CHICAGO (AP) — Federal officials say about 500,000 counterfeit N95 respirator masks have been seized in Chicago by Customs and Border Protection officers.

The agency says the shipment of masks from China were seized Sept. 10 at O’Hare International Airport. Officials say the shipment was headed to a company in Manalapan, New Jersey.

The masks are used to prevent the spread of the COVID-19 virus. Thirty of the masks were sent to a testing facility where it was determined that 10% of the respirators tested had a filter efficiency rating below 95%.

Frontline workers, hospital officials and even the Food and Drug Administration say there is a shortage of the masks in the U.S. due to pressures on the medical supply chain.