A catalytic converter is a component in your car's exhaust system that converts engine emissions from pollutants into less harmful substances.
In fact, catalytic converters are what permit today's cars to emit less than 1 percent of the carbon monoxide, nitrous oxides, and hydrocarbons they would have put out before 1975, when the first cars equipped with converters went on the market.
The converter itself is a metal can inserted into the exhaust pipe after the engine and before one or more of the mufflers and resonators that reduce the noise emitted by the engine.
The cannister contains a honeycomb mesh, made of ceramic or metal, that is coated with catalysts--including small amounts of such precious metals as platinum, rhodium, and palladium.
When exhaust gases pass over these chemicals, they react in ways that convert the pollutants into less harmful substances, notably carbon dioxide.
The unburned hydrocarbons (HC) and carbon monoxide (CO) are chemically combined, resulting in carbon dioxide (CO2) and water (H2O). Those substances are simply emitted out the tailpipe.
The first "two-way" converters in 1975 controlled just carbon monoxide and hydrocarbons. Newer "three-way" converters also process nitrous oxides (NOx), from which the oxygen is removed, converting them into nitrogen.
Carbon dioxide is now known to be a greenhouse gas, of course, but it's not hazardous to the human cardiovascular system in the same way as the three named pollutants.
In fact, in the early 1970s, when catalytic converters were created to respond to emissions-control rules from the then-new Environmental Protection Agency, the concept of climate change was not yet part of accepted science.
It only took six years for all new U.S. passenger vehicles to come with catalytic converters. The technology spread across most of the world's production of motor vehicles over the next two decades, to the point where virtually all new cars and light trucks now have them.
Some other type of small engines--in motorcycles, ATVs, lawn mowers, generators, chainsaws, and other appliances--have them. So do wood stoves.