Police arrested 19 suspects in a raid of an unlicensed liquefied petroleum gas (LPG) refilling station in Pudong New Area that used substandard cylinders, the Shanghai Public Security Bureau announced Tuesday.

The bureau said that the illicit trade of LPG, which is used to fuel gas stoves and water heaters, puts consumers at risk and has called on better supervision of the industry, which primarily serves migrants and suburban households who don't have access to city natural gas service.

Police officers descended on the refilling station Friday after receiving a tip about the operation the previous Tuesday, according to a press release from the public security bureau.

They confiscated an LPG tanker truck, five trucks, 381 propane cylinders and 50,000 yuan ($7,910) in cash.

The ring had been operating since May 2012, according to police.

They ran the unlicensed business by buying between 15 and 20 tons of LPG from a city in northern China for about 5,700 yuan a ton, which they then sold in Shanghai for 6,300 yuan to 6,500 yuan a ton.

The ring used substandard cylinders, which posed a safety hazard to their customers, police said.

Ring members bought the cylinders for 50 yuan each, primarily from recycling centers and households that were about to be relocated.

After refilling the substandard cylinders, they transported them back to the suburbs for sale. Most of their customers were migrant workers and small restaurants.