Without an emissions control system, harmful gases from your car's exhaust would be pumped into the earth's atmosphere without any control. The emissions control system contains several components to help keep potential pollutants from damaging the air you breathe. The main air-cleaning component is the catalytic converter. The catalytic converter chemically transforms potentially harmful gases into safe substances. The e-g-r, or exhaust gas recirculation valve reduces the temperature of combustion in the engine. The thick haze , or smog, that often hangs over cities is thought to be caused by nitrous oxides in exhaust, which are a product of high combustion temperatures. The positive crankcase ventilation system, or p-c-v valve routes potentially dangerous crankcase gases back into the engine manifold so they'll combust. An air pump system mixes oxygen with unburned exhaust gases to help them burn easier. Your car may be equipped with some, or all of these components, and they should be checked periodically to make sure they're in good working order. Contact a qualified local mechanic for further questions about your car's emission system.