Early attempts to curb air pollution consisted of building higher smokestacks, but this just spread the pollutants to locations downwind. Now technology is being used to filter out particulates from smokestack emissions. To control sulfur oxide emissions, low sulfur fuel can be used. Also, so-called "scrubbers" can be used to remove SO_x after combustion by exposing the gases to crushed limestone (forming gypsum, calcium sulfate, as a by-product). Catalytic converters on cars use platinum beads to reduce carbon monoxide and nitrogen oxide emissions.