If you don't include ice packs in your child's sack lunch, the food he or she takes to school will warm up before lunchtime.

That can be a problem if the lunch includes a ham sandwich, chicken salad or other foods that should be refrigerated to keep bacteria from growing to illness-causing levels.

In a study published in the September issue of the journal Pediatrics, researchers in Texas measured the temperatures of foods in sack lunches of preschool-aged children. They found that only 1.6 percent of perishable foods in the kids' lunches were in the safe temperature zone — below 40 degrees or above 140 degrees.

Perishable food should not be left at room temperature for longer than two hours (or one hour if the temperature is above 90 degrees), experts say.

Child care centers in Nebraska are required to provide food to kids, and bringing food from home is discouraged, said Janet White Phelan, executive director of the Midwest Child Care Association. So any problems with warmer food are more likely to surface at schools, where kids don't have access to refrigerators.

Some tips for keeping sack lunches safe:

>> Pack just the amount of perishable food that can be eaten at lunchtime. Throw away perishable leftovers if a refrigerator is not available.

>> Insulated, soft-sided lunchboxes or bags are best for keeping food cold, but metal or plastic lunchboxes and paper bags also can be used. If using paper lunch bags, create layers by double bagging to help insulate the food.

>> Frozen juice boxes can provide another ice source in lunchboxes or bags. The juice likely will thaw by lunchtime.

>> Prepackaged combos that contain lunch meats along with crackers, cheese and condiments must be kept refrigerated. This includes lunch meats and smoked ham that are cured or contain preservatives.

>> It's fine to prepare the food the night before and store the packed lunch in the refrigerator. One mom freezes scores of peanut butter and jelly sandwiches at the beginning of a week, and doles out one per day to her kids. Her tip to keep the bread from becoming mushy: Put peanut butter on both slices of bread. Freezing sandwiches helps them stay cold, and can help other items in the sack stay cold as well. However, for best quality, don't freeze sandwiches containing mayonnaise, lettuce or tomatoes.

>> Use an insulated container to keep food like soup, chili and stew hot. Keep the insulated container closed until lunchtime to keep the food at 140 degrees or above.

>> At lunchtime, discard all used food packaging and paper bags. Do not reuse packaging.

>> Some food is safe without an ice pack. Items that don't require refrigeration include whole fruits and vegetables, hard cheese, dried fruits, beef jerky, canned or vacuum-packed meat and fish, chips, bread, bagels, crackers, peanut butter, jelly, mustard, pickles, granola bars, trail mixes, nuts and single-serve applesauce containers.

>> Include an ice pack in any lunch if the food you purchased was in the grocery store's refrigerated section. Or plan on keeping your lunch bag in a refrigerator.

>> Consider having your child eat the lunches provided by his or her school.

>> Prepare cooked food — turkey, ham, chicken and vegetable or pasta salads — ahead of time to allow for thorough chilling in the refrigerator. Divide large amounts of food into shallow containers for fast chilling and easier use. Keep cooked food refrigerated.

For general food-safety tips, go to www.foodsafety.gov