BEIJING (Reuters) - Customs officers in China’s commercial capital, Shanghai, have discovered 120,000 fake respiratory masks, supposedly made by U.S. diversified manufacturer 3M Co, state media said on Wednesday.

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News of the seizure came as air pollution has engulfed the capital, Beijing, with authorities declaring a “red alert” there because of dangerously high levels of smog.

Many people in China wear face masks in the hope of preventing the breathing of pollution and airborne viruses, or to stop themselves spreading colds and coughs to others.

The fake masks were discovered in two batches during inspections at a Shanghai customs site, the state-run China News Service said, adding that many people had been detained in the course of a broader investigation into fake masks.

China has been trying to rein in counterfeiters who have copied everything from Apple iPhones to Louis Vuitton handbags, since it joined the World Trade Organization in 2001, when it was required to adhere to global standards on intellectual property rights.

Officials at China’s 3M office were not available for comment.

The seizure of the fake masks has increased Shanghai’s tally of counterfeit cases to 30 this year, which involve more than 700,000 products, the news agency cited the Shanghai customs bureau as saying. It declined to comment to Reuters.

Beijing and four other cities in northern China have issued pollution red alerts. Beijing’s red alert is triggered when the government believes air quality will surpass a level of 200 on an air quality index that measures various pollutants for at least three days.

The U.S. government deems a level of more than 200 “very unhealthy”.

Rapid industrial growth and car ownership have led to surging levels of pollution in major urban areas and Chinese researchers have identified the problem as a major source of unrest.

A red alert triggers restrictions on vehicles while schools are closed, businesses recommended to allow flexible hours and the government advises that all “large-scale, outdoor activity” be stopped.

(The story corrects paragraph 8 to show that Beijing’s red alert is triggered when the government forecasts at least three days of bad pollution, not just when it is expected to surpass a certain level)