 percent of lunches were at room temperature.

Furthermore, only 1.6 percent of lunches with perishable items were found to be in a safe temperature zone, while more than 90 percent of the lunches ? even with multiple ice packs ? were kept at unsafe temperatures, according to the study.

"The main finding of our study is that more than 90 percent of perishable items were at an unacceptable temperature ? according to USDA guidelines ? an hour and a half before lunch," said study author Fawaz Almansour, a doctoral candidate in the department of nutritional sciences at the University of Texas at Austin, to USA Today.

"This was an eye opener," he added. "As a parent, when my child comes home with a stomachache or vomiting, I usually think it's a virus. I don't think the food I serve is the problem."

The authors suggest that parents and the public be educated on safe food packing practices so as to prevent bacteria from growing and potentially causing illness.

"The vast majority of lunches were clearly out of a safe range, but it's hard to know what the true biological impact of that is. We don't truly know how often this results in a foodborne illness," said Dr. Michael Green, a pediatric infectious disease specialist at Children's Hospital of Pittsburgh, to USA Today. "This paper raises a lot of questions, but isn't able to provide a lot of solutions."

Ruth Frechman, a Burbank-based registered dietitian and spokeswoman for the American Dietetic Association, said putting lunches in well-insulated lunch bags with cold packs surrounding perishable foods are a good place to start.

She told the Los Angeles Times that food should not be left out for more than two hours, or no more than one hour if it's 90 degrees or hotter.

"Make sure the lunch is as cold as it would be if it was stored in a refrigerator," which is about 40 degrees or cooler, she said.

That adults grew up eating unrefrigerated lunches stored in brown paper bags without cold packs, it doesn't mean kids today are safe.

"There wasn't always the media to highlight kids getting sick," Frechman told the los Angeles Times. "It was just a fact of life if someone got food poisoning. ... The potential for food-borne illnesses is there, so it's never a good idea to take a chance," she said. "Bacteria grows fast, so prevention is the key."