It’s back-to-school season — and that means kids are once again at risk for a health hazard that has nothing to do with unhealthy school lunches or being targeted by a classroom bully.

Instead, this health issue is about the weight students carry on their backs. Thanks to heftier science and math textbooks as well as electronics like laptops, students these days are toting around heavier than ever loads in their backpacks, Karen Jacobs, clinical professor of occupational therapy and past president of the American Occupational Therapy Association (AOTA), tells Yahoo Parenting. Throw in the sports gear they need for after-school activities and the bottled water many kids take to school, and that can add up to 10, 15, even 20 pounds on their backs.

“Heavy backpacks are a health risk for children of any age because their musculoskeletal systems are not fully developed,” says Jacobs. “All that weight can cause aches and pain in a child’s neck, shoulders, back, and head.” One study even found that the weight from a backpack can can trigger spinal curvature.

Heavy backpacks are a bigger problem than most parents think, and this is supported by a troubling statistic: Approximately 8,500 kids between 5 and 18 were treated for backpack injuries in 2013, according to the U.S. Consumer Product Safety Commission.

To draw attention to the dangers of a heavy backpack, AOTA has designated Sept. 16 National School Backpack Awareness Day. AOTA experts will be at schools across the country, conducting backpack “weigh-ins” and talking to students and parents.

“It’s perfectly fine for even preschoolers to have backpacks, but it’s important to keep some safety rules in mind so the backpack doesn’t cause stress and damage,” says Jacobs.

Parents should make sure their children’s backpacks fit them comfortably and don’t leave them struggling. “Don’t just go with the superhero backpack your child wants if it isn’t supportive,” she says. These seven tips will ensure that your back-to-schooler isn’t carrying the weight of the world on her (small) shoulders.

A backpack shouldn’t clock in at more than 10 percent of a child’s weight. “That means if your kid weighs 100 pounds, the backpack shouldn’t amount to more than 10 pounds,” says Jacobs. Not sure? Toss it on the bathroom scale, and do your own weigh-in.

Use different compartments to distribute weight evenly. Heavier items, like books and laptops, should go in the center, while lighter items can go in the front and on the sides.

Encourage kids to use both shoulder straps. Hanging the backpack off one shoulder puts all the weight on one side of the body, which sets a child up for aches and pains on that side.

Secure the hip strip. “If the pack is on the heavy side, fastening the hip strap will distribute the weight better than if you secured the chest strap,” says Jacobs. “The point is to keep all of the weight off the shoulders.”

Get a backpack with wheels. For kids who can’t cut back on the amount of stuff they have to carry, encourage them to roll their backpack instead of wear it.

Watch for warning signs. A backpack that poses a health threat will cause pain or numbness, leave red strap marks on the shoulders, or change a child’s posture so he walks hunched over. Notice any of these, and it’s time to do a bag check to see why it’s so heavy and what she can leave at school or home to slim it down.

Talk to your child’s teachers. If kids at your child’s school have to bring home heavy books or tote computers every day and it’s overloading them, team up with other parents to figure out a solution. “Ask teachers to coordinate so kids don’t have to carry so much every single day,” suggests Jacobs.

Why Kids’ Backpacks Are Harming Their Health