Packaged food usually sits in plastic-lined bags designed to keep air, moisture and contaminants out. That same protection can also increase the chances of chemicals from the packaging seeping into the food. When a chemical raises health concerns, industries often swap it for a molecular cousin, a substitution that may either be genuinely safer or simply be too understudied to know whether it is harmful. With thousands of chemicals in use, one-by-one regulation can't keep pace, so untested replacements can slide through unchecked.