Consumer Reports on Monday urged four chocolate producers to commit by Valentine’s Day to reduce the amounts of lead and cadmium in their dark chocolate products after testing revealed harmful levels of the heavy metals.

In letters to Hershey Co., Mondelez International Inc., Theo Chocolate and Trader Joe’s, Consumer Reports said long-term exposure to the metals can result in nervous system problems, immune system suppression and kidney damage.

It said the danger was greater for pregnant women and young children because of the risk of developmental problems. The letters were accompanied by nearly 55,000 petition signatures.

Last month, Consumer Reports said 23 of the 28 dark chocolate bars it tested included potentially harmful levels of lead, cadmium or both for people who eat more than one ounce of chocolate a day.

Consumer Reports said many consumers eat dark chocolate for its potential health benefits and relatively low sugar levels, but “there’s nothing healthy about ingesting heavy metals.”

The chocolate makers did not immediately respond to requests for comment. Consumer Reports did not immediately respond to a request for additional information.

Trader Joe’s has been sued at least nine times by consumers over its dark chocolate since Consumer Reports released its study.

Hershey’s and Mondelez have also been sued over the magazine’s findings, as have other chocolate makers, including Godiva and Lindt.