A muffler and catalytic converter, ripe for stealing. (credit: noricum)

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.