Thieves in some cities in the US have been stealing catalytic converters at an alarming rate. Why? Because the converters—which contain small amounts of valuable metals like platinum, palladium, or rhodium—command a good price as scrap metal, with some converters selling for as much as $640 on the black market (PDF).

The National Insurance Crime Bureau (NICB) released these statistics last week, showing that catalytic converter thefts are more common than one might think, with 25,394 such thefts in the US since 2008. Just last year there were 3,986 catalytic converter thefts, making it the second-highest year for such thefts. 2014 saw the most thefts, clocking in at 4,370. The NICB also notes that its numbers might be low, because the organization can only count the number of thefts that have involved the car owner’s insurance provider.

Catalytic converters are required on vehicles sold after 1975, as they mitigate the pollutants and other emissions that the car gives off when it’s running. Without a catalytic converter, your car won’t pass a smog test.

“An aggressive thief can easily collect 10-15 or more converters in a single day,” the NICB wrote in a press release. “They often target sport utility vehicles (SUVs) because their ground clearance is sufficient for the thief to gain access to the converter without having to deploy a jack. And that saves time.”

But in California, which had the most catalytic converters stolen in the eight-year tracking period of any state, "Almost all the thefts involved Priuses," according to the Los Angeles Times. That's because Priuses also have easily accessible catalytic converters. LAPD Lieutenant Tim Torsney told the Times that some crews in the LA area are capable of removing the converters in less than a minute.

Most catalytic converters sell for between $20 to $240 each. Repairs usually cost between $500 and $2,300, but the NICB noted that this number can skyrocket because often “vehicles are significantly damaged in the theft process.” One person the LA Times spoke to said an auto repair shop charged $2,700 to replace the stolen catalytic converter on their Prius and build a steel cage around the component to discourage future theft.

The NICB recommends that people in high-risk areas have their VIN or license place number etched onto the catalytic converter to deter theft. "One could park close to the entrance of buildings or shopping centers," the Bureau adds. "Lastly, you could consider modifying your vehicle with a vehicle security system or secure the catalytic converter to the vehicle frame with additional welding."

As one might expect, the number of thefts is closely tied to the price that the metals in the converter command on the market. In 2008 the US saw 3,248 reported catalytic converter thefts, but between 2008 and 2009 the prices of platinum, palladium, and rhodium dropped precipitously, causing only 1,058 catalytic converters to be reported stolen. From there, stolen converter statistics steadily rose with the prices of those metals.

As mentioned above, California saw the overwhelming majority of reported catalytic converter thefts in the last eight years, with 8,072. Texas came in second, with 1,705 and Illinois came in third, with 1,605. In cities, Chicago, Sacramento, and Los Angeles ranked highest in reported thefts, with a whopping 980 occurring in Chicago alone and 850 and 550 happening in Sacramento and Los Angeles, respectively.