With kids heading back to school, it's back to the lunch-making grind for many parents. This year, on top of all the usual worries about "litterless lunches," healthy choices, and allergenic foods there's a new worry: a too-warm lunch bag.

Last month, a study came out that suggested that many kids' packed lunches become too warm by lunchtime to be safe. The study, in the journal Pediatrics, measured the temperatures of 235 preschoolers' lunches over three days. They then counted how many of the lunch items were in the so-called "temperature danger zone" when bacteria can grow to unsafe levels -- over 5 degrees Celsius and below 60 degrees.

The tests were done about 90 minutes before lunchtime, and almost all had warmed up too much to be safe: 97 per cent of the meat was in the danger zone, as was 99 per cent of dairy. That was despite the fact that nearly half the lunch bags contained an ice pack. (The ones with two ice packs fared the best.)

Toronto-based registered dietician Cara Rosenbloom says she was a bit surprised by the study's findings, but it left her with one big question: how many of the kids who ate those too-warm lunches actually got sick?

"That's what I would like to know. And the answer most likely is none," she tells CTVNews.ca.

"I would loved to have had this study taken one step further where they watched these preschoolers over the next 10 days to see if there were any incidents of diarrhea, vomiting and stomach cramps."

The study also didn't determine how long the foods had been in the "danger zone" – a critical bit of information, says Rosenbloom. Most food safety experts say food should not sit in the danger zone for more than two hours. It's possible the lunches stayed cool for hours but entered "the danger zone" right before being tested, she says. That would still make them safe to eat 90 minutes later.

Rosenbloom thinks the worries raised by the study might be a bit overblown. After all, when she was a kid, it came in a brown paper bag, there was no ice pack, and no insulated bag.

"And I ate tuna with mayonnaise almost every day," she says.

But just because most of us brown-bagged our lunches, it doesn't mean that was the safest way to go, points out microbiologist Dr. Allison McGeer of Toronto's Mount Sinai Hospital.

"When I was a kid I used to ride in a car without seatbelts and I'm still alive too. But it's still safer with seatbelts," she said.

These days, we know that foods should be kept as cold as possible for as long as possible. That's why many of us now use insulated lunch bags and ice packs.

But perhaps this study suggests even that may not be enough? McGeer agrees that more study may be needed to answer some of the questions the study raises.

As for those who say, "My kids have never gotten sick from a packed lunch," McGeer would counter: are you sure about that?

"That is not a statement that I would make with any degree of confidence," says McGeer.

Food poisoning is hard to spot, and even she would have trouble distinguishing it from illnesses caused by such contagious stomach bugs as noroviruses or rotaviruses.

Even though most food poisonings are short-lived and not life-threatening, that also doesn't mean they're a picnic. Even a mild case can mean stomach cramps, diarrhea, vomiting and maybe a day spent in bed.

"It'll make your child miserable, and it loses you time from work. It's unpleasant, no question," says McGeer.

Since bacteria can get into our food at many points in the food chain -- at the food factory, in the store, in the kitchen -- it makes sense to do as much as we can to prevent bacteria from growing in our kids' lunch bags. That's why insulated bags with ice packs are a good idea.

Following good food hygiene while preparing those lunches is also smart. So that means:

• washing your hands before touching food

• sanitizing countertops and food prep tools

• ensuring cooked foods are cooked well

• and avoiding high-risk foods when you can, such as eggs or mayonnaise

You can also try freezing drink boxes to use as ice packs. Even some sandwiches can be frozen the night before to keep them cold for as long as possible.

So how would you know if your kids' lunches are getting too warm and their hot lunches too cold? Test them, recommends Rosenbloom.

Buy a thermometer and test your insulated bottle to see how long it takes for food inside to fall into the temperature danger zone. Run the same kind of drill on a cold lunch placed inside an insulated bag with ice pack. Better yet, she suggests, test the food a few times to see when it actually warms up.

"My guess is by 11 a.m. or 12 p.m., your food will start getting in the danger zone, but it won't be there for two hours," she says.

Rosenbloom adds that if you want to ensure that your kids eat the lunches you so carefully pack, get them involved in preparing the lunch. Even little kids can help put grapes in a plastic container, for example, or spread food onto a sandwich.

"They're going to be more likely to eat their lunch if they've had a hand in choosing and preparing it," says Rosenbloom.