Chinese authorities are cracking down on fraudulent activity involving face masks, arresting dozens of people for hoarding materials and driving up prices, and closing down a factory accused of making substandard masks.

China is a key producer of surgical masks and the advanced N95 masks used by medical workers, but countries that have bought Chinese masks and test kits, including the Netherlands and Spain, have rejected tens of thousands of them for being defective.

A total of 42 people were arrested across four provinces for criminal activities involving the fraudulent production and sale of melt-blown fabric, the synthetic polymer material used to filter out particles. Melt-blown fabric is in short supply globally because of skyrocketing demand for masks and the relative difficulty in producing the nonwoven fabric.

China’s Ministry of Public Security arrested the people in Guangdong and three other provinces for hoarding and trying to drive up prices, seizing material worth almost $5 million, the ministry said in a statement Friday.

“The public security organs will always maintain a ‘zero tolerance’ approach to crimes involving protective materials related to the epidemic,” an unnamed ministry official said.

Separately, Jiangsu provincial officials shut down factories producing melt-blown fabric in the city of Yangzhong after reports they were producing inferior-quality masks for export.

The city, northwest of Shanghai, had seen a sudden proliferation of melt-blown fabric producers as prices for the material skyrocketed. The wholesale price of melt-blown fabric has increased from $2,260 to as much as $97,500 per ton in less than six months, according to the Yicai financial news website.

The Commerce Ministry has also revoked the licenses of two exporters of personal protective equipment that it said were tarnishing the image of “Made in China.” One of the companies, Shenzhen-based AIPO, was producing earphones and microphones until it switched to producing face masks, disinfectant and protective suits in February. The other, Beijing-based Tus-Digital Group, had been a blockchain tech firm.