Amazon.com Inc. has agreed to tighten quality control standards on its marketplace following an investigation that determined dozens of children’s school supplies sold on the site had illegal levels of toxic metals, lead and cadmium.

As part of the agreement hammered out with Washington State Attorney General Bob Ferguson, the company also agreed to pay $700,000, which will be used to fund environmental protection efforts. The agreement underscores the difficulty the world’s largest online retailer has ensuring that the independent merchants who generate more than half of its sales adhere to official safety standards.

The Seattle company contacted the purchasers and issued more than $200,000 in refunds earlier this year when it learned of the investigation. To resolve the issue, Amazon (AMZN) agreed to require merchants to have their products tested for toxic materials, Ferguson said Thursday in a release.

“As a parent, when I buy products for my kids, I expect them to be safe,” Ferguson said. “All retailers must ensure that their products do not pose a threat to Washington children. If they don’t, they will hear from my office.”

Both state and federal laws prohibit the sale or distribution of children’s products that contain lead or cadmium above certain levels due to health risks.

“Customer safety is Amazon’s top priority,” Amazon said in a statement. “We worked with our selling partners to verify that the school supplies and children’s jewelry in our store are safe and enhanced our processes to verify the safety of these products moving forward. We welcome ongoing collaboration with the Attorney General and other agencies to promote customer safety.”