Button batteries are mighty small — and mighty dangerous. The round batteries injure or kill more than 3,500 people each year, according to the National Capital Poison Center (NCPC). The latest victim: a 2-year-old in Oklahoma, who died six days after swallowing one.

According to the American Academy of Pediatrics Button Battery Task Force, the most serious injuries usually are caused by batteries just 20 millimeters in diameter, about the size of a nickel. When a button battery is swallowed it often passes safely through the body, but sometimes it can get stuck in the esophagus, where it forms a corrosive chemical that damages tissue.

Related: Parents: Beware of Button Batteries

A ubiquitous threat

Button batteries are becoming more and more common. You'll find them in cameras, hearing aids, singing greeting cards, flameless candles, toys and watches. "Every car now sold has a button cell in the key fob," says John Drengenberg, consumer safety director for UL.

It's critical to be vigilant when handling them, says Drengenberg. "Even a battery that's dead can have enough charge to harm human tissue."

"When you change batteries it's important to dispose of old ones right away," says Drengenberg. That doesn't mean toss them into an open trashcan. "Put used button batteries in a plastic bag in the outdoor garbage, where a child won't be tempted by it. Some communities also accept button batteries for recycling."

Little kids aren't the only people susceptible to injury from button cell batteries. Older adults, whose eyesight may be failing, can mistake a loose button battery for a pill, warns Drengenberg. If an older parent or grandparent lives with you, make sure the batteries she uses in her hearing aid aren't kept anywhere near her medication.

In 2015 UL established standards for products that have button batteries inside. "One thing we look at," says Drengenberg, "is how the compartment where the battery is stored closes. It used to be there was a cover that you could just slide off and on. Now these compartments are secured with tiny screws." Look for the UL Mark on these products.

Related: Top Home Safety Tips for People with Kids

What to do if someone downs a battery

It’s impossible to know if a battery will pass through or get hung up in the esophagus, so if a child (or anyone) accidentally ingests one, the NCPC says to take these steps immediately.

  1. Call the 24-hour National Battery Ingestion Hotline at 202-625-3333. It’s likely an x-ray will be necessary to make sure the battery made it all the way to the stomach. The Hotline specialist will help you figure out if one is needed.
  2. If possible, give the operator the battery’s ID number, which will be listed on the package or a matching battery.
  3. Do not try to make the person throw up or allow him to eat or drink anything until an x-ray (if needed) shows the battery is past the esophagus.
  4. Watch for fever, stomach pain, vomiting or blood in stool. Call the doctor right away if any of these develop.
  5. Check stools until the battery passes.

Related: 8 Smart Ways to Toddler-Proof Your Kitchen