The Food and Drug Administration has warned the public not to consume certain frozen shrimp products sold at Walmart due to possible contamination with Cesium-137, a radioactive isotope.
It said the warning affects the Great Value brand of raw frozen shrimp sold at the superstore, adding that anyone who purchased the products should dispose of them.
A statement from the agency said the FDA was “actively investigating reports of Cesium-137 (Cs-137) contamination in shipping containers and frozen shrimp products” shipped from Indonesia.
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It said that the U.S. Customs & Border Protection (CBP) had detected Cs-137 in shipping containers at four U.S. ports: Los Angeles, Houston, Savannah, and Miami. Containers that tested positive for the isotope have been denied entry to the U.S.
The FDA added that although it had not confirmed the presence of contamination in any commercial product in the United States, the shrimp products appear “to have been prepared, packed, or held under insanitary conditions whereby it may have become contaminated with Cs-137 and may pose a safety concern.”
“If you have recently purchased raw frozen shrimp from Walmart that matches this description, throw it away. Do not eat or serve this product,” the FDA statement added.
The FDA said limited exposure to Cs-137 can cause “an elevated risk of cancer, resulting from damage to DNA within living cells of the body.”
The amount of Cs-137 the FDA detected is below its “levels of concern” for imported foods, it said. Even so, the agency said that the recall attempts to address concerns related to “longer term, repeated low dose exposure.”
The FDA recommended that anyone who has bought or who sells the products throw them out. Walmart similarly advised customers with the products in their possession to discard them. The company confirmed to TIME that it immediately recalled the products from impacted stores and said customers who had already purchased them could visit any Walmart store for a full refund.
“The health and safety of our customers is always a top priority,” a Walmart spokesperson said in a statement. “We have issued a sales restriction and removed this product from our impacted stores. We are working with the supplier to investigate.”
Cs-137 is the most common radioactive isotope of Cesium, according to the U.S. Environmental Protection Agency (EPA). It is produced via nuclear fission in order to be used in medical devices and gauges, and it is often used in radiation therapy to treat cancer.
Cs-137 can also be a waste product of nuclear reactors and has been found near nuclear accidents.
People are often exposed to small quantities of Cs-137 due to the isotope remaining in the environment following nuclear weapons testing in the mid-20th century. Cs-137 can last in soil for years, which means that it often ends up in the food cycle and in foods that people ingest. Typically this is at extremely low levels that are not harmful to people—though as the FDA emphasized in its statement, even low levels of Cs-137 radiation can eventually build up and become harmful.
Large amounts of the isotope, meanwhile, can be incredibly dangerous, and in addition to increasing cancer risks, can cause burns, radiation sickness, and even death, according to the Centers for Disease Control and Prevention (CDC).